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     Airborne Digital Frame Cameras
     The Technology is Really Improving!
     As recent visits to the respective factories in Heerbrugg and Oberkochen have shown, at the present                           offering high-quality film processing and scanning ser-
                                                                                                                                   vices - which means that the air survey companies do
     time, the pre-eminent airborne imagers used for the acquisition of image data for mapping and GIS pur-                        not have to invest in these facilities themselves.
                                                                                                                                   Examples of these bureaux include HAS Images and
     poses are the metric photographic film frame cameras from Leica (in the shape of its RC30) and Z/I                            Precision Photo & Imaging, both located in Dayton,
                                                                                                                                   Ohio, home of the Wright-Patterson Air Force Base with
     Imaging (with its RMK-TOP). Both of these film cameras are still enjoying substantial sales and the                           its Aeronautical Systems Center and its famed recon-
                                                                                                                                   naissance laboratories.
     newly-built examples join the many hundreds of their predecessors that are in everyday use world-wide.
                                                                                                                                   Besides the economies in facilities and staff that result
     They will continue to be of great service to the mapping community for a long time to come. However                           from using digital cameras, the radiometric quality of
                                                                                                                                   the 11- or 12-bit images with their greater dynamic
     digital frame cameras are now beginning to be used more widely, especially in the United States. Format                       range is (or should be) superior to those produced by
                                                                                                                                   film-based cameras - though this loudly trumpeted
     sizes and ground coverage from a given flying height and for a stated ground resolution are still quite                       advantage is not always too apparent to the user.
                                                                                                                                   Going further with this discussion, the storage, archiv-
     small in comparison with those attainable with the metric film cameras. Nevertheless, slowly but surely,                      ing and proper maintenance of numerous rolls of large-
                                                                                                                                   format film is another matter of concern and expense -
     digital frame camera technology is improving and, especially with the smaller formats, there now exists                       though the reliable handling, storage and back-up of
                                                                                                                                   the huge amounts of digital image data generated by
     a significant market for airborne digital frame cameras supported by an enthusiastic band of users. The                       airborne imaging and mapping is probably a matter of
                                                                                                                                   equal concern. On the other hand, metric film cameras
     purpose of this article is to provide an overview of some of the current developments in airborne digital                     do have some decided advantages, notably their large
                                                                                                                                   formats, which allow much greater areal coverage of
     frame camera technology as they apply to mapping and GIS operations. Not every digital camera that                            the ground in a single exposure from a given flying
                                                                                                                                   height and for a given ground resolution. Besides
     has been mounted and operated from airborne platforms will be covered in this account. As other recent                        which, at this early stage of development, those few
                                                                                                                                   airborne digital cameras that come anywhere near the
     visits to the United States have revealed, new digital cameras and applications are continually appearing.                    performance of metric film cameras cost at least twice
                                                                                                                                   as much as their already expensive film-based equiva-
     So only a representative sample of these cameras will be covered in this article. Airborne pushbroom                          lents. Hopefully this situation will improve as the digital
                                                                                                                                   technology develops and matures. However, if cameras
     line scanners such as the DLR HRSC-A series, the Leica Geosystems ADS40 and the Japanese Starlabo                             with small or medium formats are indeed suitable for a
                                                                                                                                   specific airborne image data acquisition and mapping
     Three Line Scanner (TLS) will not be considered in this account - since they are line scanners, not frame                     task, then already digital cameras offer much both to
                                                                                                                                   the organisation operating the camera and to the user
     cameras, with quite different geometric and imaging characteristics.                                                          of the resulting digital image data. In fact, it would be
                                                                                                                                   true to say that, for small formats, digital frame cam-
                                                                                                     by Prof. Gordon Petrie        eras are now approaching a dominant market position -
                                                                                                                                   though, of course, large numbers of existing small- and
     Film v. Digital - Pros and Cons                                 dedicated darkroom-cum-laboratory; and highly skilled         medium-format film cameras still remain in service with
     The advantages of digital frame cameras over film               staff. Besides which, the finally developed film needs to     air forces and other organisations.
     frame cameras are fairly obvious and quite well known           be scanned in an expensive high-quality film scanner if
     - chiefly that there is no film! Thus there is no need for      it is to be used in a digital image processing system or      The production of colour and false-colour images is a
     the organisation operating the camera to buy rolls of           a digital photogrammetric workstation (DPW) - though          special problem with digital frame imagers. With film
     expensive stable (metric-quality) wide-format photo-            not if it is to be used in an analytical plotter or ana-      frame cameras, to change from a monochrome (black-
     graphic film and to have it chemically processed after          logue stereo-plotter, which are still in use in surprising-   and-white) photographic film to a colour or false-colour
     its exposure in the camera. The film processing itself is       ly large numbers in many mapping organisations.               film is a simple matter - just insert the appropriate roll
     a demanding and quite costly procedure since it                 However, in this particular matter of film processing         of film into the empty film magazine or cassette and the
     requires an expensive processing machine; the use of            and scanning, it must also be noted that many air sur-        camera is ready for action. But a comparable procedure
     lots of different chemicals having a short shelf life; a        vey companies in the U.S.A. now use specialist bureaux        is not possible with digital cameras - since the CCD
                                                                                                                                   areal arrays that are used in digital cameras are inher-
                                            (a)                                                                             (b)    ently monochromatic. Various techniques have been
                                                                                                                                   devised to overcome this particular characteristic. One
                                                                                                                                   of the most popular is the use of an interpolation pro-
                                                                                                                                   cedure in conjunction with a mosaic of tiny (pixel-sized)
                                                                                                                                   colour filters placed over the array of detectors.
                                                                                                                                   However this affects the quality of the resulting image.
                                                                                                                                   Another commonly employed solution is to use multiple
                                                                                                                                   digital cameras - with each camera recording a specific
                                                                                                                                   spectral band to make up the final composite colour or
                                                                                                                                   false-colour image which is produced by image process-
                                                                                                                                   ing. Needless to say, having to use multiple cameras
                                                                                                                                   and filters is costly and may be inconvenient. Usually it
     Fig. 1 - The large-format (23 x 23cm) metric film frame cameras that form the present standard for airborne imaging for       results in the format size being reduced with conse-
     mapping purposes against which the performance of the new airborne digital frame cameras is being measured - (a) the          quent reductions in the ground resolution or ground
     Leica Geosystems RC30; and (b) the Z/I Imaging RMK-TOP. (Sources: Leica Geosystems & Z/I Imaging)                             coverage of the resulting aerial images. There does exist



18                                October/November 2003
                                                                                                                                                                 Article

                                                                                                                              (a)      veys and mapping of vegetation and with environmen-
                                                                                                                                       tal monitoring. Many of these DCS-420 and -460 cam-
                                                                                                                                       eras are still in productive use for airborne imaging
                                                                                                                                       today.

                                                                                                                                       By contrast, the later DCS-560, -660 and -760 series of
                                                                                                                                       cameras all utilized 3k x 2k pixel = 6 Megapixel CCD
                                                                                                                                       areal arrays based on an indium-tin-oxide (ITO) alloy
                                                                                                                                       that did not have a sensitivity in the near infra-red part
                                                                                                                                       of the spectrum. Thus they could not be used to gener-
                                                                                                                                       ate false-colour images, though, of course, they could
                                                                                                                                       still generate true- colour images. The last in the series
                                                                                                                              (b)      - the DCS-760 - has just been discontinued and is
                                                                                                                                       being replaced by Kodak's new DCS Pro 14n digital
                                                                                                                                       frame camera. Again this is built around a 35mm SLR
                                                                                                                                       camera body manufactured for Kodak by Nikon.
                                                                                                                                       However the DCS Pro 14n features a much larger CMOS
                                                                                                                                       (instead of CCD) areal array sensor providing images
                                                                                                                                       with a format size of 3,024 x 4,536 pixels = 13.89
                                                                                                                                       Megapixels. Each detector is 8µm square in size. It will
                                                                                                                                       be most interesting to see if this camera will come into
                                                                                                                                       use for airborne imaging in the same way as the previ-
                                                                                                                                       ous DCS-460, -560, -660 and -760 series.

Fig. 2 - (a) Mosaic Filters. In order to capture colour or false-colour images, a mosaic of tiny (pixel-sized) coloured filters is     (a) European Examples
placed over a single layer of CCD detectors. Each detector (pixel) can only capture radiation in a single spectral band -              In the case of the GeoTechnologies consultancy from
either red, green or blue. The values for the other colours for each specific pixel position are interpolated from the values of       Bath Spa University College in the U.K., it devised its
the surrounding pixels.
                                                                                                                                       ADPS (Aerial Digital Photographic System) based on
(b) The Foveon X3 technology comprises three separate layers of CMOS detectors, with each layer capturing a complete
                                                                                                                                       the use of the older Kodak DCS-420 and -460 cameras.
image in a different spectral band or colour. (Source: Foveon)
                                                                                                                                       To the camera was added a specially designed mount
a new digital imaging technology using a three-layer                I.1 Single Cameras with Mosaic Filters                             with anti-vibration pads, an electronics control unit and
CMOS areal array corresponding to the three-layer                                                                                      a small inexpensive GPS set that could be used for in-
colour or false-colour photographic emulsion. This is the           Kodak's DCS Series                                                 flight navigation and control purposes. A small Psion
X3 areal array developed by the Foveon company from                 Although several other amateur or professional digital             palmtop computer was also used to carry out flight
Santa Clara, California with a small format size of 2.3k x          cameras have been used to acquire images from the air              track planning and mission planning. These ADPS sys-
1.5k pixels = 3.5 Megapixels. So far, this technology has           for mapping or monitoring applications, the main small-            tems have been used very successfully from light air-
not been adopted for airborne imaging. However it is                format digital frame cameras that have been used for               craft for local use where rapid response is a key issue -
still early days for such an application, since the prod-           the purpose have been Kodak's DCS series. The DCS-                 e.g. during flooding or forest fires - or in situations
uct was only introduced to the market in 2002.                      200 from 1992 was an early example. More popular                   where frequent flights are being undertaken over limit-
                                                                    have been the later DCS-420 and DCS-460 models that                ed areas - e,g, for coastal zone monitoring. Thus the
Airborne Digital Frame Cameras                                      were first introduced in 1994 and 1996 respectively.               ADPS systems have been sold mainly to research
Since the size of the areal array of detectors is, at pre-          Both of these cameras have been out of production for              groups in universities (Cranfield, King's College London,
sent, the single most important factor that controls the            some years and have been replaced - though not with                Ulster, etc.) and environmental research agencies. In
suitability, availability and usage of digital frame cam-           quite the same functionality - by the later DCS-560, -             1998, Professor Maas, then at the Technical University
eras in the aerial mapping field, they will be considered           660 and -760 models, the last dating from 2001. The                of Delft (and now professor in Dresden), also reported
here under three main headings:-                                    DCS-200 and -420 models both incorporated Kodak's                  on his pilot studies in the Netherlands based on the
(I) small-format cameras, typically generating images               smaller M5 silicon-based CCD areal array producing                 use of the DCS-200 and -460 cameras mounted on a
     with formats of 1,000 x 1,000 to                               images with a format size of 1,012 x 1,524 pixels = 1.5            helicopter in a similar manner.
     2,000 x 3,000 pixels, i.e. between 1 and 6                     Megapixels, with each detector (pixel) on the array
     Megapixels;                                                    being 9µm square in size. The later DCS-460 cameras              (b) Canadian Examples
(II) medium-format cameras with image formats typical-              were fitted with the larger M6 silicon-based CCD areal           Yet another university developed system is that built at
     ly around 4,000 x 4,000 pixels =                               array producing images with a format size of 2,036 x             the University of Calgary in Canada. This comprises two
     16 Megapixels; and                                             3,060 pixels, amounting to 6 Megapixels in total, again          of Kodak's DCS-420 cameras, the one mounted in the
(III) large-format cameras having a format of 6,000 x               with 9µm square detectors in the array. In each case,            nadir pointing position, the other mounted in an
     6,000 pixels = 36 Megapixels or larger.                        the CCD array was housed in the body of a Nikon                  oblique position 30° from the vertical. These have been
                                                                    35mm single lens reflex (SLR) camera body.                       interfaced to a Honeywell strapdown IMU and an
                                                                    Furthermore, the arrays incorporated Bayer mosaic fil-           Ashtech GPS receiver, which provide accurate in-flight
I. Small-Format Digital Cameras                                     ters with a separate filter placed over
                                                                    each individual detector in the array.
In particular, the developments in this field have result-          With a Bayer pattern filter, the tiny
ed in a rich variety of solutions that generate multi-              green, red or blue filters are inter-
band images. Four different types of system can be dis-             mingled in a systematic pattern with
tinguished:                                                         50% of the resulting pixels being
(i) single cameras equipped with a mosaic filter and                green, 25% blue and 25% red. For
     producing their colour                                         the DCS-460, these produce, via
     or false-colour images by interpolation;                       interpolation, colour or false-colour
(ii) single cameras equipped with rotating filter wheels            images amounting to 3 x 6
     to produce multi-band images;                                  Megapixels = 18 Megapixels over the
(iii) single cameras fitted with three CCDs and a beam              three bands - either blue, green and
     splitter and suitable filters to produce colour or             red (for true-colour) or green, red
     false-colour images; and                                       and near infra-red (for false-colour).
                                                                                                              Fig. 3 - The ADPS (Aerial Digital Photographic System) based on the Kodak DCS-
(iv) multiple cameras coupled together and equipped                 This latter capability was and is         460 CIR digital frame camera which is produced by the GeoTechnologies consul-
     with the appropriate colour filters to produce multi-          especially attractive to those field      tancy. Besides the camera, the system features an anti-vibration mount and an
     band images from which colour or false-colour                  scientists such as foresters, botanists intervalometer and power pack, together with a Trimble GPS unit. (Source: Bath
     images can be produced.                                        and ecologists concerned with sur-        Spa Univ. College)



Latest News? Visit www.geoinformatics.com                                                                                            October/November 2003                                          19
                        Article

     (a)                                                             (b)                                                               These are based on Kodak's MegaPlus monochrome
                                                                                                                                       (black-and-white) digital camera range, which includes
                                                                                                                                       numerous different models, each with a different size of
                                                                                                                                       CCD areal array. They include the MegaPlus 1.0 (provid-
                                                                                                                                       ing 1 Megapixel images); 1.4i (with 1.4 Megapixels); 1.6i
                                                                                                                                       (1.6 Megapixels); 4.2i (4.2 Megapixels); 6.3i (6.3
                                                                                                                                       Megapixels); and 16.8i (16.8 Megapixels). In the case of
                                                                                                                                       SenSyTech's AA456 Airborne Digital Camera (ADC), the
                                                                                                                                       Megaplus 4.2i with 2,020 x 2,041 pixels = 4.2
                                                                                                                                       Megapixels forms the basis of the system to which
                                                                                                                                       SenSyTech has added a camera mount; a system con-
     Fig. 4 - (a) The Tetracam ADC digital frame camera developed specifically for the capture of small-format false-colour frame      trol unit (including a built-in GPS set); and an operator
     images from light aircraft, flown at low altitudes for agricultural applications.                                                 control and display unit for its use in airborne imaging.
     (b) The ADC camera uses a CMOS areal array in conjunction with a Bayer mosaic filter and features a colour LCD                    The SenSyTech ADC system generates monochrome
     viewfinder and display on the back of the camera. (Source: Tetracam)                                                              images only.

     positional and attitude data that can be used for image          to cover any substantial area of terrain. In response to         This was then followed by the AA497 Airborne Multi-
     rectification purposes. A further development, also              this particular characteristic of these very small-format        spectral Digital Camera (AMDC) which was developed
     undertaken in Calgary by the Alberta Research Council            images, image processing packages such as DIME                   by SenSyTech in the late 1990s in cooperation with
     is DORIS (Differential Ortho-Rectification System). It           (Digital Images Made Easy!!) from Positive Systems and           ERIM International. This featured the same basic units
     comprises a Kodak DCS-520 camera, an Applanix                    COBRA from Inpho's Finnish associate company have                as the AA456, but to it were added coloured optical fil-
     POS/AV IMU/DGPS unit and an on-board NovaTel DGPS                been developed that can handle the huge numbers of               ters mounted on a fast rotating filter wheel that was
     receiver. However, to this system has been added an              small-format images in a highly automated manner to              placed in front of the camera lens, together with an
     Optech airborne laser scanner (lidar). The IMU/DGPS              generate a single large and homogeneous orthoimage               IMU to provide attitude data. With the ability of the
     combination gives the data needed for direct geo-refer-          mosaic in digital form for the end-users. Of course, this        MegaPlus 4.2i camera to acquire digital images at the
     encing, while the lidar generates the DEM on which the           operation also requires the availability of a DEM, which         rate of two per second, this arrangement allows the
     final ortho-rectification of the frame images is based.          is normally supplied from some other existing source if          AA497 system to acquire between two to five individu-
     Still in Calgary, a commercial company, VeriMap, oper-           a full ortho-rectification needs to be implemented.              al images of the ground in a rapid burst with between
     ates a Kodak DCS-420 (or alternatively, a Mitsubishi                                                                              80 to 98% overlap between them. The actual overlap is
     thermal IR camera) in conjunction with a Litton IMU &            Tetracam                                                         dependent on the speed of the airborne platform and
     DGPS combination and a lidar system. This has been               Tetracam is an independent manufacturer of small                 the number of individual spectral images being cap-
     flown extensively in the U.S.A., as well as Canada.              multi-spectral cameras that is based in Chatsworth,              tured in the burst. Nominally the images are supposed
                                                                      California. Previously it produced its MCA camera sys-           to cover the same piece of ground, but obviously each
     (c) American Examples                                            tem which employed four separate imaging channels                individual spectral band image is taken from a slightly
     In a parallel but quite independent development at the           with separate lenses and filters for each channel to             different position in the air as the airborne platform
     University of Florida, a system has also been built com-         produce multi-band images from which colour or false-            flies forward. The consequence of this sequential
     prising two cameras for use in the Florida Gap Project.          colour images could be generated. This system is now             method of image capture is the requirement for special
     The first of these is a video camera linked to a video           being re-designed. However, currently the company's              software to carry out the rectification and registration
     recorder mounted in the aircraft. The second is a Kodak          main product is its ADC multi-spectral camera designed           of the individual spectral images needed to produce
     DCS-420 camera linked to an on-board computer. An                specifically for agricultural use. The ADC is a single-lens      the final multi-spectral image. This software was devel-
     Accupoint flight navigation and camera control unit is           camera featuring a Motorola CMOS areal array that gen-           oped by ERIM. The resulting images are then available
     installed that accepts inputs from a Garmin DGPS                 erates a 1,280 x 1,020 pixel = 1.3 Megapixel image. The          to the user either as separate band images or as
     receiver and a Watson digital gyro system that mea-              array is equipped with a Bayer mosaic filter. The                colour or false-colour composite images.
     sures the pitch, roll and heading of the aircraft. The           resulting colour image can be seen directly in the air-
     control unit is used to help direct the flight path of the       craft on a tiny screen fitted to the back of the camera
     aircraft and to trigger the cameras at the appropriate           and is stored on a flash card. The camera is also                I.3 Single Cameras with a
     positions. The resulting imagery is being used to collect        equipped with a USB port to allow it to be connected             Beam Splitter & 3 CCD Arrays
     land cover samples and transects to assist in the over-          to a laptop computer and a serial port that gives con-
     all classification of Florida's land cover.                      nection to a GPS. The body of the tiny battery-operated          Redlake Multi-Spectral (MS) Cameras
                                                                      camera measures 5.5 x 3 x 2 inches (14 x 7.5 x 5cm)              A quite different approach to the acquisition of digital
     Turning next to commercial developments in this field            without the lens and has proven to be very popular for           colour and false-colour images from airborne platforms
     in the U.S.A., Positive Systems of Whitefish, Montana            agricultural applications. Thus it has sold widely to            using a single camera has been taken by the Redlake
     also developed various ADAR systems that were similar            individual farmers and crop consultants as well as to            company located in San Diego, California. This is based
     to the ADPS system of GeoTechnologies. They were                 large organisations such as Ag Canada and the U.S.               on the technology developed by the DuncanTech com-
     also based on the DCS-420 and -460 digital frame cam-            Dept. of Agriculture and to agriculture and agronomy             pany that was taken over by Redlake in November
     eras. These have been sold to various universities and           departments in several American universities.                    2002, Redlake having previously acquired Kodak's
     research organisations in the U.S.A. and in Central and
     South America. Furthermore a large American company,             I.2 Single Cameras
     Landcare Aviation, based in Oriskany, New York, has              with Rotating Filter
     operated no fewer than 15 small Cessna single-engined            Wheels
     and Piper Aztec twin-engined aircraft, each equipped
     with a Kodak DCS-460 or -560 camera linked to a dual-            SenSyTech ADC & AMDC
     channel GPS set; a precision Litton 200 fibre-optic              Cameras
     based IMU; and an on-board GPS-based flight manage-              The SenSyTech company is
     ment system. These systems have been operated by                 based in Ann Arbor,
     Landcare mainly on behalf of the Emerge company, for-            Michigan and was formerly
     merly part of the large Litton electronics group and             known as Daedalus
     now part of the huge ConAgra food company. Emerge                Enterprises. It was well
     has been concerned heavily with land use management              known for its early multi-
     and with agricultural applications of the resulting colour       spectral and infra-red lines-
     and false-colour imagery.                                        can imagers that date from
                                                                      the early 1980s onwards.
     Image Processing & Rectification                                 However its current product      Fig. 5 - The SenSyTech AMDC (Airborne Multispectral Digital Camera) comprising the
     Since the ground coverage of each DCS frame image is             line also includes two digi-     digital frame camera with its filter wheel and IMU; the control unit with an integral GPS
     tiny, very large numbers of images need to be acquired           tal frame camera systems.        receiver; and a display and keyboard housed in a portable case. (Source: SenSyTech)



20                                October/November 2003
                                                                                                                                                            Article

 (a)                                       (b)                                  offered by Airborne Data Systems of               ments and the Zeiss TA-5 gyro-controlled mount to sta-
                                                                                Wabasso, Minnesota. This is based on the          bilize the multiple camera unit during flight - though
                                                                                MS3100 or MS4100 camera combined with             the addition of these systems adds considerably to the
                                                                                an electronics control unit, a GPS set and        cost of a system. Flight planning software
                                                                                flight control and navigation software.           (TerraMission); in-flight control and monitoring software
                                                                                Similarly the Australian company, Integrated      (TerraFlight); and post-flight processing software for
                                                                                Spectronics from Baulkham Hills, New South        band-to-band registration and radiometric corrections
Fig. 6 - (a) The Redlake (formerly DuncanTech) MS4100 digital frame cam- Wales - which is well known for its HyMap                to the recorded images are also provided as part of the
era fitted with a single lens and three separate CCD areal arrays to
                                                                                hyperspectral scanner - has produced its          overall system. STI Services also offer the less expen-
record images of the same piece of ground in three separate spectral
                                                                                ISAACS (Integrated Spectronics Airborne           sive DMSV multi-spectral camera system. This has a
bands simultaneously to generate either a true-colour or false-colour
image. (Source: Redlake)                                                        Acquisition Camera System) which is also          very similar configuration, but utilizes four very low-
(b) A diagram of the beam splitting and colour separating prism used in         based on Redlake's MS3100 and MS4100              noise video cameras with each producing a much
the MS4100 camera allowing the simultaneous capture of the three spec-          models. To these, the company has added a         smaller format (578 x 740 pixels) than the CCD digital
tral bands required for the production of true-colour or false-colour frame ruggedized mount, a control unit and con-             cameras used in the TS-1 and TS-3. It appears from the
images at full resolution and without interpolation. (Drawn by Mike             nections to an Applanix POS/AV unit and           Web site that the DMSV system has been developed in
Shand)                                                                          the Fugro Omnistar DGPS system, as well as        collaboration with SpecTerra Systems Pty. Ltd., based in
                                                                                providing its own image data acquisition          Perth, Western Australia. Among the users is the
EAMD division with its range of MegaPlus cameras a                 and processing software.                                       AeroMap mapping company based in Alaska.
year or so before. Unlike the various digital cameras
that have been discussed up till now - all of which use                                                                           (b) Space Imaging
a single CCD array to collect their images - the                   I.4 Multiple Camera Systems                                    Terra Systems has also been the supplier of the DAIS-1
DuncanTech/Redlake cameras use three quite separate                                                                               (Digital Airborne Imaging System-1) operated by Space
CCD areal arrays to collect simultaneously the three               A quite thriving area, especially in the United States,        Imaging for certain types of mapping and land cover
individual band images that make up a false-colour                 has been the development of multiple small-format              classification projects. Although this is a custom-built
(CIR) image. This is achieved through the use of a sin-            frame cameras that are coupled together with their             system, it is, in many ways, very similar to the TS-1
gle lens in conjunction with a beam splitting and                  optical axes set parallel to cover the same piece of           system. It comprises four Dalsa CA-D7-1024T digital
colour separating prism that is located immediately                ground simultaneously to form a multi-spectral imaging         frame cameras, each equipped with a 1,024 x 1,024
behind the lens. The three CCD areal array sensors cap-            system. The basis for all of these systems has been to         array of CCD detectors, with each detector 12 x 12µm in
ture the entire frame image simultaneously in different            use relatively low-cost small-format monochrome CCD            size. As with the other TS systems, the spectral filters
spectral bands - which is a necessity when the images              cameras and fit them with appropriate filters to gener-        allow the acquisition of individual band images in the
are being captured from a moving airborne platform.                ate the multi-band images from which the final true-           blue, green, red and near infra-red parts of the spec-
These cameras overcome the limitations of the two pre-             colour or false-colour frame images can be generated.          trum. The DAIS-1 is mounted on a Zeiss TA-S gyro-sta-
vious techniques where the true-colour or false-colour             Currently a considerable number of these systems are           bilized mount and has an on-board differential GPS
images were being generated either by using a Bayer                operational.                                                   receiver and an Applanix POS/AV-510 unit to provide
or other mosaic filter screen and subsequent interpola-                                                                           measured values of the external orientation parameters
tion or through the use of sequential images via a                 (a) STI Services                                               (position and attitude) for each frame. As with many of
rotating filter wheel. Furthermore, the use of this                A centre for the development of these multiple camera          these multiple small-format camera systems, the
arrangement means that three full planes of colour sep-            systems has been Terra Systems which is based in               images are then ortho-rectified using the position and
arated image data are being collected and integrated               Hawaii and was taken over by STI Services in 2001.             attitude values from the DGPS and IMU instruments in
to form a single true-colour or false-colour image                 The company's original system was called TerraSim-1            conjunction with a standard USGS-NED DEM obtained
instead of using the interpolation process where the               (TS-1) and comprised four small-format (1,024 x 1,024          from an internal data bank. After which, tonal balancing
final true-colour or false-colour images are made up               pixel) CCD cameras equipped with synchronized elec-            is carried out to remove the variations in tone between
only partially of actually measured values, while the              tronic shutters. Each camera was also fitted with a dif-       the different frame images within a block due to illumi-
remaining values have been interpolated.                           ferent colour filter to allow their images to be collected     nation geometry and viewing geometry. This is followed
                                                                   in different spectral bands - usually blue, green, red         by the mosaicing of the individual rectified and tonally-
Several different models of this DuncanTech camera                 and near infra-red. The TS-1 camera system is still            balanced image frames.
design featuring three CCD areal arrays and a single               being offered by STI Services. Its second generation
lens used in conjunction with a beam splitter are avail-           system, called TerraSim-3 (TS-3), again comprises four         (c) Airborne Data Systems
able, each model being equipped with a different size              cameras, each equipped with a larger 2,048 x 3,072             This company (already mentioned above in the section
of CCD areal array. These include the MS2100 (with 494             pixel CCD areal array and an appropriate colour filter.        on Redlake's cameras) is based in Wabasso, Minnesota
x 656 pixels); MS2150 (582 x 780 pixels); MS3100                   These individual camera units are mounted together in          and has been designing and building its own
(1,040 x 1,392 pixels); MS4000 (1,200 x 1,600 pixels);             a single box, together with the associated electronics         SpectraView digital multi-spectral systems since 1992.
and MS4100 (1,080 x 1,920 pixels). Although the cam-               and an external GPS antenna that receives differential         Again these systems are based on the use of multiple
eras have been sold directly to some airborne users, a             GPS signals supplied via a satellite. Optional are an          small-format cameras coupled together and operated
special integrated system called Agri-View is also                 inertial measurement unit (IMU) for attitude measure-          as a single unit. A wide range of options is offered
(a)                                  (b)                                                                             (c)




Fig. 7 - (a) The principle of multi-band images being captured simultaneously over the same piece of ground by multiple small-format digital frame cameras. (Drawn by Mike Shand)
(b) The TerraSim-1 (TS-1) multi-band imaging system from STI Services comprising four small-format digital frame cameras, each equipped with different colour filters to record images of
the same piece of ground in different spectral bands simultaneously.
(c) The DMSV unit comprising four low-noise video frame cameras also equipped with different colour filters to record multi-band images simultaneously. (Source: STI Services)

                                                                                                                                                                                              21
Latest News? Visit www.geoinformatics.com                                                                                       October/November 2003
                        Article

                                                                                                    The company's AirCam                ple camera system like UND's AEROCam which is
                                                                                                    multi-spectral system               operated from single-engined or twin-engined aircraft
                                                                                                    comprises four individ-             owned by its School of Aerospace Sciences.
                                                                                                    ual cameras coupled
                                                                                                    together with a navi-               (g) IGN, France
                                                                                                    gation and control unit             During the 1990s, the French IGN national mapping
                                                                                                    and an electronic dis-              organisation carried out a research and development
                                                                                                    play - along much the               programme investigating the possibilities of using air-
                                                                                                    same lines as the                   borne digital frame imagery for some of its mapping
                                                                                                    other multiple camera               work. Different types of digital camera were used to
                                                                                                    systems being dis-                  acquire the necessary images. The first of these has
                                                                                                    cussed in this section.             again involved the use of multiple cameras - in IGN's
                                                                                                    The four cameras used               system, three monochrome digital cameras were used.
                                                                                                    in the AirCam system                These were aligned with parallel optical axes and fitted
                                                                                                    have either 1k x 1k or              on to a shock absorber mount. Each camera used a
                                                                                                    2k x 2k CCD areal                   Kodak 3k x 2k CCD areal array and each was fitted
                                                                                                    arrays. These are then              with a different filter - blue, green, red - to ensure that,
     Fig. 8 - Space Imaging's DAIS-1 multiple digital frame camera system showing the individual
                                                                                                    fitted with the usual               when the three individual bands were combined, a
     components of the overall system. (Source: Space Imaging)
                                                                                                    blue, green, red and                true-colour image would result. A second system has
                                                                     near infra-red filters. However some examples have                 involved the use of two cameras, each fitted with a
                                                                     only three of the cameras equipped with blue, green                Kodak 3k x 2k areal array using the Bayer mosaic filter
                                                                     and red filters, while the fourth camera is a colour               and subsequent interpolation to generate colour
                                                                     camera equipped with a Bayer mosaic filter. While a                images. The two cameras were mounted in the aircraft
                                                                     GPS is supplied as standard, a ring-laser based IMU                in a "split-vertical" (i.e. oblique) configuration with the
                                                                     has only been supplied to a few customers - since                  optical axis of the first camera being tilted to the right
                                                                     it is of course an expensive addition to the overall               of the flight line and that of the second camera to the
                                                                     system.                                                            left of the line. While this increased the lateral coverage
                                                                                                                                        of the twin camera system, a preliminary rectification of
                                                                          (e) ImageTecK                                                 the two tilted images was necessary prior to their use
                                                                          This company, based in Chatsworth, California, has sup-       for mapping. IGN's reports also mention the use of a
                                                                          plied five multiple camera systems to the Boeing              third type of digital camera having a 4,096 x 4,096
                                                                          backed Resource21 company that specializes in the             pixel array that generates black-and-white imagery, but,
     Fig. 9 - This particular example of the SpectraView multi-
                                                                          supply of remotely sensed imagery to the agricultural         strictly speaking, this falls into the class of medium-for-
     spectral frame camera system from Airborne Data
     Systems is equipped with four cameras - though up to                 industry. Each system consists of four cameras coupled        mat digital cameras that will be dealt with in the next
     seven cameras can be supplied with the system. (Source:              together with each individual camera being equipped           section.
     Airborne Data Systems)                                               with a different narrow-band filter - like the other cam-
                                                                          era systems in this group. The main application has           Summary
     using cameras equipped with CCD areal arrays having                  been to supply farmers with colour images for the anal-       In summary, it can be seen that numerous small-format
     either 1,024 x 1,024 pixels (Model SV4P); 2,048 x 2,048              ysis of growth patterns and disease.                          airborne digital cameras are currently in operational
     pixels (SV4S2); or 4,096 x 4,096 pixels (SV4S4).                                                                                   use. Indeed, it can be said that the technology is now
     Airborne Data Systems also informs potential cus-                    (f) American University Developed Systems                     quite well established. However it can also be seen
     tomers that a total of between four and seven cameras                Besides the commercially-built and operated systems           that the main emphasis has been on the production of
     can be fitted into their standard box, though the four-              described above, other similar multiple camera sys-           true-colour and false-colour images for environmental
     camera version is that most commonly supplied, pro-                  tems have been developed and built by research                monitoring and agricultural applications over relatively
     viding separate blue, green, red and near infra-red                  groups in different American universities. One of             small areas. Even then, very large numbers of images
     band images. All of the SpectraView systems incorpo-                 these is based in the University of North Dakota              have to be handled and processed.
     rate a DGPS set and an IMU. The IMU can either be a                  (UND) and forms part of the Upper Midwest
     more precise unit (type -L) combining a ring laser gyro              Aerospace Consortium. This group has developed, in
     with the DGPS or a less precise unit (type -M) offering              partnership with a private company, Digit Inc., its           II. Medium-Format Digital Cameras
     a more limited 20m positional accuracy. As with the                  Airborne Environmental Research Observation Camera
     other multiple camera systems, after the image data                  (AEROCam). Again this comprises four Dalsa CA-D4              Whatever the undoubted success of small-format digital
     has been acquired, first band-to-band registration of                digital cameras, each of which is based on a 1,024 x          cameras, the biggest drawback has always been the
     the individual images is carried out. This is followed by            1,024 pixel CCD areal array of 12 x 12µm detectors.           very limited size of the format itself - with the resulting
     the use of the DGPS and IMU data to geo-reference the                This AEROCam system is also equipped with a DGPS              severe limitations in the ground coverage of a single
     composite images. Further processing steps allow the                 and an IMU. Still another university that has built a         frame image. So, during the late 1990s, with the com-
     production of mosaics or ortho-mosaics.                              small-format airborne camera system is that devel-            mercial availability of larger CCD areal arrays of up to
                                                                          oped at the Environment Remote Sensing Center                 4,000 x 4,000 pixels = 16 Megapixels at a not too
     (d) Kestrel Corporation                                              (ERSC) at the University of Wisconsin located at its          exorbitant price, there was an immediate response on
     The Kestrel Corporation is based in Albuquerque,                     main campus in Madison. However this appears to be            the part of the airborne imaging community. This has
     New Mexico and is very active in the biomedical                      based on small digital cameras from Dycam in                  mainly gone in two different directions.
     imaging field as well as developing multi-spectral                   Chatsworth, California for use in a tiny electrically-        (i) The first has been the development of digital backs
     and hyperspectral imagers for remote sensing use.                    powered UAV, called DigiDot, rather than a full multi-            that could be fitted to existing high-quality film cam-
      (a)                                                           (b)                              (c)                                                       (d)




     Fig. 10 - The AirCam system from the Kestrel Corporation showing (a) the four frame camera unit; (b) the display unit, together with a laptop computer; (c) the electronics unit; and (d) a
     false-colour image of Fort Bliss, Texas acquired by an AirCam system. (Source: Kestrel Corporation)



22                                October/November 2003
                                                                                                                                                         Article

(a)                                                                     (b)                                                      (c)




Fig. 11 - (a) The GeoTechnologies MF-DMC medium-format frame camera system. The actual camera is in the middle of the photo. It is based on the body of a Hasselblad 555 ELD film cam-
era and is equipped with an f = 40mm lens and a Dicomed digital back using Bayer colour mosaic filters. On the left is an Apple Mac laptop computer; on the right, a Garmin GPS receiver
sits on top of a Fugro OmniSTAR DGPS unit.
(b) A Bell 412 helicopter modified with a special nose fairing in place protecting a MF-DMC frame camera system.
(c) The protective nose fairing has been removed from the helicopter to show the MF-DMC frame camera system with its roll and pitch adjustable mount. (Source: Bath Spa Univ. College)

    eras generating 6cm wide images such as those              sured positional and attitude data from the DGPS/IMU            Fuji mosaic filter with interlocking octagonal-shaped
    manufactured by Hasselblad, Rollei, Mamiya and             combination showed a good accuracy, though, as we               pixels (instead of the square-shaped pixels used by
    Contax.                                                    now know, not quite of a standard to eliminate the              Kodak) to generate colour images by interpolation.
(ii) The second has been the development of specifical-        need for ground control points (GCPs) and aerial trian-
    ly designed digital cameras using the medium-for-          gulation, but still providing very valuable auxiliary data      (a) European Examples
    mat arrays, the most obvious example being the             for the geo-referencing process. Further tests carried          European examples of these converted film cameras for
    Kodak MegaPlus 16.8i camera already mentioned              out in collaboration with the EarthData mapping com-            airborne imaging include the MF-DMC (Medium-Format -
    above.                                                     pany involved the use of the AIMS system together               Digital Mapping Cameras) from the GeoTechnologies
                                                               with an airborne laser scanner (lidar) over a range in          consultancy at Bath Spa University College in the U.K.
                                                               Maryland. This proved to be an excellent combination            whose earlier ADPS system has been already discussed
II.1 Modified Film Cameras                                     with the digital imagery and the lidar data complement-         above. These MF-DMC cameras utilize the body of the
                                                               ing one another quite strongly.                                 Hasselblad 555ELD SLR camera equipped with an elec-
AIMS                                                                                                                           trically driven shutter and available with a range of
A pioneering and very influential development in this          Digital Backs                                                   Zeiss lenses. The MF-DMC(1) model uses the Dicomed
particular area has been the Airborne Integrated               The undoubted success of the AIMS project has led to            16MPx array, while the MF-DMC(2) model makes use of
Mapping System (AIMS) that was first proposed by the           many similar systems being created. On the digital              the Kodak ProBack. A number of MF-DMC cameras have
Center for Mapping at Ohio State University in 1995.           camera side, a number of digital backs have been                been supplied to mining companies in Indonesia and
The original proposal was for the development of a             developed for fitting to film cameras with 6 x 6cm or 6         South America; yet another has been supplied to an
fully digital acquisition system for mapping purposes.         x 4.5cm formats from which the film magazines have              organisation in Uganda which uses it to monitor exces-
This would comprise a digital camera together with a           been removed. These digital backs were developed by             sive vegetation growth on Lake Victoria. In the U.K.,
tightly integrated DGPS/IMU system capable of produc-          various companies such as MegaVision, Sinar,                    further examples have been supplied to NPA Ltd. and
ing high-accuracy positional and orientation data for          PhaseOne, Linhof and Jenaoptik. Most of them have               to the government's Environmental Agency. This agency
the digital camera and the platform on which it was            been based on Kodak's ITO-based 4,080 x 4,080 pixel             operates its MF-DMC camera in conjunction with an
mounted. As finally realized in prototype form in 1997,        CCD areal array with its 9µm square detector size, as,          Optech airborne laser scanner for coastal monitoring
the digital camera part of the overall system consisted        of course, is Kodak's own DCS ProBack product. Other            purposes. The lidar produced DEM is used as the basis
of a Hasselblad 553ELX camera body equipped with a             suppliers have used the Dicomed silicon-based CCD               for the ortho-rectification of the digital frame images
Zeiss lens and fitted with a "BigShot" digital camera          areal array which also has a format of 4k x 4k pixels,          being acquired by the camera.
back. This utilized a 4k x 4k pixel CCD areal array with       but features the rather larger detector size of 15 x
a 15µm square detector size, manufactured by                   15µm and is considerably more expensive than the                Another quite similar conversion is that carried out by
Lockheed Martin Fairchild Semiconductors. The integrat-        Kodak product. On the other hand, the Kodak array is            the Latvian mapping company, SIS Parnas. This com-
ed positioning and attitude elements of the overall            limited to true-colour only, whereas the Dicomed prod-          prises a Mamiya RZ67 Pro II camera which has also
AIMS system comprised two dual-frequency Trimble               uct is also sensitive to radiation in the near infra-red        been equipped with the Kodak ProBack. The camera
GPS receivers and a strapdown Litton LN-100 IMU.               and can be used to produce false-colour images. Both            was then calibrated at the photogrammetry laboratory
                                                               the Kodak and Dicomed arrays use Bayer mosaic filters           of the Moscow State University of Geodesy &
As demonstrated by many tests and published reports            with interpolation to produce the required colour               Cartography to allow it to be used for photogrammetric
from the period 1999 to 2001, the AIMS system was a            images. Another supplier of digital backs for fitting to        mapping. As with many of the systems discussed
distinct success. The digital camera produced high-qual-       medium-format Hasselblad and Mamiya cameras is the              above, a DGPS receiver is fitted to provide accurate 3D
ity monochrome (panchromatic) images having a much             Japanese Fuji company. However its Luma II digital back         positions of the camera stations. The final photogram-
better ground coverage than those produced by the              utilizes a somewhat smaller-sized rectangular CCD areal         metric processing of the imagery is being carried out
small-format cameras - though still quite limited by the       array - with 4,008 x 2,672 pixels = 11 Megapixels -             using the PHOTOMOD DPW supplied by the Russian
standards of metric film cameras. The use of the mea-          than the Kodak and Dicomed products. It also uses a             Racurs company.

 (a)                                                          (b)
                                                                                                                               (b) American Examples
                                                                                                                               In the United States, many more medium-format digital
                                                                                                                               frame cameras have been constructed and come into
                                                                                                                               productive use. Prominent among these is the camera
                                                                                                                               system that originally has been produced for the
                                                                                                                               Emerge company, which, as previously discussed, has
                                                                                                                               been an extensive user of the Kodak DCS-460 small-
                                                                                                                               format digital cameras. This new Digital Sensor System
                                                                                                                               (DSS) has been constructed on the basis of a Contax
                                                                                                                               645 camera, modified to take the MegaVision digital
                                                                                                                               back based on the Kodak 4k x 4k CCD areal array with
Fig. 12 - (a) The Emerge DSS medium-format digital frame camera - which utilizes the body of the Contax 645 film camera        its Bayer mosaic filter. The camera is available with
and a MegaVision digital back - placed on its mount.                                                                           Zeiss lenses having a range of focal lengths with f =
(b) The complete DSS system with its Applanix supplied electronics package, including its POS-AV DGPS/IMU unit. The actu-      35, 45 or 80mm. A metal frame attached to the camera
al IMU is fitted to the top of the metal frame that fits over the camera. (Source: Applanix)                                   mount has been constructed to fit over the camera like



Latest News? Visit www.geoinformatics.com                                                                                   October/November 2003                                          23
                        Article

      (a)                                         (b)                                                                                 Azimuth Corporation, now part of Leica Geosystems.
                                                                                                                                      However the digital camera has been designed and
                                                                                                                                      built by the company's R&D section located in
                                                                                                                                      Albuquerque, New Mexico. This uses a Rolta lens in
                                                                                                                                      combination with a Kodak 4k x 4k CCD areal array.
                                                                                                                                      Originally only panchromatic imagery was generated,
                                                                                                                                      but the company has now produced a new version of
                                                                                                                                      the camera fitted with a similar sized (4,080 x 4,080
                                                                                                                                      pixel) CCD areal array with a Bayer mosaic filter to pro-
                                                                                                                                      duce colour frame images having a 48 Megabyte (3 x
                                                                                                                                      16 Megabytes) file size after interpolation. Under a
                                                                                                                                      newly announced re-organisation, the 3Di and
                                                                                                                                      EnerQuest companies have been amalgamated and will
                                                                                                                                      operate the three DATIS systems and the RAMS system
                                                                                                                                      under the banner of the newly merged company, which
                                                                                                                                      is called Summit Mapping LLC.

     Fig. 13 - (a) The DATIS II system developed by 3Di - with its medium-format digital frame camera and its laser scanner           (c) Digital Camera & Lidar - EarthData
     together with its IMU and DGPS units - all integrated together within a single mount (seen in the foreground). The accom-
                                                                                                                                      Finally, within this group, it is worth noting the
     panying electronics package is located in the background.
                                                                                                                                      extensive and high-profile use of another airborne
     (b) The RAMS system developed by EnerQuest, comprises a medium-format digital frame camera (on the right); the laser
     scanner which is integrated with an IMU and a DGPS receiver (in the middle); and an on-board electronics package that            laser scanner and digital camera combination operat-
     records the data acquired by the digital camera and laser scanner units (on the left).                                           ed by the EarthData mapping company of
                                                                                                                                      Gaithersburg, Maryland. In the wake of the terrorist
     a cap and carries an embedded IMU that is set directly          various other users, including Nortrek Geomatics in              attack of September 11th, 2001 on the World Trade
     over the camera. The partnership that developed the             Canada and AeroDigital Chile.                                    Center in New York, EarthData flew its system on 19
     system under the leadership of Emerge included Pixel                                                                             occasions between 15th September and 22nd
     Physics of Rochester, New York, which has carried out           (a) Digital Camera & Lidar - DATIS                               October 2001 to collect lidar data and digital frame
     the modification and radiometric calibration of the cam-        Several other medium-format frame camera systems                 imagery to produce DEMs and digital orthophotos
     era. The other major partner in the project has been            that have been designed specifically to complement air-          that were used extensively during the rescue and
     Applanix, which has developed the electronics package,          borne laser scanners have been produced for American             recovery operations that followed the attack. The
     including the integration of its POS-AV DGPS/IMU sys-           photogrammetric mapping companies. An early example              imager that was used for this work was the Kodak
     tem and a flight management control and display sys-            was the DATIS (Digital Airborne Topographical Imaging            Megaplus 16.8i digital frame camera producing
     tem. Recently Applanix has taken over the whole pro-            System) produced by the 3Di company at its Boulder,              panchromatic (black-and-white) imagery with 256 (8-
     ject (including the patent rights) and is now marketing         Colorado facility. Two complete DATIS systems are now            bit) grey values. Quite a number are being used in
     the DSS system world-wide. Pixel Physics is also offer-         operational; a third has just been built. DATIS uses             combination with airborne laser systems that pro-
     ing the camera only under the name TerraPix DACS                lidar systems that have been designed and built by 3Di           duce DEMs, on the basis of which, the digital frame
     (Digital Aerial Camera System). The latest news (from           in-house. The overall DATIS system employs a Systron-            images can be ortho-rectified.
     May 2003) is that the DSS system will also be offered           Donner IMU together with a DGPS constructed on the
     by Leica Geosystems. Several of the DSS systems have            basis of OEM cards rather than a complete unit bought            Summary
     already been sold, including two to users in Japan.             in from another supplier. The digital camera compo-              It can be seen that there is a strong move
                                                                     nents of the system have also been designed in-house,            towards the airborne use of medium-format digital
                                                                     although their actual construction has been sub-con-             cameras which offer a substantially better ground
     II.2 Medium-Format Digital Cameras                              tracted to an outside supplier. The cameras are fitted           coverage than the small-format cameras. Quite a
                                                                     with Rollei lenses and feature a 4k x 4k Kodak CCD               number are now being used in combination with
     So far, the main examples of purpose-built airborne             areal array producing panchromatic (monochromatic)               airborne laser scanners that produce DEMs, on the
     medium-format digital cameras have been those based             frame images. These images are referenced directly to            basis of which, the digital frame images can be
     on the top-end of the MegaPlus range, first manufac-            the DEM produced by the laser scanner, allowing rapid            ortho-rectified.
     tured by Kodak and now produced by Redlake. A typi-             production of digital orthophotos.
     cal example is the FSE Aerial Imaging System offered
     by ImageTecK of Chatsworth, California. This is based           (b) Digital Camera & Lidar - RAMS                                III. Large-Format Frame Cameras
     on the MegaPlus 16.8i monochrome camera ruggedized              The RAMS (Remote Airborne Mapping System) operated
     for flight and provided with a control unit and a com-          by EnerQuest of Denver, Colorado is another integrated              As mentioned in the introduction to this article, the
     puter with removable disk drives and special acquisi-           system comprising an airborne laser scanner and a dig-              quality standard for the large-format frame cameras
     tion software, Previously ImageTecK had supplied simi-          ital frame camera. In this case, the lidar with its inte-           used for photogrammetric mapping and interpretation
     lar systems using lower-resolution MegaPlus models to           grated DGPS and IMU has been supplied by the                        has been set by the current models of the metric film
                                                                                                                                                           frame cameras produced by Leica
                                                                                                                                                           (RC30) and Z/I Imaging (RMK-TOP).
         (a)                                         (b)                                       (c)                                                         Equipped with suitable gyro-con-
                                                                                                                                                           trolled mounts and featuring for-
                                                                                                                                                           ward image motion compensation in
                                                                                                                                                           conjunction with fine-grained high-
                                                                                                                                                           resolution film, these cameras can
                                                                                                                                                           produce images with resolutions of
                                                                                                                                                           50 to 60 lp/mm (amounting to 20 to
                                                                                                                                                           17µm) on the film plane. If one
                                                                                                                                                           takes the relationship between the
                                                                                                                                                           film resolution in lp/mm and the
                                                                                                                                                           equivalent pixel size as being 1:2
                                                                                                                                                           (as given by the Kell-Factor), then
                                                                                                                                                           the smallest possible pixel size on
     Fig. 14 - (a) This diagram shows the arrangement of the 8 cameras making up the Z/I Imaging DMCS frame camera system. The 4 pan cameras
                                                                                                                                                           the film will lie between 7 to 10µm.
     located within the central part of the diagram are all tilted from the vertical outwards in a star-shaped configuration. The other 4 cameras locat-
     ed at the corners of the diagram all point in the vertical direction with parallel optical axes to produce overlapping images of the same piece of    This relationship is also reflected in
     ground in the red, green, blue (RGB) and near-IR parts of the spectrum.                                                                               the smallest pixel size that is pro-
     (b) The ground coverage of the four tilted pan cameras.                                                                                               vided for in high-accuracy pho-
     (c) The production of a single "virtual" vertical pan image from the four tilted sub-images. (Drawn by Mike Shand)                                    togrammetric film scanners - the



24                                October/November 2003
                                                                                                                                                                  Article

 (a)                                                              (b)                                                                                         company. Each of the four cam-
                                                                                                                                                              eras has a Zeiss f = 120mm
                                                                                                                                                              lens. In combination, this four
                                                                                                                                                              camera configuration produces a
                                                                                                                                                              rectangular 13.5k x 8k = 108
                                                                                                                                                              Megapixel panchromatic image
                                                                                                                                                              giving an angular coverage of
                                                                                                                                                              74° (cross-track) x 44° (along-
                                                                                                                                                              track) over the ground.

                                                                                                                                                            For the acquisition and genera-
                                                                                                                                                            tion of true-colour or false-
                                                                                                                                                            colour frame imagery, Z/I
                                                                                                                                                            Imaging's solution incorporated
                                                                                                                                                            in the DMCS again utilizes multi-
                                                                                                                                                            ple cameras - either three or
                                                                                                                                                            four as required. However, in
                                                                                                                                                            this case, these cameras are
                                                                                                                                                            arranged with parallel optical
Fig. 15 - (a) The Z/I Imaging DMC (Digital Modular Camera) on its Zeiss T-AS gyro-controlled mount.                                                         axes to cover the same piece of
(b) The various elements of the DMCS (Digital Mapping Camera System) installed in an aircraft carrying out demonstration flights in the U.K. The T-         ground. Each individual camera
AS gyro-controlled mount and the cradle on which the camera unit will sit are in the lower right part of the photo. The electronics unit is located at      has a filter defining a different
the left edge of the photo. (Source: Z/I Imaging)                                                                                                           spectral band placed in front of
                                                                                                                                                            it. The size of the individual CCD
Leica DSW500 with 5µm; Z/I Imaging PhotoScan with              III.1 Large-Format Digital Frame Imagery                            areal arrays used in these multi-band cameras is much
7µm; Vexcel Imaging VX4000 with 7.5µm; Wehrli RM-              Using Multiple Cameras                                              smaller - 2k x 3k = 6 Megapixels - than those used in
3 with 8µm; and Vexcel Austria UltraScan 5000 with                                                                                 the DMCS pan cameras. So the cameras have been fit-
5µm. Scanning a high-resolution monochrome film                The two commercial developments that are under way                  ted with short focal length (f = 25mm) wide-angle lens-
with a 10µm pixel size on any one of these devices             in this particular area are the multiple camera systems             es to provide a reasonable ground coverage. The
results in an image having 23,000 x 23,000 pixels =            of Z/I Imaging and Vexcel Austria.                                  penalty for this is, of course, a very considerable reduc-
529 Megapixels for the standard film frame format                                                                                  tion in the ground resolution (cf. f = 25mm v. f =
size of 23 x 23cm. In commercial practice, in order to         (a) Z/I Imaging                                                     120mm) compared with the DMCS pan camera when
keep down the file sizes, the film is often scanned at         The Digital Modular Camera (DMC), now re-named the                  operating from the same flying height. The final true-
a 15µm pixel size - which still results in an image            Digital Mapping Camera System (DMCS), has been                      colour or false-colour image is formed later through the
with 15,333 x 15,333 = 235 megapixels. As we have              under development by Z/I Imaging for some years The                 co-registration and fusion of the individual images
seen, these pixel sizes lying in the range 7 to 15µm           initial announcement of its development took place in               recorded by each of the three or four component cam-
that can be obtained from scanned film images lie in           1999, while the prototype was first shown at the ISPRS              eras.
the same range as those commonly found on CCD                  Congress held in Amsterdam in July 2000. Now the first
areal arrays. Thus there is little difference in the geo-      deliveries of the DMCS are under way to customers in                (b) Vexcel Austria
metric resolution values that are encountered in the           the United States (3001), Japan (KKC) and Germany                   The Vexcel Austria UltraCam D camera was announced
focal plane with film frame cameras and with digital           (IFMS). The DMCS comprises two separate multiple                    in April 2003. It also features multiple cameras operat-
frame cameras. The main difference lies in the much            camera systems which are integrated together inside a               ed in two groups of four. The first group of four cam-
smaller format sizes that are possible with digital            single box so that they can be operated as an integrat-             eras generates panchromatic (black-and-white) images,
frame cameras due to the small sizes of CCD areal              ed unit sitting within a Zeiss T-AS gyro-controlled                 while the second group of four generates true-colour
arrays that are available at the present time.                 mount. The first of these two multiple camera systems               and false-colour images. However the generation of the
Undoubtedly the situation will improve with time.              forming the DMCS is designed to produce monochrome                  panchromatic images is carried using a quite different
                                                               (panchromatic) imagery; the second system produces                  configuration to that used in the Z/I Imaging DMCS.
Current developments in large-format digital frame cam-        colour or false-colour imagery of a somewhat lower                  Instead of the four tilted cameras of the DMCS, the
eras are taking place in two main directions:-                 ground resolution. To generate the panchromatic (black-             UltraCam D has its four cameras all pointing in the ver-
(i) The first involves the use of multiple medium-format       and-white) images, the DMCS integrates four individual              tical direction with parallel optical axes. The four cam-
    cameras producing multiple images that are synthe-         cameras arranged (i.e. tilted outwards) in a star-type              eras are mounted in a line pointing in the flight direc-
    sized into a single large-format image. This is the        configuration. The resulting four individual tilted images          tion. The shutters of each camera are triggered
    approach is that being followed by the commercial          overlap slightly and are exposed simultaneously. They               sequentially with a very tiny time delay of 1 or 2 mil-
    vendors of large-format digital cameras.                   are then processed (i.e. rectified) to form a single per-           liseconds between them. This means that each sub-
(ii) The second approach involves the construction of          spective image. Each of the four component cameras is               image making up the complete frame image is essen-
    single cameras fitted with the very largest CCD areal      fitted with a 7k x 4k = 28 Megapixel CCD areal array                tially taken from a single exposure station in the air.
    arrays that are available at the present time. So far,     originally manufactured by Philips in Eindhoven in the              Thus the components of the final composite image,
    these cameras have all been constructed for military       Netherlands - though this part of the Philips organisa-             which is made up of the different sub-images, all share
    customers and applications.                                tion has now been taken over by the Canadian Dalsa                  the same projection centre. One of the cameras is des-

 (a)                           (b)                                                                                     (c)                                  (d)




Fig. 16 - (a) The basic concept of the Vexcel Austria UltraCam D system with the four pan cameras all pointing vertically towards the ground with parallel optical axes. These four cameras
are triggered in very rapid succession to record a set of sub-images taken from a single projection centre in the air. The resulting sub-images are stitched together to form a single large-for-
mat pan image.
(b) The three main units of the UltraCam D system with the digital frame camera at left; the computing and storage unit in the middle; and the display unit at right.
(c) The UltraCam D multiple frame camera unit with its 8 cones - 4 of these are located in-line to produce the final large-format pan frame image; the remaining 4 cones acquire the multi-
band images covering the red, green, blue (RGB) and near-IR parts of the spectrum to form true-colour and false-colour images.
(d) A colour image of Salzburg Cathedral taken with the UltraCam D at 1:5,500 scale and a ground pixel size of approximately 5cm. (Source: Vexcel Austria)



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                                                                                                                                                            Article

(a)                                  (b)                                                  company's well-known KS-87              con wafer. The pixel size is 8.75 x 8.75µm and the
                                                                                          reconnaissance film frame cam-          array provides monochrome images with a resolution of
                                                                                          era with its 5 inch (12.5cm) wide       57 lp/mm (similar to that of metric film cameras). The
                                                                                          film producing images with a 4.5        array also allows a 2 frames per second read-out rate.
                                                                                          x 4.5 inch (11.5 x 11.5cm) format.      More than half-a-dozen F-979F and F-985C cameras
                                                                                          The CA-260/4 conversion dating          have been built that utilize this particular array. When
                                                                                          from 1993 used a 2k x 2k = 4            fitted with an f = 75mm lens, the F-979F camera pro-
Fig. 17 - (a) The Recon/Optical CA-270 "dual-band" digital frame camera providing         Megapixel CCD array with built-in       vides an angular coverage of 55.8°. As with all of these
day and night optical imaging capabilities.
                                                                                          IMC capability. It could be fitted      cameras fitted to military reconnaissance aircraft that
(b) Pan (visible) and medium-IR frame images taken simultaneously with the CA-270
                                                                                          with f = 75, 150 or 300mm lens-         need to operate at high speeds and low altitudes, IMC
camera. (Source: Recon/Optical)
                                                                                          es. The later CA-260/25 model           is standard. The work to develop an "Ultra High
ignated as the "master cone" and features four CCD                from 1996 used a much larger 5k x 5k = 25 Megapixel             Resolution Reconnaissance Camera" based on this
areal arrays located in the corners of its overall field of       CCD areal array which could be used to take stereo-             technology has been sponsored by the U.S. Naval
view. The other three cameras (or "cones") acquire the            cover of larger areas. A still later model in the series is     Research Labs (NRL). However, in presentations and
sub-images that fill the gaps between the four images             the CA-260/50 which features a rectangular 10k x 5k =           papers given at the ASPRS conferences held in
that are produced by the "master cone". The final                 50 Megapixel CCD areal array developed by Dalsa. This           Washington and Denver last year (2002), it appears
panchromatic image, after being stitched together from            array provides images with a resolution of 49 lp/mm             that BAE Systems are considering the development of a
the master images and the sub-images, is 11.5k x 7.5k             with a 2.5 frames per second read-out rate. Using an f          metric version of this camera designed specifically for
pixels in size = 86 Megapixels, with each pixel being             = 75mm lens, this array provides an angular coverage            mapping applications. It is already apparent that, if four
9µm square in the focal plane. The four cameras all               of 68.4° x 37.5° over the ground. Another digital cam-          of these 9k x 9k pixel areal arrays can be packaged
use an f = 100mm lens from Schneider Kreuznach as                 era that is a conversion of a film frame camera is the          together, a format of 18k x 18k pixels = 338.6
standard, the angular coverage of the final image over            KS-127 LOROP (Long-Range Oblique Photographic) cam-             Megapixels would result. This would give a ground cov-
the ground being 55° x 37°.                                       era equipped with an f = 66 inch (1.67m) lens. The              erage and a geometric resolution that is comparable
                                                                  conversion also allowed it to record its images using a         with that of metric frame cameras.
In the case of the second group of multiple cameras               digital back.
that generate the true-colour and false-colour images,                                                                            Summary
once again, these are arranged with parallel optical              Besides these conversions, Recon/Optical has also built         The present situation with regard to large-format air-
axes, with each of the lenses of the four cameras hav-            a series of purpose-built digital frame cameras. These          borne digital cameras is extremely interesting with sev-
ing the appropriate spectral filter placed in front of it.        include the CA-265 IR digital frame camera which uses           eral cameras having either just entered service or will
Each of these four cameras has a CCD areal array of 4k            a 2k x 2k = 4 Megapixel array that is sensitive to radi-        do so in the near future. It will be fascinating to see
x 2.7k pixels = 10.8 Megapixels and is fitted with a              ation in the medium IR (Ï = 3 to 5µm) part of the spec-         the results that can be obtained when using these new
lens having f = 28mm. These provide an angular cover-             trum that allows image collection both during the day           cameras for mapping operations. Nevertheless it should
age of 65° (cross-track) x 46° (along-track). The proto-          and during the night. Other Recon/Optical models                be borne in mind that this new generation of airborne
type UltraCam D camera has already been flown and                 include the CA-261 Step Frame Camera which provides             digital cameras will produce monochrome images that
sample images are available from Vexcel Austria - which           extended cross-track coverage by fast-stepping move-            are equivalent to those from film cameras utilizing a 5
has now been merged with the Vexcel Corporation of                ments of the camera in that direction. Some of the lat-         inch (12.5cm) wide film and producing images that are
Boulder, Colorado in May 2003.                                    est digital frame cameras from Recon/Optical are the            4.5 x 4.5 inches (11.5 x 11.5cm) is size. When films are
                                                                  so-called "dual band" or "dual-spectrum" models such            scanned using a pixel size of 10µm (close to the pixel
                                                                  as the CA-270 (low altitude), CA-279 (medium altitude)          size of the CCD areal arrays), this produces images that
III.2 Large Format Digital Cameras                                and CA-295 (high altitude). These allow simultaneous            are 11.5k x 11.5k = 130 Megapixels in size. If, instead, a
                                                                  images to be acquired in both the visible and infra-red         pixel size of 15µm is used, this results in an image with
The current state-of-the-art in digital frame cameras is          parts of the spectrum allowing day and night acquisi-           7.67k x 7.67k pixels = 59 Megapixels. Thus the current
represented by the reconnaissance aerial cameras that             tion of image data. It should be noted that all of these        generation of large-format airborne digital frame cam-
have been constructed in the U.S.A. by Recon/Optical              Recon/Optical cameras are generating monochromatic              eras lie in the same region in terms of geometric reso-
and BAE Systems for American military air forces.                 (black-and-white) images.                                       lution and format size as those aerial film cameras
                                                                                                                                  using 5 inch (12.5cm) wide film.
Recon/Optical                                                    BAE Systems                                                      Conclusion
This company, originally known as Chicago Aerial                 Those framing cameras produced by BAE Systems that               As this article has attempted to show, real progress in
Industries (CAI), has been a major supplier of recon-            incorporate 9,216 x 9,216 pixel = 85 Megapixel CCD               airborne digital frame camera technology has been
naissance camera systems for over 70 years. During the           areal arrays give the highest resolution digital frame           made over the last decade. Small-format digital frame
early 1970s, the company developed a pushbroom                   images from a single array that are known about pub-             cameras have already become well established for air-
scanner equipped with a linear CCD array for the USAF.           licly. The part of BAE Systems that produces these cam-          borne imaging purposes. Medium-format digital frame
However this type of imager was not favoured by the              eras is called the Reconnaissance & Surveillance                 cameras have now entered service in some numbers.
air force, since it meant that the reconnaissance aircraft       Systems Division and is based in Long Island, New                Almost certainly, they will come into still more
had to be flown in a relatively stable and non-manoeu-           York. Previously this particular facility formed part of         widespread use. Large-format digital frame cameras are
vring path while acquiring the linescan imagery, making          Lockheed Martin's Aerospace Electronics Systems (AES)            now being developed and they represent the current
them vulnerable to defensive fire. So the company has            operation which was purchased by BAE Systems' North              state-of-the art - though, as yet, they still do not pro-
since concentrated its efforts into cameras equipped             American business in 2000. In fact, prior to its owner-          vide the format sizes and coverages that are standard
with large CCD framing arrays which allow the coverage           ship by Lockheed Martin, the facility was a part of              with metric film cameras. However the advent of com-
of an area to be acquired while the reconnaissance air-          Fairchild with a long tradition in the construction of film      parable digital frame cameras does appear to be on
craft is manoeuvring. Recon/Optical now makes numer-             cameras. The CCD areal array, which is manufactured in-          the horizon.
ous cameras that are based on this technology. The CA-           house by BAE Systems, measures 8 x 8cm in size and
260/4 was an upgrade (or conversion) of the                      is fabricated on a single 5 inch (12.7cm) diameter sili-         Professor G. Petrie, Department of Geography &
                                                                                                                                  Topographic Science, University of Glasgow, Glasgow, G12
                                                                                                                                  8QQ, Scotland, U.K.
                                                                                                                                  Internet: http://www.geog.gla.ac.uk/~gpetrie
                                                                                                                                  E-mail: gpetrie@geog.gla.ac.uk




Fig. 18 - Unit Assemblies for the BAE Systems 9.2k x 9.2k Ultra High Resolution Reconnaissance System (Source: BAE Systems)



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