Vicarious calibration: A reflectance-based experiment with AirMISR

A vicarious reflectance-based calibration experiment for the Multiangle Imaging SpectroRadiometer (MISR) airborne simulator, AirMISR, is described as one precursor experiment of this type planned for postlaunch application to MISR itself. The experiment produces a set of multiangle near-top-of-atmosphere radiances that are compared with the multiangle AirMISR radiances, established using a laboratory calibration. The field and aircraft data were collected as part of an engineering test flight at Moffett Field, CA, on November 5, 1997. A concrete tarmac was used as the field target. Atmospheric optical depth data were collected adjacent to the target throughout the actual overflight period using a single Reagan solar radiometer. For logistical reasons, the surface hemispherical directional reflectance factor (HDRF) was determined 7 days later using the Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere III (PARABOLA III), along with the areally averaged spectral HDRF at normal incidence, obtained with an Analytical Spectral Devices (ASD) FieldSpec moderate resolution field spectrometer. AirMISR overflew the target under clear sky conditions though the aerosol turbidity was high (∼0.3 at 550 nm). Good to fair agreement has been obtained at all angles and wavelengths between the top-of-atmosphere (TOA) radiances calculated for the measured atmospheric and surface conditions and the radiances incident at AirMISR as determined from the laboratory calibration. Some systematic disagreements are present. The largest disagreements (∼15% in the blue) are found at the highest view angles and the smallest at nadir viewing (<5%). Possible explanations for the differences in radiances at large view angles are discussed.