By Joe Predina, Laura Jairam, Randall Bass, and Mary Beth Crile

REMOTE CONTROL

n the past, using weather satellite data archives to establish long-term climate trends has been diffi cult

and fi lled 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 diffi cult to develop indisputable climate records. However, technology currently in development will fi nally 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 refl ected 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 refi nement 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 fi ndings as part of the Climate Change Research Initiative. These recommendations suggest that the remote sensor stability per decade should be at least fi ve 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

Space-based sensors for monitoring global climate trendsAdvances in calibration technology are enabling sensors in space to detect minute changes in Earth’s climate accurately and effectively

Satellite data

26 • METEOROLOGICAL TECHNOLOGY INTERNATIONAL 2010

I

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 scientifi c models. The spectral resolution achieved must be fi ne 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 sensorsWeather 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 fi lter radiometers was broad in comparison with today’s standards and incapable of resolving fi ne 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 fi lters that were characterized in detail for each instrument prior to launch.

Satellite data

“Technology has evolved to the point where prior limitations associated with satellite observations can be eliminated”

METEOROLOGICAL TECHNOLOGY INTERNATIONAL 2010 • 27

Satellite data

To date, space-based remote sensors have been spectrally and radiometrically calibrated on the ground. Once on orbit, the calibration tended to slowly degrade over time due to normal aging and drift processes associated with the hardware. Additionally, despite detailed ground calibration and characterization, the spectral response of one instrument was usually slightly different from other instruments in its series.

Radiometric calibration and the brightness temperature measurement uncertainty associated with these instruments depended on the quality of blackbody reference targets carried on board the satellite or, in the case of visible sensors, by the quality of an onboard diffuser that used solar radiation as a reference. Both these methods were subject to degradation over time, since the reference target properties were determined on the ground. This calibration could not be renewed routinely after launch except by inference from many earth observations or comparison with simultaneous balloon observations known as radiosondes. Neither of these methods can achieve the necessary calibration accuracy/stability to produce undisputed climate records from space.

High-resolution coverageFuture satellite architectures and technologies under development at ITT Space Systems Division, such as the Advanced Baseline Imager (ABI), the Cross-track Infrared Sounder (CrIS), Climate Absolute Radiance and Refractivity Observatory (CLARREO), and Active Sensing of CO2 Emissions over Nights,

Days, and Seasons (ASCENDS), are overcoming this problem. Technology has evolved to the point where prior limitations associated with satellite observations can be eliminated. The new class of instruments under development is capable of providing an order of magnitude reduction in measurement uncertainties, better stability over time, fi ner spectral resolution, and more precise knowledge of the spectral response function.

Rather than sampling portions of the spectrum, these new sensors provide continuous spectral coverage at high resolution. The calibrated output of hyperspectral infrared sensors such as CrIS and CLARREO will not differ from one instrument to the next in its series. It will no longer be necessary to adapt science analysis to the unique signature of a space-based remote sensor. Instead, radiance measurements will be consistently mapped to an identical user spectral grid that is invariant from one sensor to the next and has identical spectral response shapes for all channelizations across a band. Digital syntheses of spectral response functions inherent in Fourier transform spectrometers (FTS) are replacing inconsistent and inaccurate analog optical fi lter techniques.

Satellites bring NIST on boardBreakthrough 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 hyperspectral radiometer hardware employing FTS and associated software calibration techniques, as well as linearity characterization and compensation methods unique to FTS that can achieve 50ppm or better radiometric linearity while on orbit over the full brightness temperature measurement range of a radiometer.

Other capabilities involve visible calibration techniques to achieve measurement accuracies approaching 0.2%

compared with the 2-3% currently accepted as standard; new detector technologies that push into the far infrared (15-50µm), making possible space-based measurements of earth emissions in this important wavelength range; and visible and ultraviolet hyperspectral methods using diffraction grating or FTS technology.

Active lidar sensors such as ASCENDS, which use space-based lasers to probe the atmosphere for greenhouse gas signatures to accurately determine total column concentration, are a fi nal technology.

Remote sensing from space, air, and the ground There are benefi ts and drawbacks in choosing a space-based approach to climate monitoring. One downside, perhaps the most signifi cant,

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

“Space-based monitoring enables uniform, global measurements to be taken with fi xed temporal periodicity, regardless of ground access”

28 • METEOROLOGICAL TECHNOLOGY INTERNATIONAL 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: even a small problem in implementation can cause a launch anomaly or operational glitch that drastically shortens sensor lifetime, such as the recent loss of the Orbiting Carbon Observatory Satellite.

As outlined previously, calibration is another key challenge in carrying out climate monitoring from space. Achieving the necessary precision and accuracy to detect minute, slowly changing climate trends requires onboard calibration systems that exact additional engineering costs. Further costs stem from the complex data acquisition systems required to collect and organize the pertinent auxiliary information for climate analysis, with ground receiver stations that may need to be coordinated across international borders. Finally, for space monitoring to be effective in the long term, a well-managed archival database is needed to store records and disburse data to users in a timely manner. Fortunately, these challenges are not insurmountable. In many cases, such as with onboard calibration, technical solutions are already under development, and the benefi ts of climate monitoring from space are numerous and compelling.

Currently, monitoring of greenhouse gas emissions and changes in Earth’s climate system is accomplished primarily by ground-based systems, such as sniffers and buoys. Ground-based sensors measure localized climate-driving parameters such as temperature, humidity, pollution, aerosols, spectral radiance, winds, and atmospheric concentrations of greenhouse gases. However, their deployment is usually limited. Terrain, harsh conditions, and political boundaries can inhibit deployment at many locations and the density of sensors at others. Furthermore, ground sensors are typically point source systems, which measure parameters only at and immediately around them. Interpolations must be made to infer concentrations of parameters between the ground-based sensors.

Sensor integrationSensors on airborne platforms can make measurements to help fi ll the gaps between ground sensors, and can measure localized emission sources of greenhouse gases and aerosols that may be missed by ground sensors because of windspeed or altitude. Limitations of airborne sensors include the inability to provide persistent surveillance and the diffi culty of synoptic or global coverage, not to mention sensitive instruments being at the mercy of weather conditions and aircraft vibration occurring at the time of fl ight.

Space-based monitoring enables uniform, global measurements to be taken with a fi xed temporal periodicity, regardless of restricted ground access. Remote areas, or those that are inaccessible due to political tensions, can be monitored and studied anonymously and without interference from adversarial parties. Ocean and land phenomena can be treated with equal priority. Combining suites of microwave, hyperspectral UV/Vis/IR instruments with GPS technology will enable satellite platforms to provide a wide range of climatologically relevant information geolocated to any region. Most importantly, space-based remote sensors complement ground-based and airborne sensors to form independent networks of checks and balances that essentially can be used to validate each system’s performance through inter-comparison.

ITT is well positioned to support the technological advancement needed to make climate observation from space a reality. The company has a long and successful legacy of building weather satellite imagers and

sounders that helped to form the basis of today’s weather forecasts worldwide.

During 2009, ITT completed the prototype for the most advanced space weather instrument ever built to measure and track severe storms. The Advanced Baseline Imager will monitor and measure three times the number of atmospheric conditions, provide data in seconds rather than minutes or hours, and enable forecasters to zoom in on specifi c storms while monitoring the rest of the hemisphere.

An important advancement in atmospheric sounding capability will be available soon when the CrIS instrument joins the National Polar-orbiting Operational Environmental Satellite System (NPOESS). CrIS is a hyperspectral infrared sensor that profi les atmospheric temperature, moisture, and pressure with better accuracy and much fi ner vertical resolution than previous generations of operational space-borne sounding instruments.

ITT is helping to create space-based and airborne sensors to measure greenhouse gases such as carbon dioxide and methane. ASCENDS will actively sense the diurnal and seasonal variations of CO

2 in the

atmosphere – an advantage over traditional passive systems. Overall, ITT is poised to play an active role in delivering the innovation needed for the next generation of satellite sensors, and the company looks forward to this challenge. ◗

Joe Predina is from Systems Engineering Integration and Test. Co-authors: Laura Jairam is an image scientist, Randall Bass is senior meteorologist, and Mary Beth Crile is a geoscientist at ITT Corporation Space Systems Division. www.itt.com

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Left: Prototype model of Advanced Baseline Imager. Right: Image depicting ocean and atmospheric data generated by IDL, ITT’s computing environment for data visualization and analysis