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An article I wrote with several colleagues on remote sensing.

An article I wrote with several colleagues on remote sensing.

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    MTI Article MTI Article Document Transcript

    • 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
    • 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
    • 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
    • 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