Vicarious Calibration with a Field Spectroradiometer


Vicarious radiometric calibration provides a means for monitoring the performance of on-board satellite sensors. Many applications, such as the effects of human activities on surface ecology, vegetation monitoring, climate studies and ocean research depend on the long-term stability of satellite radiometric calibrations of sensors to produce useful estimates of long-term variability. Satellite sensors are calibrated pre-launch in the laboratory using integrating spheres. The radiometric and spectral performance of sensors degrades over time post-launch  due to the change in environment and sensor use and aging.

Vicarious radiometric calibration methods include reflectance-based, airborne radiance-based, and irradiance-based calibration methods. Reflectance-based and radiance-based approaches predict the at-sensor radiance using radiative transfer model calculations, using measured surface reflectance or radiance and atmospheric conditions. The irradiance-based approach measures atmospheric characteristics as well as diffuse-to-global irradiance.

Vicarious radiometric calibration uses in situ measurements over pseudo-invariant calibration sites at the time of satellite or airborne overpass. Calibration sites can be natural (desert, ocean) or man-made (target tarps of different reflectance values). The ideal site for vicarious radiometric calibration is a location where surface reflectance properties are constant and there is minimal atmospheric condition variation.

A NIST-traceable calibrated field spectroradiometer such as the SR-6500, RS-8800, RS-5400, PSR+ or RS-3500 can be used to measure the reflectance of the target site as the satellite passes overhead. With high resolution, high sensitivity and unmatched stability and reliability, these spectroradiometers can provide the measurements required to correlate to the energy measurements produced by the satellite sensors, thereby providing correction coefficients. Fast and easy operation allow you to take a large number of scans during the limited time of the satellite overpass.

Solar radiance scan

Blue line is surface scan taken with a SPECTRAL EVOLUTION field spectrometer. The red line
represents the solar radiance of the reflected light.

Vicarious calibration in the field

Vicarious radiometric calibration can be done with a PSR+ spectroradiometer.