Abstract
Current in-flight inter-sensor vicarious calibration methods either rely on extended stable instrumented calibration sites that allows prediction of at-sensor radiance at the time of the image collect or with pseudo-invariant sites that allow transfer of the calibration from one sensor to another based on available spectral shape information of the sites surface reflectance. In the first case it requires measurements of the spectral surface reflectance and atmospheric properties that match the time and the view geometry of the site by the sensors under calibration. The second approach requires that the sensors image the pseudo-invariant site with not only the same view geometry but also the same solar illumination as well. These restrictions attempt to reduce uncertainties inherent with directional reflectance effects typically associated with ground reference targets. Such approaches are problematic, creating infrequent opportunities to inter-compare multiple sensors over their mission lifetimes. Presented in this paper is the Specular Array Radiometric Calibration (SPARC) Methodology; a vicarious calibration approach that overcomes this directional limitation for inter-sensor calibration. The SPecular Array Radiometric Calibration (SPARC) method employs convex mirrors to create two arrays of calibration targets for deriving absolute calibration coefficients of Earth remote sensing systems in the solar reflective spectrum. The first is an array of single mirrors used to oversample the sensor’s point spread function (PSF) providing necessary spatial quality information needed to perform the radiometric calibration of a sensor when viewing small targets. The second is a set of panels consisting of multiple mirrors designed to stimulate detector response with known at-sensor irradiance traceable to the exo-atmospheric solar spectral constant. The outcome is improved radiometric performance knowledge compared to other in-flight vicarious techniques through reduced uncertainties in target reflectance, atmospheric effects, and temporal variability. The only ground truth needed is the measurement of atmospheric transmittance. Using mirrors provides the advantage that directional reflectance effects are small, stable and accurately known. In fact, over very frequent operational view angles for typical collection events, variations in reflectance are negligible relative to the nominal reflectance of the target. A demonstration of the absolute vicarious SPARC method will be presented based on data collected with the IKONOS and GeoEye-1 satellites operated by GeoEye.
The Specular Array Radiometric Calibration (SPARC) Technique As A Vicarious Methodology For Accurate Inter-Sensor Calibration
Current in-flight inter-sensor vicarious calibration methods either rely on extended stable instrumented calibration sites that allows prediction of at-sensor radiance at the time of the image collect or with pseudo-invariant sites that allow transfer of the calibration from one sensor to another based on available spectral shape information of the sites surface reflectance. In the first case it requires measurements of the spectral surface reflectance and atmospheric properties that match the time and the view geometry of the site by the sensors under calibration. The second approach requires that the sensors image the pseudo-invariant site with not only the same view geometry but also the same solar illumination as well. These restrictions attempt to reduce uncertainties inherent with directional reflectance effects typically associated with ground reference targets. Such approaches are problematic, creating infrequent opportunities to inter-compare multiple sensors over their mission lifetimes. Presented in this paper is the Specular Array Radiometric Calibration (SPARC) Methodology; a vicarious calibration approach that overcomes this directional limitation for inter-sensor calibration. The SPecular Array Radiometric Calibration (SPARC) method employs convex mirrors to create two arrays of calibration targets for deriving absolute calibration coefficients of Earth remote sensing systems in the solar reflective spectrum. The first is an array of single mirrors used to oversample the sensor’s point spread function (PSF) providing necessary spatial quality information needed to perform the radiometric calibration of a sensor when viewing small targets. The second is a set of panels consisting of multiple mirrors designed to stimulate detector response with known at-sensor irradiance traceable to the exo-atmospheric solar spectral constant. The outcome is improved radiometric performance knowledge compared to other in-flight vicarious techniques through reduced uncertainties in target reflectance, atmospheric effects, and temporal variability. The only ground truth needed is the measurement of atmospheric transmittance. Using mirrors provides the advantage that directional reflectance effects are small, stable and accurately known. In fact, over very frequent operational view angles for typical collection events, variations in reflectance are negligible relative to the nominal reflectance of the target. A demonstration of the absolute vicarious SPARC method will be presented based on data collected with the IKONOS and GeoEye-1 satellites operated by GeoEye.