Inflight wavelength correction of Thermal Infrared Multispectral Scanner (TIMS) data acquired from the ER-2

Abstract

In 1991 one flightline of Thermal Infrared Multispectral Scanner (TIMS) data was acquired over Castaic Lake, CA, and in 1992 four flightlines of TIMS data were acquired over Death Valley, CA, and one flightime of TIMS data over Lake Tahoe, CA. All datasets were obtained from an altitude of 20 km. To produce surface radiance the data were first calibrated, and then corrected for atmospheric effects using MODTRAN. The surface temperature was then extracted from the radiance data assuming a constant emissivity of 0.985 in all the TIMS channels. The surface temperatures for a spectrally flat area in the Castaic Lake data were then examined. Since the emissivity of the area was constant, the surface temperatures in the six TIMS channels should also have been similar. However, this was not the case, the values in channel 4 were much higher (/spl sim/3/spl deg/C) and the values in channel 3 much lower (/spl sim/2/spl deg/C) than those in the other channels. This difference can be explained by an inflight shift in the system response functions of the six TIMS channels. The amount of shift can be determined by incremental shifting of the preflight system response functions to longer wavelengths, then recalculating the surface temperature using the above method until the temperatures for channels 3 and 4 agree. A shift of 85 nm (the narrowest TIMS channel is /spl sim/500 nm wide) was required to make the brightness temperatures of channels 3 and 4 agree to within +/-1/spl deg/C within the Castaic Lake data. The same procedure was undertaken with the TIMS data from the four flightlines acquired at Death Valley that overlapped a spectrally flat target and over Lake Tahoe. The surface temperatures for all the channels were found to be in good agreement after shifts of 79, 88, 91, and 100 nm, respectively, for the four Death Valley flightlines and 98 nm for the Lake Tahoe flightline. The amount of shift increased as a function of the time that TIMS was airborne. The Castaic Lake data were acquired after TEMS had been airborne for a similar amount of time to the Death Valley flightline which showed a shift of 91 nm. The results from this study clearly demonstrate that the system response functions of the six TIMS channels shift to longer wavelengths compared to their preflight values with data acquired from the ER-2. A method is provided for determining the amount of shift permitting its correction. The method could also be used to verify the inflight wavelength calibration of multispectral thermal infrared data acquired from other airborne or spaceborne scanners with a similar optical configuration to TIMS.