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Short-range surveillance radar systems

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					       Short-range surveillance
            radar systems
                            by C. J. Baker and B. D. Trimmer

 Small, light-weight, coherent radar systems have been successfully exploited in the
   military domain for many years. Recent advances in both technology and signal
processing techniques are enabling the production of more versatile systems aimed at
  a wider range of applications spanning both the military and civil markets. In this
 paper the fundamental design concepts underpinning this class of radar are briefly
  introduced to place in context current and emerging technical developments. The
 relationships between these developments, an increasing range of future potential
   applications and the areas of most significant technical challenge are discussed.

1   Introduction                                                 Thirdly, radar is inherently coherent and consequently
                                                                 backscatter from a distributed scene will exhibit
In the modern military environment it is universally             constructive and destructive interference, leading to the
recognised that accurate, timely and reliable knowledge of       well-known ‘speckle’ phenomenon. This manifests itself as
enemy activity is a vital ingredient in the provision of         scene-induced multiplicative noise.
overall situational awareness, potentially making the               These characteristics result in the radar returns being
difference between success and failure of operations.            presented to an operator as a single bright spot on a
   The detection and location of movement of, amongst            display. This may lead to the return from a vehicle, for
others, vehicles, men, enemy fire and ‘the fall of shot’ are     example, being indistinguishable from other bright spots
key situation awareness constituents leading to an               on the display caused by similar returns from other man-
understanding of enemy dispositions, actions and intent.         made objects or the surrounding terrain. However, the
The availability of this class of information on an all-         deployment pattern of a group of vehicles and other radar
weather, day-and-night basis in a hostile and extremely          derived data such as their speed and direction of travel may
complex environment can only be provided by radar                make their identity and intent clear to a trained operator. In
systems. This advantage together with the ability to view        order to aid the interpretation task the raw radar data is
relatively large areas makes radar a vital component within      processed into a form making its assimilation as simple as
a commander’s overall portfolio of surveillance and target       possible. Overall the radar designer must have a deep
acquisition assets. Indeed, soon after its initial invention     understanding of hardware, processing algorithms,
radar was used for military surveillance of the battlefield      display of processed data, the role of the radar operator and
and this continues to the present day.                           the operating environment and all of their interactions if
   These advantageous characteristics of radar systems           applications are to be addressed successfully.
have been exploited in numerous other applications.                 The underlying design principles which determine
Examples which fall into the category ‘short-range               detection and classification performance, location
surveillance’ include harbour surveillance, border control,      accuracies and overall effectiveness of short-range
traffic monitoring, airport and building security.               surveillance radar systems are briefly introduced in the
   Humans are used to viewing scenes at optical                  next section. This is followed by a review of past and
frequencies and our natural abilities to interpret context       current battlefield radars, highlighting the proliferation of
and content are extraordinarily good. However, for radar         small light-weight systems. This background places
the situation is somewhat more complicated for three main        current and likely future developments in a context which
reasons. Firstly, radar operating frequencies are much           clearly demonstrates their importance and leads naturally
lower (up to 100 GHz) and result in objects appearing            to the increasingly comprehensive capability and widening
relatively smooth with little or no textural information.        range of applications to which these systems are being put.
Secondly, radar resolutions are typically of the order of
10 m by a few hundred metres and consequently the                2   Basic design principles
detected backscatter will include contributions from both
objects of interest and their background; there is therefore     All radar systems work on the principle of transmitting and
an effective loss of definition and of contextual information.   receiving electromagnetic radiation, which may take the

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000                                                                181
Table 1: Typical battlefield radar requirements                  of 10 km. Using techniques such as monopulse or azimuth
                                                                 matched filtering this accuracy can be improved by
Detection ranges                                                 approximately an order of magnitude. There are many
 Man walking                           3 km
                                                                 excellent introductory radar textbooks available that
 Moving vehicle                       10 km                      establish and describe these relationships (e.g. References
 Fall of shot (artillery)              8 km                      1–3). Fig. 1 shows a very simplified schematic diagram of
Range accuracy                        20 m                       the major components comprising a radar system. Note
Bearing accuracy                      0.3º                       that a reference signal is used to retain a knowledge of
Environment                                                      phase by comparison of the outgoing transmission and
 Rain                                 up to 4 mm/h               subsequent incoming reception.
 Heavy clutter                        > – 10 dBm2/m2                The overall design of a radar system to meet a particular
                                                                 application must take into account the complex
Weight                                30 kg
                                                                 interactions between the characteristics of the transmitter,
Volume                                man-portable loads
                                                                 receiver, antenna, target, environment, size, weight, power
Power consumption                     30 W                       consumption, affordability and role of the operator. All of
                                                                 these design aspects need to be carefully considered
form of a train of pulses or may be a continuous                 together in order to provide the most cost-effective
transmission and reception. The presence of a target is          systems. Next, the design of an example radar system is
determined from the amount of incident radiation that is         considered against the requirements of a hypothetical
reflected back in the direction of the radar receiver and this   man-portable battlefield surveillance radar as in the outline
is a complex function of radar specification, target type,       specification listed in Table 1. This will highlight the
characteristics and environment. The range to the target is      significance of the role of the parameters described above
a function of the round-trip transit time from transmitter to    in system design options.
target and back to receiver. The cross-range resolution and         The technical specification of a radar system, at the
accuracy are functions of the radar’s beamwidth.                 simplest level, is expressed by the well known radar
   Used together these measurements enable a target or           equation, which can be used to indicate the high-level
object of interest to be detected and its position to be         system parameters and the effects of their interactions.
located in a convenient co-ordinate system. By examining         The radar equation for noise-limited detection may be
the returns from moving targets over a period of time,           written as:
radial velocity can be calculated. Usually this is achieved in
modern radars by measurement of the phase history of the           R4 =      PG2 σ λ2
radar returns as this can be related directly to radial                   (4π)3 LSkTBF
velocity. As stated above, target location accuracy in the
cross-range dimension is a function of beamwidth. For a          where R is the detection range, P is the peak power, G is the
1.5° beam and no additional processing, this implies a           antenna gain, σ is the radar cross-section of the target, λ is
location accuracy of approximately 160 m at a radar range        the wavelength, S is the minimum detectable signal, L
                                                                 represents the system losses and kTBF is the noise power
Table 2: Typical system parameters                               in bandwidth B and with noise figure F.
                                                                    Clearly, to detect a target of a given cross-section at a
Radar system parameter                         Value
                                                                 chosen range there are various combinations of power,
Average transmitted power                      1W                wavelength and antenna gain that have to be optimised
Transmitter duty radio                         0.1               against the constraints of external factors. One example,
Receiver noise figure                          5.0 dB            for a man-portable system that shows this in a simple way,
                                                                 is the contrasting needs to propagate through adverse
System losses                                  7.0 dB
                                                                 weather whilst maintaining a small and manageable
Elevation beam loss                            1.0 dB
                                                                 antenna size. The former will limit the upper frequency
Scan rate                                      36 deg/s
                                                                 whilst the latter will tend to impose a limit on the lower
Antenna size                                   1 × 0.5 m         frequency.
Antenna gain                                   38.5 dB              A further example is provided by the way in which a
Azimuth 3dB beamwidth                          1.5°              predetermined antenna gain may be realised in practice for
Elevation 3dB beamwidth                        3.0°              the chosen application. This will be subject to external
Wavelength at 15 GHz                           0.02 m            factors such as coverage optimisation, terrain masking, the
Dwell time at 15 GHz                           0.0625 s          likelihood of third party detection and jamming. For
                                                                 example it may be advantageous to use a fan beam that is
System losses breakdown                                          narrower in azimuth than elevation so that the radial extent
Frequency weighting and scalloping             2.0   dB          of the area of interest (as projected onto typical terrain) can
Receiver range matching and scalloping         2.0   dB          be increased to provide an extended field of coverage. This
Detection thresholding                         1.0   dB          will also improve cross-range location accuracy, which is a
                                                                 direct function of azimuth beam width. Alternative
In-service degradation                         2.0   dB
                                                                 applications may demand a wider azimuth beam, which

182                                  ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000
can scan an area more quickly, or a
pencil beam, which concentrates
energy onto a smaller area of
intersected ground.
                                                 oscillator and                 power                 transmit
   By optimising the principal radar              modulator                   amplifier
design parameters against a well
understood requirement in this way the
high-level design goals of the radar may                              reference
                                                                      signal
be outlined. This can then be used as
preparation for further, more detailed
                                                 amplifier and
refinement. Table 2 shows a                       quadrature                 preamplifier             receive
representative set of the principal                detector
parameters able to meet the
requirement for a man-portable
battlefield surveillance radar system
meeting the requirements in Table 1.
The      relative    importance      and            signal
significance of the parameters in design          processor
optimisation can now be seen. For
example, consider the case of two
classes of targets — moving vehicles        Fig. 1 Schematic diagram of radar system
and moving people — which are
assumed to have arbitrary radar cross-sections of 10 m2         3 Systems
and 0.5 m2, respectively. The radar equation can now be
used to examine performance characteristics, such as the        From its beginnings in the early 1940s the development of
influence of frequency on detection range. It is relatively     short-range surveillance radar has been mainly aimed at
straightforward to calculate range and cross-range              providing battlefield targeting information from a man-
accuracies, unambiguous Doppler bands and Doppler               portable platform for forward artillery observers. Notable
resolutions.                                                    radars have included the ZB298, which entered service in
   Consider the calculation of the optimum operating            the UK in the late 1960s. It operates in the I and J bands
frequency, for which some typical results are summarised        using largely solid-state technology with the exception of
in Table 3. The optimum frequency is shown for detection        the magnetron power source. It is man-portable in two
range, range and cross-range accuracy and Doppler               segments with the radar head tripod-mounted and the
ambiguity. The frequency range over which the                   control system able to be located up to 20 m away. The
performance falls by 6 dB is also shown, which in turn          system displays target returns on a gallium phosphide
reduces the range by approximately 30% and doubles the          screen and additionally has an audio output used by the
measurement errors. A frequency of around 15 GHz                operator to aid detection and act as a crude means of
probably offers the best compromise all-round                   classification. The ZB298 has been incorporated into two
performance but is clearly not optimal in all cases. For        tracked armoured personnel carriers, the FV 103 and the
example, at longer ranges or in adverse weather                 FV 432 with the antenna mounted on a telescopic mast.
conditions, lower frequencies would be more suitable, or, if       A similar system, the AN/PPS-15 radar, is in service in
ranges of approximately 7 km or less are required, this         the USA and worldwide.There are currently many varieties
could be achieved at a higher frequency, around 35 GHz,         of battlefield surveillance radar. A number (not all) of these
which might have size and weight advantages.                    are listed in Table 4, which shows the enormous variety of


Table 3: Optimum operating frequencies showing a 6 dB performance spread

               Performance                                                Frequency, GHz
                                                                No rain                           4mm/h rain

                                                       10 km          20 km             10 km              20 km
                                                       range          range             range              range
Detection range            Optimum                       15             15                 11                 8
                           6 dB spread                 4 – 35         4 – 35             2 – 20            2 – 16
Range accuracy             Optimum                       15             15                 11                 8
                           6 dB spread                 4 – 35         4 – 35             2 – 20            2 – 16
Cross-range accuracy       Optimum                      35              35                 15                12
                           6 dB spread                12 – 35         11 – 35            7 – 25            6 – 19
Doppler ambiguity          Spread (±100 km/h)          0 – 32         0 – 16             0 – 32            0 – 16



ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000                                                               183
Table 4: Battlefield and short-range ground surveillance radars. Detection ranges are given for large shells (e.g. 155 mm).
Data is taken from open literature sources.

Name          Country Radiated        Weight, Frequency Azimuth Azimuth Azimuth Detection range, km   Minimum
                       power           kg       band† bandwidth, coverage, accuracy,                   detect
                                                          deg       deg      deg     Men Vehicle FOS* velocity,
                                                                                                       km/h

SCB 2130A Belgium 70 W pk                      I and J           1      10 – 360     0.5        15       30             1.5
RASIT-E       France     3 kW pk        90        I                     10 – 240 0.1 – 0.6      20       40     20
RB12B         France 25 mW av.          32        J                     45 – 180      1          3       6.4             3
RATAC-S       Germany 7 kW pk          100        I                     30 – 140     0.1        18       24     25
EL/M02140 Israel         70 W pk       100     I and J           1      10 – 360                15       30              1
SCAT-20       Italy      7 kW pk               I and J                               0.1        10       20
RD 170 BT     Norway 5 kW pk           62.4       I          2.7
MSTAR &     UK         0.7 – 10W pk     30        J                      5 – 360     0.3        10       25              2
derivatives
AN/PPS-5      USA        1 kW pk       58.9       J                                  0.6         5       10             1.6
                          1 W av
AN/PPS-       USA       50 mW pk        15        J                     22 – 180                1.5      3
15A/B
Type 386      USA        100 mW        12.6       J                      22 – 90                         6
GSR                        CW
MSR-20        USA        11 W pk        30                       5                   0.6        5.2      11             0.7
AN/TPS-       USA                              I and J                               1.4
21/33
AN/TPS-74     USA                      <45        J          1.4                             >2.5 – 12   30
AN/PPS-24     USA                              B and C                     120             >300 m foliage pen



*FOS = fall of shot
† I Band: 8–10 GHz. J Band: 10–20 GHz. B Band: 250–500 MHz. C Band: 0.5–1 GHz.



extant systems and widely ranging radar specifications. It           against a typical ground environment. Fig. 2 shows the
is worth noting that it is often quite small changes in the          MSTAR equipment as a typical instance of the man-
operational requirements for these systems that have                 portable ground surveillance radar.
resulted in very significant differences in the final design.           MSTAR uses a solid-state power amplifier and retains
Table 4 illustrates that although there is a clear correlation       phase information for subsequent coherent processing.
between weight, power and detection range it is not                  The radar detects men and vehicle targets with a location
necessarily the case that the heavier and the more                   accuracy of approximately 0.3°, enables classification from
powerful the radar the longer the detection range. A sound           the Doppler signature (presented as an audio signal to the
understanding of the radar design fundamentals and the               operator) and is able to detect and correct artillery fire.
electromagnetic backscattering properties of the target              MSTAR has a low peak power to minimise hostile detection
and environment can result in much more efficient,                   by electronic surveillance measures and operates from a
compact designs resulting in greater all-round operational           standard field battery4.
versatility and effectiveness. A note of caution should also            One of the most important aspects of this class of radar
be sounded as these specifications do not necessarily tell           design is the required simplicity of the display to the
the whole story and other characteristics, such as usability,        operator. In general, he or she is under high stress and
cost, reliability and technology vintage, also need careful          cannot assimilate complex information presentations. Figs
consideration.                                                       3 and 4 show typical MSTAR displays, representative of the
   Advances in technology beyond that incorporated in the            displays available from this general class of radar system.
ZB298 have allowed the capability of later generation radar          Fig. 3 shows the area surveillance mode, where some
systems to improve further and have led to manportable               targets have been detected at the edge of the scanned
surveillance radar systems such as MSTAR (Moving and                 sector. Fig. 4 shows the display in acquisition mode;
Stationary Target Acquisition and Recognition), currently            effectively it is an enlarged section of the surveillance
in service with the British Army and RASIT (Radar                    display using a ‘B Scope’ format and showing the trails
d’Acquisition et de Surveillance dans les Intervalles                from two moving targets that have been observed over a
Terrestres) in service with the French Army.                         period of time. Note the absence of extraneous data that
   MSTAR is a pulse Doppler scanning radar system                    could distract the operator.
optimised for the detection of moving men and vehicles                  Current developments in research and technology are

184                                   ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000
Fig. 2 MSTAR deployed: (a) man-portable and (b) vehicle-mounted roles

resulting in systems with an ever-increasing radar               Technology
capability with ever-reducing sizes and weights. Not only
can this lead to an increase in the capability of man-portable   The design of radar systems for short-range surveillance
battlefield surveillance radar systems but it is also enabling   applications needs to strike a balance between leading-
systems to be hosted on a greater range of platform types        edge technology and application-driven requirements. For
and a greater range of applications to be addressed.             many applications weight, volume, power consumption,
However, the key to the successful realisation of any            performance, ease of use and cost are the principal design
system lies in optimisation of the radar performance in the
context of the operating environment, including the
interaction of the system with the human operator. This
must be achieved so that targets of interest are easily
distinguished from background clutter and must be
accomplished at an affordable cost. In the next section
some recent developments are described that are leading
to the next generation of small, lightweight, coherent
radars for short-range surveillance.

4   Recent developments

The majority of the research underpinning the next
generation of radar systems can be thought of as
                                                                 Fig. 3 MSTAR surveillance display showing a 6 km range
comprising three principal elements:

4 technology or hardware that enables the area of
  interest to be illuminated and the reflected energy
  detected
4 signal processing, whose function is to interpret the
  reflected energy so that targets may be optimally
  distinguished from the background clutter
4 human factors, the display of the processed
  information, and the interaction of the human with the
  system.

This section is split into technology and signal processing,
with the human factor elements included within the               Fig. 4 MSTAR acquisition display showing 1 km square
technology topic.                                                box


ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000                                                            185
drivers that severely constrain the radar designer’s                generated electromagnetic power to the areas of interest
options. For military systems low probability of intercept          and is the first point of reception of the backscattered
by hostile ESM (Electronic Support Measures) systems                energy. Current systems use a mechanically scanned dish
and electronic protection measures is also important. It will       antenna that is relatively simple to fabricate and is efficient
be seen in this section how technology advances are now             and easy to operate. However, it has the disadvantage that
increasing the freedom of the radar designer from these             the beam shape is fixed and therefore not always optimised
constraints.                                                        for purpose. The pointing of the beam is slow and not very
                                                                    agile and the size and shape of the antenna make it
Transmitters and receivers                                          cumbersome and physically vulnerable in many
   All radar systems have to generate and amplify an                applications. Many of these disadvantages may be
electromagnetic source prior to transmission and it is in           overcome with phased-array antennas, which can be
the design of power amplifiers where most recent progress           operated either passively or actively.
has been made. There has been an evolving trend to higher              There is much research ongoing into phased arrays and
power devices (>10 W peak in high J-band, with                      they are beginning to find application in larger systems.
corresponding advances scaled to the other radar bands)             The single biggest difficulty to be overcome in order to be
in smaller packages with higher efficiencies. The                   able to use phased arrays in small radars is cost. Current
waveform generator and power amplifier combination of               phased arrays are prohibitively expensive. Conventional
tomorrow will also be capable of enhanced agility and will          phased arrays, although reducing in cost, seem destined to
be able to host a wider variety of waveform types which can         be out of reach of the class of radar considered here. There
be tailored to a given application. For example, this may           is research ongoing into low-cost phased arrays that on
enable the duty cycle to be varied such that when coupled           maturity would revolutionise the capability of short-range
with power management techniques the system is able to              surveillance radar systems. These will enable nonlinear
minimise the likelihood of intercept by ESM systems at all          scanning so that, for example, dwell times on targets of
ranges by automatically controlling the output power to             interest could be increased whilst still maintaining an all-
suit the operating conditions. CW (continuous wave)                 round scanning capability. Laboratory prototypes have
operation may also be easily supported resulting in                 been fabricated, however much research is still required
significant potential cost savings that accompany this              before they will be mature enough for production systems.
simpler radar design approach.
   Future envisaged amplifiers will be compatible with              Power generation
substantial increases in the resolution of the radar, which            If the radar is to be self contained then a source of power
will be enhanced by more than an order of magnitude to              will be required, most likely a battery if the radar is to be
less than 1 m. This offers the potential for improved target        used remotely from a mains-like power source. Battery
detection and clutter rejection and additionally opens up           technology is advancing, with longer lifetimes available
one method for the classification of targets (discussed later       from smaller, lighter weight packages largely developed
in this paper). The sensitivity of receivers (coupled with          for a wide range of applications.
improvements in receiver signal processing) has also                   The advances in battery technology as applied to this
improved notably, resulting in overall system losses being          type of radar can be seen in the 24 V ‘Clansman’-type power
reduced by several decibels. Together these                         source suitable for an MSTAR-like radar. The typical Ni-
improvements enable the detection range of systems to be            Cad form has been augmented with lithium or nickel-
extended by up to a factor of two without weight, volume or         metal-hydride based disposable and rechargeable units.
cost penalties. Indeed, future systems may be envisaged             Typical parameters of available military batteries are
with overall weights of considerably less than 30 kg. These         shown in Table 5. Quite clearly there has been a factor of
advances instantly translate into significant improvements          two improvement in power available per unit weight,
in the capability of current systems. Coupled with                  particularly if the user is prepared to consider a disposable
corresponding improvements in signature understanding               battery.
and processing they will allow future systems to increase              This is a very active field of engineering and significant
greatly the versatility of short-range surveillance radars.         improvements are expected over the next few years as the
                                                                    commercial imperatives of computer and automotive
Antennas                                                            systems force battery development.
  The radar antenna is a key component that directs the                It is also important to manage power with the utmost

Table 5: Military battery parameters

                                        Capacity, Ah            Weight, kg             Size (L x W x H), inches

Clansman (1988)                          4                      3                      7 × 2.75 × 5
Nickel metal hydride rechargeable        4                      1.75                   4.4 × 2.5 × 5
Lithium ion rechargeable                 4                      1.3                    4.4 × 2.5 × 5
Disposable lithium                      10                      0.5                    7 × 2.75 × 5


186                                 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000
care to ensure that it isn’t being used when not needed and     hardware. These and other signal processing issues are
that processing algorithms are designed in a way that           considered next.
makes them as power efficient as possible. This is an
excellent example of the importance of taking a holistic        Signal processing
view of the radar system design to optimise performance in
all operational conditions.                                     Ever increasing signal processing power from ever more
                                                                efficient devices has been a major technology trend for
Display technology and human computer interaction               many years and is set to continue for the foreseeable
   Display technology is advancing at a considerable rate,      future, driven again by the requirements of the mass
largely driven by the demands of the mass consumer              consumer market. The consumer market may not,
market. Research prototypes of light-weight, low-power-         however, progress the processing power per watt as fast as
consumption high-resolution flat screens have been              it increases processing power per chip. A telling example is
developed and there is the possibility of ultra-thin liquid     the increasing need for on-chip fans as the power
crystal display (LCD) screens that could be rolled or folded    consumption of the PC processor rapidly expands.
for ease of storage and carriage. Clearly these forms of            There is some hope for improvements in this area via
display have to be tailored to the operating environment to     laptop PC developments, but the pace of development of
ensure that they can be viewed in all likely conditions. The    processing per watt can be illustrated by the example of an
way in which data is displayed and the method by which          update to an existing radar, currently using ten year old
the operator interacts with the radar system are also key       technology. In this case the processing available (for the
factors in ensuring that the system meets its overall design    same power consumption) by using all of the modern
goals. This may become simplified through the adoption of       technological advances has only increased by a factor of
PC standards such as Windows NT, although it is not clear       five. This contrasts with the processing power per chip,
that this necessarily provides a route to the most efficient    which has increased over the same period by a factor
system performance, nor will the commercial human               between 30 and 100. Clearly, when prime power is at a
interface necessarily be well suited to operation in extreme    premium, this issue is of fundamental importance.
conditions (for instance, operation in the pouring rain, at         Of much more importance is the move from function-
night, in a foxhole, and under attack may not be the best       specific hardware to general-purpose processing.
time to be finding the right submenu or fighting the            Although this seems at first to be just another way of
mouse).                                                         achieving the required process, it does offer a qualitatively
   The battlefield of the future will exploit digital           different approach.
technology and next-generation system designs will need             In this new scheme a pre-process operating at the signal
to consider carefully electronic forms of reporting and         processing rate can cue a much more complex algorithm,
communicating with the commander. It will also be               but at the much lower information rate. Critically, because
important to examine interoperability issues, particularly      of the availability of large memories accessible to the signal
as most future conflicts are likely to be coalition based.      processor devices, these ‘cued’ complex algorithms can
                                                                now work on the raw data stored while the cueing
Implications of technology advance                              algorithms operated.
   Overall, the above advances in technology are enabling           This ability to mix algorithm types on a single
future radar systems to function at longer ranges in            processing engine allows the designer to use much more
smaller, lighter more cost-efficient packages offering a        sophisticated radar algorithms without greatly increasing
much more versatile set of operating modes. For example,        the power consumption, size or weight.
the man-portable radar of tomorrow will be lighter, easier
to carry and able to operate at longer ranges but able to       Target detection
manage its radiated power so that the likelihood of hostile        As discussed earlier, in the principal mode of operation
intercept is minimised. Further it will be able to adjust its   of current systems targets are distinguished from their
operating parameters so that they are automatically             background by virtue of differences in the magnitude and
tailored to the task in hand. An example might be using a       radial Doppler velocity of the backscatter. When the radial
lower resolution wide-area surveillance mode to                 velocity of the target with respect to the background is
automatically cue a high-resolution classification mode         high, this is not too demanding. However the radial
towards detected targets of interest. Naturally these radar     velocity of many targets of interest will naturally be low, for
attributes translate into advantages in vehicle mounting        example a man walking or a vehicle moving close to a
where real estate for sensors is at a premium but demands       cross-radial direction. Consequently the resulting Doppler
on performance are equally severe. The likely future            velocity may be comparable with the background,
performance of these radar systems also makes them              particularly if the latter consists of wind-blown vegetation.
prime candidates for mounting in either manned or                  If this is the case complex adaptive detection schemes
unmanned air platforms. The advantages are that the field       must be employed that take account of the unpredictable
of regard is greatly enhanced and also that the forward         and ill-behaved nature of the background clutter whilst,
motion of the platform can be used to add a synthetic           simultaneously, positively exploiting the characteristics of
aperture imaging mode to the current moving-target              target signatures, the overall objective being to maximise
detection mode without major changes to the basic radar         target detection whilst minimising false alarms. Fig. 5.

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000                                                               187
Fig. 5 Example of adaptive
thresholding: (a) real data;
(b) targets found using
constant thresholds;
(c) targets found using
adaptive thresholds (CFAR —
constant false alarm rate)




                                           range




                                                       frequency               frequency               frequency
                                                           a                      b                       c




shows a comparative example of fixed and adaptive                  resolutions the nature of the clutter returns will change.
thresholding. In the fixed case the threshold removes all          The energy backscattered will be reduced but its nature
the clutter but conceals two targets with radial velocities        will become progressively more ill-behaved. This requires
close to that of the clutter. However, in the adaptive case,       a detailed understanding so that detection algorithms can
where prior knowledge of clutter behaviour is embodied in          be adjusted accordingly. Various empirical and statistical
the algorithm, all the targets are detected with no false          models have been proposed. One of these, the compound
alarms. This leads to a less confusing radar display, and          form of the K-distribution, has been successfully used to
allows the operator to concentrate their attention on              describe the amplitude and correlation properties of
evaluating targets rather than being distracted by false           clutter5. However, when man-made clutter dominates a
alarms.                                                            resolution cell the K-distribution model breaks down and
   It is well recognised that the behaviour of the clutter is a    an alternative must be used. The fact that more than one
function of both environment and radar specification. As           model is needed to represent all forms of clutter illustrates
future systems may be capable of operating at higher radar         the complexity of the operating environment.

Fig. 6 Time records of
radar targets




188                                  ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000
Understanding           this
environment and target
signatures is one of the
major challenges for the
future. Overall detection
performance at high
resolutions will generally
improve as there will be
less clutter in a resolution
cell competing with the
target. However there
will come a point where
the target will start to
traverse resolution cells
during the period of the
processing interval and
this will cause its energy      a
to be smeared out;
consequently detection
will not be enhanced.
The detailed relationship
governing the detect-
ion performance under
these      conditions      is
also the subject of on-
going research but the
performance of prototype
systems is extremely
encouraging.

Target classification
   There can be little
doubt that if detected
targets could be classified
into type, such as men
and vehicles, the utility of       b
the information would
be greatly enhanced,             Fig. 7 Spectrograms (Doppler frequency against time): (a) of man walking; (b) of tank
enabling the battlefield
commander to form a much better understanding of                 may be built into automatic or semi-automatic algorithms,
enemy dispositions. The utility would be improved still          either to make the classification decision or to cue the
further if the type of vehicle, for example, could also be       operator to targets of greatest interest. Recent research
discerned. Having detected a target of interest one method       has taken this form of classification a stage further.
of classification is to use the magnitude of the time-varying       To understand the machine view of the target returns we
backscatter to modulate an audio system and provide an           must view them with the processing available to the
acoustic representation of the target Doppler signal to the      machine. In general the Doppler radar sees the target as a
operator. Experience with ZB298 and MSTAR show this to           series of Doppler spectra against time. This representation
be effective in certain conditions but overall the               of typical targets of interest is shown in Fig. 7. The two
performance is variable and the relationship between             spectrograms clearly show the difference between a radar
signature and classification is ill understood.                  return from a man walking and a vehicle (in this case a
   Doppler classification: Fig. 6 shows two examples of the      tank).
time-varying intensity (as might be used to modulate a              Of particular interest for our understanding of this form
speaker) of a single resolution cell of a target return over a   of classification is the periodic nature of the returns from
period of a few seconds. It is possible to observe gross         men (Fig. 7a), representing the movement of arms, legs
movement of the target as evidenced by the modulation            etc.; the returns from a vehicle (Fig. 7b) have much less
and it is this that the human listener hears and uses to         periodicity, representing a relatively smooth motion. In
make the classification decision. The experienced                addition to the longer term modulation behaviour of the
operator is able to make quite subtle distinctions between       backscattered radiation there is important target-related
differing target modulations. Research work is attempting        information contained in the shorter-term fluctuations. For
to identify the key classification criteria in order that they   example, reflections from wheeled vehicles may exhibit a

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000                                                             189
characteristic Doppler signature, enabling them to be             the smaller land-based targets provide a further challenge
distinguished from other targets, such as tracked vehicles.       due to the greater variability of target types and
   Machine classifiers working on short-term Doppler              background environments. Early research results are
classification have been shown to be capable of                   encouraging but algorithms are far from being either
distinguishing classes of vehicle, and capable of achieving       optimal or robust and target signature understanding is
this with very short bursts of information (such as could be      still a relatively immature science with much further work
gathered as the scan of a radar passes a target — a few tens      required. For example, little research has yet been carried
of milliseconds).                                                 out to establish the trade-offs between algorithm
   High range resolution classification: Another technique        performance,        processing      demands      and     power
for target classification is to increase the spatial resolution   consumption. The role of the radar operator is key in
by increasing the radar bandwidth. If the radar resolution        overall systems operation and must be included in
is sufficiently high it is possible that the target can be        performance trade-offs.
classified spatially. The simplest approach is to form a             Target classification is a statistical technique, and future
‘range profile’, which is in effect a one-dimensional target      systems are likely to employ multiple methods to classify
signature formed from a series of sequential range gates. It      targets in an attempt to make the results more robust.
is one dimensional in the sense that the second dimension         Current developments in Doppler and high range
is bounded by the antenna beamwidth, which is relatively          resolution techniques will complement each other in
large and therefore provides little or no additional              future ground surveillance radar systems.
information. The range profile will contain backscatter
from the different parts of the target under interrogation. If    Imaging for target detection and classification
the nature of the profile is characteristic then it is possible     If there is relative motion (or angular change) between
to perform classification. The simplest method would be to        the radar and the target or area of interest then an image
build a library set of ‘known’ targets and compare the            can be formed by synthesising a virtual antenna that
profile under test with the library sets. Library sets can be     results typically in a cross-range resolution that matches
compiled either from measurements or from simulation.             that of the radial direction. Two of the more common forms
   Pattern recognition techniques for classification: Standard    of this technique are known as ISAR (inverse synthetic
pattern recognition or matched-filtering techniques can be        aperture radar) and SAR (synthetic aperture radar).
used to perform the classification for either the Doppler or        In ISAR relative motion is provided via the velocity or
high range resolution technique. For Doppler                      rotation of the target. However, if the motion of the target is
classification, the simpler techniques appear adequate to         unknown this results in defocusing and uncertainty in
achieve an acceptable and robust performance.                     the scaling of the image. Techniques aimed at
   For improved performance in high range resolution              removing these imaging errors have been developed
systems, more advanced neural network or genetic                  for ship targets but their successful application to
programming approaches can be employed. This has been              smaller land targets is still in its infancy.
demonstrated successfully for maritime applications but             An obvious extension to the military utility offered by
                                                                                   this class of radar is to mount it on an
                                                                                   airborne platform (such as an unmanned
                                                                                   aerial vehicle or a helicopter), which affords
                                                                                   a much less inhibited view of the battlefield.
                                                                                   In addition, the forward motion of an
                                                                                   airborne platform can be used to synthesise
                                                                                   a large antenna and achieve high resolution
                                                                                   in two dimensions. This is known as SAR
                                                                                   and typically the direction of view is
                                                                                   perpendicular to that of flight of the air
                                                                                   platform. The attributes of the radar
                                                                                   systems described here lend themselves
                                                                                   ideally to this form of imaging. The
                                                                                   techniques for producing a correctly
                                                                                   focused image are relatively mature and
                                                                                   have been demonstrated to resolutions of a
                                                                                   few metres; in principle it is possible to form
                                                                                   an image with a resolution of the order of 1
                                                                                   m or less. Fig. 8 shows an example SAR
                                                                                   image. At very high resolutions an image of
                                                                                   a target can be formed as a basis for
                                                                                   classification. This is the subject of intense
                                                                                   worldwide research and is likely to lead to
                                                                                   techniques that can be used reliably and
Fig. 8 Example of SAR imagery                                                     effectively in operational systems.

190                                   ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000
   Airborne short-range surveillance radar systems can be
used in both moving-target and imaging modes to provide           Chris Baker graduated from the
a more comprehensive surveillance capability. Indeed they         University of Hull in 1980 with a
can be used simultaneously so that the moving targets can         first class honours degree in
                                                                  Applied Physics. After completing a
be overlaid on the SAR image, thus providing a valuable           PhD in laser physics (also at Hull)
context in which interpretation decisions can be made. Not        he joined DERA (formerly RSRE)
surprisingly there are penalties to be paid, largely in the       in 1984. Initially, Chris worked in
                                                                  maritime radar, pioneering pulse-
form of extra system complexity and consequently in cost.         by-pulse analysis techniques aimed
For example, the rate at which data is produced,                  at characterising electromagnetic
particularly by imaging radars, is somewhat prodigious            backscatter from the sea surface. In
                                                                  1990 he was appointed leader of the
(up to several hundred megabits per second) and a means           Battlefield surveillance research
of communicating this to the ground for exploitation must         group and played a key role in the
form part of the overall system. This of course also implies      development of airborne radar techniques, including ultra-high-
                                                                  resolution SAR, STAP and interferometry. This culminated in the
a ground segment and operations will require mission              highly successful MoD procurement programme ASTOR. Chris is
planning and a much higher level of all-round                     currently head of short-range radar research, specialising in
sophistication and support. Clearly the increased                 seeker sensors and surveillance and target acquisition systems.
                                                                  Chris is a Fellow of the IEE, is chairman of professional group E15
capability offered has to be carefully balanced against the       (Radar, Sonar and Navigation) and is a member of the organising
increased complexity and system costs. Nevertheless the           committee for RADAR 2002
potential for using small lightweight coherent radars to
                                                                  Address: PE 304, DERA Malvern, St. Andrews Rd., Malvern,
perform many functions is considerable and likely to grow         Worcs. WR14 3PS, UK.
in the near future. Implicit reductions in production costs       Email: Cbaker@mail.dera.gov.uk
should increase further the range of applications and
                                                                  Barry Trimmer graduated with a
accelerate developments in this important category of             degree in Physics from Warwick
radar. Applications of SAR imaging include 3-D mapping,           University in 1978 and was awarded
flood management, oil exploration, volume estimation,             a Master’s degree in Astronomy by
                                                                  Sussex University in 1979. He
treaty verification, surveying, urban planning, ocean             joined the radiation laboratory at
monitoring and coastal erosion monitoring.                        EMI Electronics, Hayes, working
                                                                  on radar antennas for the
                                                                  Searchwater radar and naval ESM
                                                                  systems. During the 1980s, he
5   Summary                                                       developed      RF      and      system
                                                                  modelling within EMI, leading to
                                                                  system      design       of     ground
In this paper the essential design parameters of short-           surveillance and weapon locating
range battlefield surveillance radars have been outlined          radars. He was responsible for the development of the highly
and examples of operational systems given. Against this           successful MSTAR radar based on the original design by Barry
                                                                  Priestley. Since 1990 he has originated the design of a wider range
backdrop current and likely future technological and              of sensors in the fields of airborne radar, EW, multisensor systems
signal processing advances have been described pointing           and (more recently) UAV payloads. He is presently radar and
the way to future system developments. Rapid advances             ISTAR design authority for Racal Defence Electronics at Crawley,
                                                                  and manages the Advanced Systems group. He is a Member of the
are impacting almost every aspect of the system and are           IEE and a chartered engineer.
quickly increasing the capability of man-portable
battlefield radars whilst simultaneously finding application      Address: Advanced Systems, F Building, Racal Defence
                                                                  Electronics, Manor Royal, Crawley, West Sussex, RH10 2PZ, UK.
in vehicle and air platform based surveillance. The quality       Email: barry.trimmer@rdel.co.uk
of information about target type and status is likely to
enable the operator to perform increasingly wide ranging
and sophisticated tasks, so enabling the battlefield
commander to gain a more comprehensive understanding           References
of his situation. This trend is set to continue. As costs
continue to be driven downwards it is likely that additional   1 SKOLNIK, M. I.: ‘Introduction to radar systems’ (McGraw Hill,
commercial applications, from harbour surveillance to the        1980)
                                                               2 STIMSON, G. W..: ‘An introduction to airborne radar’ (Sci Tech,
protection of buildings and other facilities requiring
                                                                 1998, 2nd edn.)
security measures, will emerge. The range of applications
                                                               3 NATHANSON, F. E.: ‘Radar design principles’ (McGraw Hill,
clearly increases greatly with the ability to form SAR          1969)
imagery, although airborne systems will be accompanied         4 WATTS, S., TRIMMER, B., PRIESTLEY, B., and BAKER, C. J.:
by higher manufacturing and operating costs.                     ‘Battlefield   surveillance     radars’    (Defence     Systems
                                                                 International, 1996)
                                                               5 WARD, K. D., BAKER, C. J., and WATTS, S.: ‘Maritime
Acknowledgments                                                  surveillance radar part 1: Radar scattering from the ocean
                                                                 surface’, IEE Proc. F, Radar Signal Process, 1990, 137, pp.51–62
The authors would like to thank Professor Mike Dean and
Dr. Simon Watts for their helpful comments and assistance      ©IEE:2000
during the writing of this paper.                              Received 11th January 2000


ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000                                                                         191

				
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