Abstract
Vicarious calibration is the determination of an on-orbit sensor’s radiometric response using measurements over test sites such as Railroad Valley (RRV), Nevada. It has the highest accuracy when a remote sensor’s view angle is aligned with that of the surface measurements, namely at a nadir view. For view angles greater than 10°, the dominant error is the uncertainty in the off-nadir correction factor. The factor is largest in the back-scatter principal plane and can reach 20%. The Orbiting-Carbon Observatory has access to a number of datasets to determine this deviation. These include measurements from field instruments such as the Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere (PARABOLA), as well as satellite measurements from Multi-angle Imaging SpectroRadiometer (MISR) and MODerate resolution Imaging Spectroradiometer (MODIS). The correction factor derived from PARABOLA is consistent in time and space to within 2% for view angles as large as 30°. Field spectrometer data show that the correction term is spectrally invariant. For this reason, a time-invariant model of RRV surface reflectance, along with empirically derived coefficients, is sufficient to use in the calibration of off-nadir sensors, provided there has been no recent rainfall. With this off-nadir correction, calibrations can be expected to have uncertainties within 5%.
Highlights
Vicarious calibration (VicCal) is one methodology by which the ratio of incident radiance to output digital numbers (DN) can be determined for an on-orbit sensor
If a nadir-measured surface reflectance is used in the calibration, without correction for bi-directional reflectance factor (BRF) with view angle, an error of up to 10% can be incurred for sensors that view up to 30◦ off-nadir
The off-nadir correction term can be obtained from the Multi-angle Imaging SpectroRadiometer (MISR) Level 2 Surface product, where the normBRF agree with PARABOLA to within 1%
Summary
Vicarious calibration (VicCal) is one methodology by which the ratio of incident radiance to output digital numbers (DN) can be determined for an on-orbit sensor. Being a desert playa in a locally arid environment, sabkha processes dominate soil moisture [3] including evaporative pumping, which causes a dryer surface during the day at the same time experiencing a higher evaporative throughput of water from below The combination of these compositional, hydrological, and structural effects could cause differences in the apparent bi-directional reflectance factor (BRF), but the off-nadir reflectance, normalized to its value at nadir, is shown here to have minimal variability in time and location within the playa. A correction is needed to adjust the radiances measured by the reference sensor into effective radiances that would have been seen at the view angles of the sensor to be calibrated This technology is not specific to RRV, but the validation of the MISR BRF data product over desert sites, as shown here, is relevant to those making these cross-sensor comparisons
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