The Use of High–Spatial–Resolution Satellite Data to Validate Low–Spatial–Resolution Automated Vicarious Calibration Results

Jeff Czapla–Myers, Kurtis Thome – University of Arizona

ABSTRACT: The Remote Sensing Group (RSG) at the University of Arizona developed an instrumentation suite and methodology in 2004 to measure the surface and atmospheric characteristics of a test site in the absence of personnel. The Radiometric Calibration Test Site (RadCaTS) methodology is based on the reflectance–based approach, which has been used by RSG since the 1980s. RSG field campaigns occur at a rate of once per month during the academic year, and longer campaigns are held during the summer months. In an attempt to increase the number of data sets collected throughout the year, the RadCaTS approach was developed so that surface and atmospheric data can be collected during every overpass of sensors that are capable of continuous operation.

Using the RadCaTS methodology, the surface bidirectional reflectance factor of the site is measured using four nadir–viewing radiometers. Their locations are chosen based on the topography of the site in an effort to sample the 1–kmˆ2 large–footprint area used with such sensors as MODIS and AVHRR. This work quantifies the uncertainty in predicting the surface reflectance of the 1–kmˆ2 area based on the point measurements of the automated methodology. It also determines if the number of radiometers, and their positions, are suitable to characterize the site in a spatial sense. These uncertainties are determined through the use of portable spectroradiometers, and high–spatial–resolution QuickBird imagery.

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Terra and Aqua MODIS Thermal Band Calibration Consistency Through Observations of a Ground Target

Brian Wenny, Jennifer Dodd – Science Systems and Applications, Inc.; Jack Xiong – NASA Goddard Space Flight Center

ABSTRACT: The two MODIS instruments currently operating on the Terra and Aqua spacecrafts have successfully delivered Earth view science products since launch in 2000 and 2002, respectively. MODIS has a total of 36 spectral bands, 16 of which are thermal emissive bands (TEB), covering wavelengths from 3.7 to 14.4 micrometers.The derived long–term science data set has proven useful for land, mmatmosphere and ocean applications. A method to track the relative TEB calibration stability and consistency between the two MODIS instruments using long–term ground target observations was developed previously and applied to MODIS bands 31 (11 micrometers) and 32 (12 micrometers). The ground target of Dome Concordia, Antarctica (75.1 S, 123.4 E) is a high altitude, flat, extensive area that is generally recognized to be surface temperature and emissivity homogeneous over satellite remote sensing footprint spatial scales. MODIS bands 31 and 32 were used for the initial study, as they are atmospheric window bands that view the surface and require minimal atmospheric corrections due to the cold, dry, rarefied atmosphere overlying Dome Concordia.No noticeable calibration drift between the two MODIS sensors was observed for bands 31 and 32 over the 5 years of overlapping observations. In this work, the technique is extended to the other MODIS thermal bands to investigate long-term relative calibration stability and consistency.  The results indicate additional corrections for cloud coverage and atmospheric contributions are required for these bands.

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SDLXR (Space Dynamics Laboratory Transfer Radiometer)

Kendall Johnson, Alan Thurgood, Jeff Blakeley – USU/Space Dynamics Laboratory

ABSTRACT: Over the past several years the Space Dynamics Laboratory has been building its calibration capabilities. One of the larger of these projects has been the design and building of the SDLXR (Space Dynamics Laboratory Transfer Radiometer), which is nearing completion. The instrument has a 70 mm entrance beam with a 1 milli–radian field of view. To maintain a low background, the system operates between 30 and 50 K. The detector, supplied by NIST, is an SiAs bib detector operated at 10 K. The instrument spectral range is from 2 to 30 microns, limited by the detector response. There are two operating modes. One is a filter based radiometer mode and the other is an interferometer (spectrometer) mode. By moving several mirrors the optical beam can either pass through an interferometer or by pass it (radiometer mode). The interferometer is a cryogenic Fourier Transform Spectrometer built at SDL and has a KBr (potassium bromide) beamsplitter. It will provide relative spectra with 0.5 wavenumber unapodized resolution. Various filters for the radiometer are provided in 4 filter wheels with 8 positions in each and cover wide and narrow bands in various wavelengths ranges of interest. This system will be able to spectrally characterize IR sources. The spectral information obtained from the interferometer combined with the radiometric information will provide higher accuracy source and chamber calibrations which will allow a better understanding of radiometric sources thus reducing the uncertainties in sensor calibrations. We plan to have the SDLXR calibrated at NIST in the future using the same equipment used to calibrate the NIST BXR and SXR. This will shorten the calibration chains thus decreasing errors and increasing calibration accuracy.

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Satellite Sensor Inter–comparison and Validation Studies

Allen Larar, Dan Zhou, Xu Liu – NASA Langley Research Center; William Smith – Hampton University / University of Wisconsin

ABSTRACT: Measurement system validation is critical for advanced satellite sensors to reach their full potential of improving observations of the Earth’s atmosphere, clouds, and surface for enabling enhancements in weather prediction, climate monitoring capability, and environmental change detection. Experimental field campaigns, including satellite under–flights with well–calibrated FTS sensors aboard high–altitude aircraft, are an essential part of the validation task. This presentation focuses on inter–comparison and validation studies performed using data from recently–completed field experiment campaigns with the National Polar–orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounding Testbed—Interferometer (NAST–I).

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Intercalibration of AVHRR and IASI Radiances using Radiative Transfer Calculations

Xu Liu, Allen Larar, Daniel Zhou – NASA Langley Research Center; William Smith – Hampton University / University of Wisconsin; Peter Scluessel – EUMETSAT

ABSTRACT: We will discuss a method for intercomparing AVHRR and IASI radiances. A radiative transfer model will be used to take into account of instrument functions and scene dependencies.

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Infrared Spectroradiometry for Climate Benchmark Traceability: Approach and Demonstration Study

Sergey Mekhontsev, Leonard Hanssen, Jinan Zeng – NIST; Vladimir Khromchenko, Alexander Prokhorov – USU/Space Dynamics Laboratory; John Dykema – Harvard University; Fred Best – University of Wisconsin–Madison

ABSTRACT: Long term absolute measurements of the Earth spectral radiance in the infrared are essential for climate change modelling and prediction. A proposed CLARREO mission envisages use of original on–board reference blackbodies with built–in means of controlling stability of cavity emissivity and temperature sensors. It is important, though, that this concept and its implementation undergoes a rigorous testing and is made traceable to the international standards prior to launch. Paper deals with our approach to pre–flight element– and system–level calibration of the payload and validation of the on–board monitoring techniques. A brief update on the current status of NIST thermal IR metrology and forthcoming capabilities is provided. Paper also includes first results of the demonstration study, which has been recently started at NIST with participation of all involved laboratories.

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Inter–comparison Between AIRS and IASI Through Retrieved Parameters

Daniel Zhou, Xu Liu, Allen Larar – NASA Langley Research Center; William Smith – Hampton University; Jonathan Taylor – Met Office; Peter Schlüssel – EUMETSAT; Larrabee Strow – University of Maryland Baltimore County; Stephen Mango – NPOESS Integrated Program Office

ABSTRACT: The Joint Airborne IASI Validation Experiment (JAIVEx) was conducted during April 2007 mainly for validation of the Atmospheric InfraRed Sounder (AIRS) on the Aqua satellite launched on 4 May 2002 and the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp satellite launched on October 19, 2006. Ultra–spectral resolution infrared spectral radiance obtained from near nadir observations provide atmospheric, surface, and cloud property information. An advanced retrieval algorithm with a fast radiative transfer model, including cloud effects, is used for atmospheric profile and cloud parameter retrieval. A one–dimensional (1–d) variational multi–variable inversion solution is used to improve an iterative background state defined by an eigenvector–regression–retrieval. This physical inversion scheme has been developed, dealing with cloudy as well as cloud–free radiance observed with ultra–spectral infrared sounders, to simultaneously retrieve surface, atmospheric thermodynamic, and cloud microphysical parameters. Retrievals of atmospheric soundings, surface properties, and cloud optical/microphysical properties with AIRS and IASI observations are obtained and presented. These retrievals are further inter–compared with those obtained from airborne FTS system, such as the NPOESS Airborne Sounder Testbed Interferometer (NAST–I), dedicated dropsondes, radiosondes, and ground based Raman Lidar. The capabilities, in terms of measurement and/or calibration accuracies, of current satellite ultra–spectral sounders such as the AIRS and IASI are investigated through their retrieval parameters.

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AVHRR Lunar Observations and Calibration Challenges

Changyong Cao – NOAA/NESDIS; Tom Stone – US Geological Survey; Jack Xiong – NASA Goddard Space Flight Center

ABSTRACT: In this study, we investigated the lunar observations from NOAA–14 and –16 AVHRR to evaluate their usefulness for instrument stability trending. Several hurdles must be overcome before these AVHRR lunar observations can be used. The peculiar behavior of AVHRR space clamp has to be well understood to restore the lunar observations from the space–view data. For AVHRR, not only the moon was observed at different phase angles, but also in many cases only part of the moon is observed, due to the small field of regard cross–track in its space–view. Despite these complications, analysis of NOAA–14 and –16 AVHRR lunar observations show good correlation between the instrument output count and the moon phase angle, and methods are developed to mitigate the effect of partial moon observations. Stability trending and irradiance analysis using these lunar observations are investigated in this study with moderate success. Limitations in the AVHRR lunar observations also provide lessons learned for future sensor design toward climate quality instrumentation.

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Long Term Stellar Variability in the Infrared

Ray Russell, Richard Rudy, Trishana Prater, David Lynch, Kirk Crawford, Daryl Kim, David Gutierrez, Brandon Kaneshiro – The Aerospace Corporation; Michael Sitko – University of Cincinnati; Heidi Hammel – Space Science Institute; Mark Skinner – Boeing LTS

ABSTRACT: The accurate study of stellar variability in the infrared from ground–based observatories requires a stable, well–characterized sensor, uniform and high quality observing techniques, and accurate correction for the effects of the intervening atmosphere. The Aerospace Corporation’s Broadband Array Spectrograph System (BASS) is a liquid–Helium–cooled dual prism array spectrograph that has remained essentially unchanged since 1995, with relatively minor modifications in the time frame from its creation in 1988 until 1995. Observing techniques, while being refined during that time, are also essentially unchanged, and the observing team has maintained its core observers for the entire time frame. The BASS sensor is the workhorse instrument for the creation of the Aerospace Spectral Energy Distribution (ASED) catalog whose primary function is to supply absolutely calibrated spectra for use in calibrating IR sensors in space, and those results have been discussed in this conference on several occasions.

This presentation provides an ancillary result – the nature of the variability of the spectra of certain stars in amplitude and shape over a time frame of more than a decade. In order to provide these comparative data, one must accurately correct for instrumental effects as well as the atmosphere. Consideration must be given to (and corrections applied for) the effects of not only atmospheric absorption, but also of seeing, tracking, and guiding. These corrections will be presented through a discussion of extinction coefficients obtained for two observatories.

This work is supported at The Aerospace Corporation by the Independent Research and Development program.

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On–orbit Solar Calibrations using the Clouds and Earth's Radiant Energy System (CERES) In–flight Mirror Attenuator Mosaic (MAM) Calibration System

Robert Wilson – Science Systems and Applications, Inc.; Kory Priestley – NASA Langley Research Center; Phillip Hess, Susan Thomas – Science Systems and Applications, Inc.

ABSTRACT: The Clouds and Earth's Radiant Energy System (CERES) scanning thermistor bolometers measure earth–reflected solar and earth–emitted longwave radiances, at the top– of–the–atmosphere. The bolometers measure the earth radiances in the broadband shortwave solar (0.3–5.0 microns) and total (0.3 –>100 microns) spectral bands as well as in the 8–>12 microns water vapor window spectral band over geographical footprints as small as 10 kilometers at nadir. December 1999, the second and third set of CERES bolometers was launched on the Earth Observing Mission Terra Spacecraft. May 2003, the fourth and fifth set of bolometers was launched on the Earth Observing Mission Aqua Spacecraft. Ground vacuum calibrations define the initial count conversion coefficients that are used to convert the bolometer output voltages into filtered earth radiances. The mirror attenuator mosaic (MAM), a solar diffuser plate, was built into the CERES instrument package calibration system in order to define in–orbit shifts or drifts in the sensor responses. The shortwave and total sensors are calibrated using the solar radiances reflected from the MAM's. Each MAM consists of baffle–solar diffuser plate systems, which guide incoming solar radiances into the instrument fields of view of the shortwave and total wave sensor units. The MAM diffuser reflecting type surface consists of an array of spherical aluminum mirror segments, which are separated by a Merck Black A absorbing surface, overcoated with silicon dioxide. Thermistors are located in each MAM plate and baffle. The CERES MAM is designed to yield calibration precisions approaching .5 percent for the total and shortwave detectors.

However, in their first year of operation the Terra and Aqua MAMs showed shifts in their calibrations larger than expected. Shifts of this nature have been seen in other Solar viewing instruments in the past. A possible explanation has attributed the gain/spectral throughput change to pre-orbit or on-orbit contamination combined with solar ultraviolet induced chemical changes to the contaminate during solar exposure. In the subsequent year of operation all instruments begin to stabilize within the .5 percent precision range.

In this presentation, the MAM solar calibration procedures will be presented along with on-orbit measurements for the eight years the CERES instruments have been on-orbit. A switch to an azimuth rotation scan of the Sun rather than a elevation scan will also be discussed. Comparisons are also made between the Terra CERES instruments and the Aqua instruments during their MAM solar calibrations and total solar irradiance experimental results to determine how  precise the CERES solar calibration facilities are at tracking the sun's irradiance.

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Online Catalog of World–wide Test Sites for Sensor Characterization

Gyanesh Chander – SGT at US Geological Survey

ABSTRACT: In an era when the number of Earth–observing satellites is rapidly growing and measurements from these sensors are used to answer increasingly urgent global issues, it is imperative that scientists and decision makers can rely on the accuracy of Earth–observing data products. The characterization and calibration of these sensors are vital to achieving an integrated Global Earth Observation System of Systems (GEOSS) for coordinated and sustained observations of Earth. The U.S. Geological Survey (USGS), as a supporting member of the Committee on Earth Observation Satellites (CEOS) and GEOSS, is working with partners around the world to establish an online catalog of prime candidate test sites for the postlaunch characterization and calibration of space–based optical imaging sensors. The online catalog provides easy public Web site access to this information for the global community. This paper describes the catalog, the test sites, and the methodologies to use the test sites. It also provides information regarding access to the online catalog and plans for further development of the catalog in cooperation with calibration specialists from agencies and organizations around the world. Through greater access to and understanding of these test sites and their use, the validity and utility of information gained from Earth remote sensing will continue to improve. The online catalog can be accessed at http://calval.cr.usgs.gov/sites_catalog_map.php.

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Investigation of GOES–13 Imager Channel 6 Cold Bias

X. Wu, T. Schmit – NOAA/NESDIS/STAR; R. Galvin – ITT Industries; M. Gunshor – University of Wisconsin; D. Blumstein – Centre National d’Etudes Spatiales; Y. Li – IMSG, Inc.; S. Sohn – Korea Meteorological Administration; M. Goldberg – NOAA/NESDIS/STAR

ABSTRACT: A cold bias for GOES–13 Imager Band 6 (13.3 µm), on the order of –2 K, was found through inter–satellite calibration. The ensuing investigation of the root cause of the bias ruled out many possible factors, including uncertainties in blackbody temperature and emissivity, instrument nonlinearity response to signals, and f–number and temperature differences between pre–launch test and on–orbit operation. It is concluded that the most likely root cause is error in the spectral response function. Furthermore, it is found that the error in spectral response function may not be entirely due to uncertainties in pre–launch characterization; the on–orbit contamination could contribute to the change in effective spectral response function. The inter–satellite calibration tools, developed for the Global Space–based Inter–Calibration System (GSICS), played a crucial role in this investigation.

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Characterization of High–Flux Light Emitting Diodes for Radiometric Space Applications

Mark Helmlinger, Butch Miller – NGST; Tania Darnton – University of California Santa Barbara; Chris Durell – Sphere Optics

ABSTRACT: A wide range of narrow, mixed, and “white” spectral outputs are available from commercial LED manufacturers. Each product is optimized for terrestrial display or lighting applications, not for vacuum, radiation or temperature extremes. As well, the long-term response of phosphors and packaging to the space environment is not well understood.

Open literature to date reflects this, although space radiation effects have been investigated for component LEDs; Nieke et. Al “Spaceborne spectrometer calibration with LEDs”. Informed by the current state of the art within industry and academia; Yuqin Zong and Yoshi Ohno, “New practical method for measurement of high-power LEDs”, a targeted study at NGST is investigating the nature and degree of characterization necessary to implement LEDs as calibrated radiometric light sources in the space environment. Custom mixed–spectra integrating sphere LED–based calibrators in use today are an example of a proposed final application; Brown, et. Al, “Advanced Radiometric Sources for Ocean Color”. This poster documents progress to–date at NGST and outlines our current understanding of critical parameters.

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