1) Advances in satellite sensor calibration technology have enabled more accurate monitoring of global climate trends from space. 2) Prior satellite sensors had limitations like spectral resolution and radiometric accuracy that made establishing long-term climate records difficult. 3) New satellite instruments under development can provide continuous high-resolution spectral coverage, onboard calibration, and reduced measurement uncertainties, enabling undisputed climate monitoring from space.

1. 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
2 6 • ME TEOROLOGICAL TECHNOLOGY INTERNATIONAL 2010

2. Satellite data
“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.
ME TEOROLOGICAL TECHNOLOGY INTERNATIONAL 2010 • 2 7

3. 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, fi ner 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,
2 8 • ME TEOROLOGICAL TECHNOLOGY INTERNATIONAL 2010

4. 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 fi xed 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 INTERNATIONAL 2010 • 2 9