As part of an effort to reduce uncertainties in the radiometric calibrations of integrating sphere sources and standard lamp irradiance sources, the Goddard Space Flight Center (GSFC) Radiometric Calibration Facility (RCF) primary radiometer was characterized at the NIST facility for Spectral Irradiance and Radiance Calibrations with Uniform Sources (SIRCUS). Specifically, the radiometer's slit spectral function was measured and the magnitude of out-of-band stray light was determined. The characterization also revealed significant contributions of spectral stray light due to fluorescence of the radiometer's input sphere. The RCF examined the effects of stray light and sphere fluorescence in the radiometer on source radiance calibrations along with approaches to reduce those sources of measurement error.
Two critical requirements of any calibration source are short and long-term operational stability and repeatability. Source monitoring is necessary in quantifying overall source performance including stability and repeatability. The NASA GSFC Code 920.1 Radiance Calibration Facility (RCF) developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of its integrating sphere calibration sources. FRMS bands are in the 0.4 -2.4 μm region, with several bands selected to coincide with common remote sensing bands. The FRMS was designed and fabricated in the year 2000. Early in 2001, the FRMS was reconfigured prior to being deployed on the RCF 180cm integrating sphere. This paper describes the instrument modifications resulting from the FRMS reconfiguration and presents FRMS monitor data for three RCF integrating sphere sources.
Two critical requirements of a calibration source are short- and long-term operational stability and repeatability. Monitoring the source is an asset in determining the origin of temporal changes, thus increasing confidence in source performance and quantifying repeatability. Monitor data which fall outside established parameters indicate that the source requires maintenance or re-calibration against the standard. The NASA GSFC Code 920. 1 Radiance Calibration Facility (RCF) has developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of its integrating sphere calibration sources in the 400-2400nm region. Sphere output change mechanisms include lamp aging, coating (BaSO4) deterioration, and water vapor level. FRMS wavelength bands are selected to be sensitive to changes caused by these mechanisms. Several FRMS bands coincide with common remote sensing instrument bands.
Multiple photon scatterings inside an integrating sphere can result in significant path lengths compared with line-of- sight sources. In strong water vapor absorption channels, such as those on MODIS and the MODIS airborne simulator, these internal path lengths can result in a significant reduction in sphere output radiance. Path length probability distributions for photons exiting a sphere are determined using Monte Carlo calculations. Approximate analytic expressions are also derived. Results are used to determine the effect of water vapor absorption on integrating sphere sensor calibrations in several pertinent channels.
NOAA-11 Advanced Very High Resolution Radiometer (AVHRR) and associated ground-based data have been collected at NOAA/NESDIS, on a daily basis and for 600 days, using five stations within the continental United States in the NOAA solar radiation (SOLRAD) monitoring network. The data have been filtered to include only uniformly overcast conditions and analyzed along the lines described by Paris and Justus (1988). Results from this first long-term pilot operational application of the method are presented. The method is potentially useful for establishing yearly-averaged trends in the radiometric gain of AVHRR Channels. The relatively small data base examined here suggests a precision in the 600 day mean gain of 5 percent or worse, with a significant part of this uncertainty being driven by poor knowlege of the bidirectional reflectance properties of clouds. The results suggest that the method in its present formulation has insufficient precision to be used as a primary method for the measurement of in-orbit gains of reflected-solar radiometers aboard polar orbiting satellites. Intrinsic limitations to the precision and time resolution of the method are discussed, and suggestions are offered for improving the precision of future results.
In February 1989 NASA announced the projects it will fund for an eighteen month definition phase study as part of its planned Earth Observing System program. One of these projects was the " Clouds and the Earth''s Radiant Energy System Instrument" (CERES-I). The instrument is composed of two scanning components each very similar in design to the successful ERBE scanner. One operates continuously in the ERBE " cross-track" scan mode while the other rotates the scan plane through 180 degrees every 30 to 45 seconds. A computer simulation code for ERB measurement developed at NOAA/NESDIS has been used to evaluate the potential performance of the cross-track component of this instrument. Specifically we have estimated the error in ERB instantaneous flux measurements for five different CERES-I designs and for two different earth surface resolutions 1 and 2. 5degree latitude and longitude regions. This study is limited to the effects of spatial sampling errors by assuming that the Earth/atmosphere system reflects radiation isotropically. A subsequent study will treat the effects of angular sampling errors and evaluate the value of the rotating azimuth scanner in minimizing this error. To allow for a margin in meeting user requirements (RMS error 10 W*M2) it has been concluded that a triangular (bi-directional) scanner with a field-of-view of 1. 8 degrees (half the ERBE design) averaged into 2. 5 degree target areas is the preferred design (RMS error
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