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					The InTernaTIonal revIew of weaTher, ClImaTe, and hydrology TeChnologIes and servICes




Meteorological
T e c h n o l o g y                      i n t e r n a t i o n a l




                                                                           l A ISS
                                                                              U Ue
                                                                                n !
                                                                                  c
                                                                                    h
  2010
  SHOWCASE
Satellite data
by Joe Predina, Laura Jairam, Randall Bass, and Mary Beth Crile




remote control
Space-based sensors for
monitoring global climate trends
Advances in calibration technology are enabling sensors in space to
detect minute changes in Earth’s climate accurately and effectively
          n the past, using weather satellite
   I      data archives to establish long-term
          climate trends has been difficult
and filled with controversy, because the
platforms were not designed for the
purpose. Limitations related to spectral
resolution, spectral range, radiometric
accuracy, long-term stability, and calibration
differences between various sensors made it
difficult to develop indisputable climate
records. However, technology currently in
development will finally enable accurate and
reliable measurements from space. These
advances have set the stage for a paradigm
shift in climate monitoring, where satellite
measurements will play a more important
role in the future.
    Satellite sensors are critical in measuring
and trending Earth’s radiation budget and
balance. Because changes in the reflected and
emitted radiation are small in comparison
with the enormous magnitude of radiation
exchanged between Earth, space and the sun,
the absolute accuracy achieved by remote
sensors in making these measurements
becomes paramount.
    The measurement accuracy needed to
perform indisputable climate trending from
space has undergone refinement over time,
with the most notable consensus reached in
2002, when scientists from NASA, NOAA,
NIST (National Institute of Standards and
Technology), NPOESS-IPO, and various
universities published their findings as part of
the Climate Change Research Initiative. These
recommendations suggest that the remote
sensor stability per decade should be at least
five times smaller than the climate parameter
trended. The recommendations have
remained largely unchanged since their
introduction in 2002.
    These recommendations are forcing new
ways of thinking when designing satellite
sensors for measuring climate trends. Since
the energy exchange between the sun,
Earth, and space spans ultraviolet to the far
infrared (generally between 0.2µm and
50µm), the observations from space must
also produce a continuous spectrum over


ME TEOROLOGICAL TEChnOLOGy In TERn ATIOn AL 2010
“technology has evolved to the point
where prior limitations associated with
satellite observations can be eliminated”
                              ITT has supplied multispectral imagers and
                              sounders to weather forecasting services for more
                              than 50 years. Hurricane Floyd, 1999 (Photo
                              courtesy NASA Goddard Space Flight Center)

                               the same range to validate scientific models.
                               The spectral resolution achieved must be
                               fine enough to enable development of better
                               spectroscopic atmospheric models and to
                               improve the knowledge of Earth surface
                               properties. New hardware architectures,
                               calibration methods, and associated ground
                               processing are required to support the
                               accurate parameter measurements.
                                   For example, radiance observations in
                               the visible spectrum must be calibrated
                               about 10 times more accurately than current
                               methods. Brightness temperature
                               uncertainty for radiometers must approach
                               current NIST characterization limits of 0.01-
                               0.03K in the infrared bands and maintain
                               this level of calibration for at least 10 years
                               while on orbit. Spectral calibration and high
                               spectral resolution of radiance observations
                               are essential for identifying changes in the
                               concentration of trace gas species such as
                               greenhouse gases. Finally, trending of Earth
                               cloud fraction, aerosol content, and surface
                               vegetation characteristics will be vitally
                               important to future climate models.

                               Limitations of early space sensors
                               Weather satellite sensors designed in the
                               past primarily exploited information-rich
                               segments of Earth’s spectrum for the
                               purpose of short-term weather forecasting or
                               real-time nowcasting. The number of
                               spectral channels spanning the infrared,
                               visible or microwave bands was typically
                               very small for any particular remote sensor.
                               The typical channel bandwidth of filter
                               radiometers was broad in comparison with
                               today’s standards and incapable of resolving
                               fine spectral features of the atmosphere. In
                               addition, the detailed spectral response
                               shape of these radiometers was primarily
                               governed by one or more optical band pass
                               filters that were characterized in detail for
                               each instrument prior to launch.
                                                                                                                                Satellite data




                                                                                                     Typical radiation balance between Earth, sun and
                                                                                                     space averaged over 24 hours (Reprinted with
                                                                                                     permission of Trenberth)


                                                                                                      Satellites bring NIST on board
                                                                                                      Breakthrough technologies under
                                                                                                      development at ITT are changing how
                                                                                                      space-based remote sensing will be
                                                                                                      performed for climate trending. It essentially
                                                                                                      brings NIST capabilities on board the
                                                                                                      spacecraft. Some of these include greater
                                                                                                      than 0.999 emissive broadband infrared
                                                                                                      blackbody reference targets that provide
                                                                                                      international standard traceability to within
                                                                                                      0.015K over the life of the reference target.
                                                                                                      Other NIST capabilities include
    To date, space-based remote sensors have        Days, and Seasons (ASCENDS), are                  hyperspectral radiometer hardware
been spectrally and radiometrically calibrated      overcoming this problem. Technology has           employing FTS and associated software
on the ground. Once on orbit, the calibration       evolved to the point where prior limitations      calibration techniques, as well as linearity
tended to slowly degrade over time due to           associated with satellite observations can be     characterization and compensation methods
normal aging and drift processes associated         eliminated. The new class of instruments          unique to FTS that can achieve 50ppm or
with the hardware. Additionally, despite            under development is capable of providing         better radiometric linearity while on orbit
detailed ground calibration and                     an order of magnitude reduction in                over the full brightness temperature
characterization, the spectral response of one      measurement uncertainties, better stability       measurement range of a radiometer.
instrument was usually slightly different from      over time, finer spectral resolution, and             Other capabilities involve visible
other instruments in its series.                    more precise knowledge of the spectral            calibration techniques to achieve
    Radiometric calibration and the brightness      response function.                                measurement accuracies approaching 0.2%
temperature measurement uncertainty
associated with these instruments depended          “Space-based monitoring enables
on the quality of blackbody reference targets
carried on board the satellite or, in the case of   uniform, global measurements to be
visible sensors, by the quality of an onboard
diffuser that used solar radiation as a
                                                    taken with fixed temporal periodicity,
reference. Both these methods were subject to       regardless of ground access”
degradation over time, since the reference
target properties were determined on the                Rather than sampling portions of the          compared with the 2-3% currently accepted
ground. This calibration could not be renewed       spectrum, these new sensors provide               as standard; new detector technologies that
routinely after launch except by inference          continuous spectral coverage at high              push into the far infrared (15-50µm),
from many earth observations or comparison          resolution. The calibrated output of              making possible space-based measurements
with simultaneous balloon observations              hyperspectral infrared sensors such as CrIS       of earth emissions in this important
known as radiosondes. Neither of these              and CLARREO will not differ from one              wavelength range; and visible and ultraviolet
methods can achieve the necessary calibration       instrument to the next in its series. It will     hyperspectral methods using diffraction
accuracy/stability to produce undisputed            no longer be necessary to adapt science           grating or FTS technology.
climate records from space.                         analysis to the unique signature of a space-          Active lidar sensors such as ASCENDS,
                                                    based remote sensor. Instead, radiance            which use space-based lasers to probe the
High-resolution coverage                            measurements will be consistently mapped          atmosphere for greenhouse gas signatures to
Future satellite architectures and                  to an identical user spectral grid that is        accurately determine total column
technologies under development at ITT               invariant from one sensor to the next and         concentration, are a final technology.
Space Systems Division, such as the                 has identical spectral response shapes for all
Advanced Baseline Imager (ABI), the Cross-          channelizations across a band. Digital            Remote sensing from space, air, and
track Infrared Sounder (CrIS), Climate              syntheses of spectral response functions          the ground
Absolute Radiance and Refractivity                  inherent in Fourier transform spectrometers       There are benefits and drawbacks in choosing
Observatory (CLARREO), and Active                   (FTS) are replacing inconsistent and              a space-based approach to climate monitoring.
Sensing of CO2 Emissions over Nights,               inaccurate analog optical filter techniques.      One downside, perhaps the most significant,


                                                                                     ME TEOROLOGICAL TEChnOLOGy In TERn ATIOn AL 2010
Satellite data




is the high cost of building and launching a
satellite system. A large amount of highly
skilled labor, specialized equipment, and
facilities are required. Furthermore, the risk
of mission failure can be precariously binary:                                                    Left: Prototype model of Advanced Baseline
even a small problem in implementation can                                                        Imager. Right: Image depicting ocean and
cause a launch anomaly or operational glitch                                                      atmospheric data generated by IDL, ITT’s
that drastically shortens sensor lifetime, such                                                   computing environment for data visualization
as the recent loss of the Orbiting Carbon                                                         and analysis
Observatory Satellite.
    As outlined previously, calibration is        Sensor integration                               sounders that helped to form the basis of
another key challenge in carrying out climate     Sensors on airborne platforms can make           today’s weather forecasts worldwide.
monitoring from space. Achieving the              measurements to help fill the gaps between           During 2009, ITT completed the
necessary precision and accuracy to detect        ground sensors, and can measure localized        prototype for the most advanced space
minute, slowly changing climate trends            emission sources of greenhouse gases and         weather instrument ever built to measure
requires onboard calibration systems that         aerosols that may be missed by ground            and track severe storms. The Advanced
exact additional engineering costs. Further       sensors because of windspeed or altitude.        Baseline Imager will monitor and measure
costs stem from the complex data acquisition      Limitations of airborne sensors include the      three times the number of atmospheric
systems required to collect and organize the      inability to provide persistent surveillance     conditions, provide data in seconds rather
pertinent auxiliary information for climate       and the difficulty of synoptic or global         than minutes or hours, and enable
analysis, with ground receiver stations that      coverage, not to mention sensitive               forecasters to zoom in on specific storms
may need to be coordinated across                 instruments being at the mercy of weather        while monitoring the rest of the hemisphere.
international borders. Finally, for space         conditions and aircraft vibration occurring          An important advancement in
monitoring to be effective in the long term, a    at the time of flight.                           atmospheric sounding capability will be
well-managed archival database is needed to           Space-based monitoring enables uniform,      available soon when the CrIS instrument
store records and disburse data to users in a     global measurements to be taken with a           joins the National Polar-orbiting Operational
timely manner. Fortunately, these challenges      fixed temporal periodicity, regardless of        Environmental Satellite System (NPOESS).
are not insurmountable. In many cases, such       restricted ground access. Remote areas, or       CrIS is a hyperspectral infrared sensor that
as with onboard calibration, technical            those that are inaccessible due to political     profiles atmospheric temperature, moisture,
solutions are already under development, and      tensions, can be monitored and studied           and pressure with better accuracy and much
the benefits of climate monitoring from space     anonymously and without interference from        finer vertical resolution than previous
are numerous and compelling.                      adversarial parties. Ocean and land              generations of operational space-borne
    Currently, monitoring of greenhouse gas       phenomena can be treated with equal              sounding instruments.
emissions and changes in Earth’s climate          priority. Combining suites of microwave,             ITT is helping to create space-based and
system is accomplished primarily by ground-       hyperspectral UV/Vis/IR instruments with         airborne sensors to measure greenhouse
based systems, such as sniffers and buoys.        GPS technology will enable satellite             gases such as carbon dioxide and methane.
Ground-based sensors measure localized            platforms to provide a wide range of             ASCENDS will actively sense the diurnal
climate-driving parameters such as                climatologically relevant information            and seasonal variations of CO2 in the
temperature, humidity, pollution, aerosols,       geolocated to any region. Most importantly,      atmosphere – an advantage over traditional
spectral radiance, winds, and atmospheric         space-based remote sensors complement            passive systems. Overall, ITT is poised to
concentrations of greenhouse gases. However,      ground-based and airborne sensors to form        play an active role in delivering the
their deployment is usually limited. Terrain,     independent networks of checks and               innovation needed for the next generation of
harsh conditions, and political boundaries can    balances that essentially can be used to         satellite sensors, and the company looks
inhibit deployment at many locations and the      validate each system’s performance through       forward to this challenge. ◗
density of sensors at others. Furthermore,        inter-comparison.
ground sensors are typically point source             ITT is well positioned to support the        Joe Predina is from Systems Engineering Integration and
systems, which measure parameters only at         technological advancement needed to make         Test. Co-authors: Laura Jairam is an image scientist,
and immediately around them. Interpolations       climate observation from space a reality. The    Randall Bass is senior meteorologist, and Mary Beth Crile
must be made to infer concentrations of           company has a long and successful legacy of      is a geoscientist at ITT Corporation Space Systems
parameters between the ground-based sensors.      building weather satellite imagers and           Division. www.itt.com



ME TEOROLOGICAL TEChnOLOGy In TERn ATIOn AL 2010

				
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