Credit: Earth as Art

Ground-based vicarious calibration is presently the most accurate method to independently perform the absolute radiometric calibration of airborne and spaceborne Earth-observing sensors. The one obvious limitation is that personnel and equipment must be deployed to the test site to measure surface and atmospheric properties during the sensor overpass. The Remote Sensing Group has developed a methodology for vicarious calibration where personnel are no longer required to be present during a sensor overpass. The atmospheric and meteorological properties are measured using a suite of equipment that remains permanently situated at Railroad Valley Playa, Nevada. The surface bidirectional reflectance factor (BRF) is measured using an array of inexpensive, robust, all-weather radiometers based on LEDs as detectors.

LEDs were chosen as detectors for these nadir-viewing radiometers because they are tough, cost less than silicon detectors, and have their own spectral filter by nature. The LED radiometers deployed at Railroad Valley have three spectral channels centered at wavelengths of approximately 535, 622, and 845 nm. The spectral bandwidth of each channel is on the order of 40-60 nm. These radiometers have been calibrated and characterized in RSG’s calibration facility for such parameters as spectral responsivity, temperature dependence of spectral responsivity, field of view, and absolute radiometric responsivity. The absolute radiometric calibration is measured in the laboratory with a 1-m spherical integrating source (SIS), and outside using the solar-radiation-based calibration (SRBC) method, which uses the Sun as a source.