An automated ground facility, for vicarious radiometric calibration of airborne and spaceborne sensors of visible and infrared light has been established. In the term “vicarious calibration,” “vicarious” is used in the sense of “in place of another,” signifying “in place of laboratory calibration.” Vicarious calibration involves the use of ground truth in the form of measurements by ground-viewing radiometers, a Sun- viewing photometer, and meteorological instruments positioned in a ground target area. Typically, the target is a dry lakebed or other relatively homogeneous area. (The value of a relatively homogeneous target is that it minimizes effects of errors of registration between the target and the fields of view of sensors.) The measurement data are processed by a radiative- transfer computer code to estimate spectral radiances at the position of a sensor known to be overhead at the time of the measurements. These radiances can be compared with the sensor readings to calibrate the sensor.
Previously, in order to perform vicarious calibration, it was necessary to dispatch field teams on expensive measurement campaigns to target sites, scheduled in accordance with sensor overpass times and weather conditions. Difficulty was compounded by remoteness and limited accessibility of typical targets. The present ground facility nearly eliminates the need for field measurement campaigns by acquiring data nearly continuously and making the data available to all interested parties via the World Wide Web.
The present ground facility occupies a target site consisting of the Frenchman Flat dry lakebed located north-northeast of Mercury, Nevada. The instrumentation at the facility includes a light-emitting-diode spectrometer (LSpec), which consists of eight tripod-mounted, ground-viewing radiometer units containing LEDs biased to operate as photodetectors (instead of light emitters) at their respective wavelengths. The LSpec provides an essentially continuous stream of measurements at eight discrete wavelengths. These are merged with spectral surface-reflectance measurements made on occasional site visits to obtain temporally continuous coverage with high spectral resolution. Other equipment at the site includes a weather station and a tracking sunphotometer (see figure).
Measurement data are acquired at intervals of 5 minutes under all daylight conditions. The data are entered into a database maintained on a Jet Propulsion Laboratory server computer. A remote user can log into a Web- based interface and request information specific to the overpass time of a given sensor. The data can be fed as input to the radiative-transfer computer program to obtain radiances for calibration of the sensor.
This work was done by Carol Bruegge and Shannon Jackson of Caltech and Mark Helmlinger of Northrop Grumman Space Technology for NASA’s Jet Propulsion Laboratory.