Unmanned vicarious calibration for large-footprint sensors

The Remote Sensing Group (RSG) at the University of Arizona is currently developing inexpensive, unmanned radiometers based on light-emitting diodes (LEDs). This work describes these radiometers, which are now used as part of the extensive vicarious calibration research that has been conducted since the mid-1980s and presently includes such sensors as MODIS, ASTER, AVHRR, Landsat-7 ETM+, Ikonos, and Quickbird. RSG performs a typical vicarious calibration with on-site personnel measuring atmospheric and surface conditions at a test site during actual sensor overpass. A radiative-transfer code is used to calculate a top-of-atmosphere radiance, which is then compared to that reported by the sensor under test. Data collection can be limited by poor weather conditions, and in addition, it is generally difficult to collect data during every sensor overpass due to the large travel distances to the test sites. The LED radiometers are being developed as a solution to the temporal sampling limitations seen in the past. They are used in a nadir-viewing configuration to measure the surface reflectance in three spectral bands, while the atmospheric conditions are measured using a Cimel sun photometer. The data from these two instruments are used to produce a top-of-atmosphere radiance during overpass when no personnel are present. Results of laboratory calibration measurements of the radiometers are described, and include the spectral responsivity, temperature dependence of the spectral responsivity, and calibration coefficient. Finally, the top-of-atmosphere radiances produced by the unmanned vicarious instrumentation are compared to those reported by Aqua MODIS for three days in March 2005.