Bi-Directional Reflectance Factor Determination of the Railroad Valley Playa

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%.

Monitoring the TROPOMI-SWIR module instrument stability using desert sites

“…It is well documented that off-nadir soundings are affected by non-Lambertian effects (e.g. Bruegge et al, 2019a). Although correction routines can be applied, these often rely on ground measurements to derive the correction factors within a modified Rahman-Pinty-Verstraete model (Rahman et al, https://doi.org/10.5194/amt-2020-433 Preprint.…”

Characterization of the angular response of a multi-directional spectroradiometer for measuring spectral radiance

“…This shortcoming is of particular concern under cloudy-sky conditions within which short-term changes in the solar radiation are likely [9]. A more complete instrument called PARABOLA [10][11][12], capable of measuring radiance in a wide spectral range (350nm to 1650 nm) and in a relatively short time (about 10 seconds), which can measure both in the upper and lower hemisphere, can only record radiation at certain wavelengths (444, 551, 650, 860, 944, 1028 and 1650 nm) or at most in a continuous range of 300 nm in the visible spectral range (400-700 nm centred at 581 nm). Spectroradiometers that scan the sky require (too) long measuring times.…”

Monitoring the Tropospheric Monitoring Instrument (TROPOMI) short-wave infrared (SWIR) module instrument stability using desert sites

“…The BRDF of desert sand, in particular its accuracy and effect at off-nadir viewing angles, has been described and characterized by Bruegge et al (2019a). They find that desert sand has a characteristic "hot spot" in the backwards direction of the incoming solar radiation.…”

“…They also show that these BRDFs are not well characterized at large off-nadir viewing angles, with differences of 5 % in a normalized solution with respect to the zenith shown as typical. The Moderate Resolution Imaging Spectroradiometer (MODIS) BRDF product (Schaaf et al, 2011) provides world-wide coverage with a 16 d interval but also suffers from inaccuracies as described in Bruegge et al (2019a) due to saturation of the detector. These are relevant for TROPOMI, due to its very wide swath.…”