MISR: In-Flight Radiometric Calibration and Characterization Plan

• MODIS has bands similar to the four MISR bands, which will facilitate the crosscomparison;• a measure of the bias between radiance scales will be made preflight. This latter workis facilitated by the round-robin experiments where a transfer radiometer compares theMISR and MODIS laboratory standards.Difficulties in comparing MODIS and MISR radiances occur primarily due to band shapedifferences, and the nature of the radiance product 17 . MISR will report in-band weightedradiances, whereas MODIS will report the total detected radiance. This necessitates the selectionof reference sites that are as spectrally neutral as possible, in addition to being of high spatialuniformity over 3x3 pixel areas. Prior to the comparison, MISR will condition MODIS radiancesto estimate the radiances MISR would report, per our formulism.Although of lower accuracy, the cross-calibration of sensors on different platforms is ofimportance in at least two cases: 1) to cross-calibrate two models of the same sensor (e.g. MISR-AM1 to MISR-AM2), 2) to cross-calibrate sensors that are providing long-term global-changedata sets. MISR will support these studies, as the need arises.8.1.2 Desert scenesStable desertic sites have been used for the calibration verification of optical satellitesensors. By assuming a surface and atmospheric climatology, radiances can be computed withoutthe overhead of intensive field campaigns. The SPOT team has investigated 20 desertic zonesselected in Saharan North Africa and Saudi Arabia 46 , selected using a criterion of spatialuniformity in a multitemporal series of cloud-free METEOSAT-4 visible images. For these scenesthey have verified the temporal stability of these sites at hourly and seasonal time scales. We planto make use of these desert sites in the verification of MISR radiometric calibration.8.1.3 Lunar observationThe Moon cannot be viewed by the MISR sensors unless there is dedicated lunar/ deepspace Calibration Attitude Maneuver (CAM). The MISR instrument was designed to meet itsperformance specifications without the reliance on any CAMs. Since the Moon is not an extendedtarget sufficient to fill the MISR field-of-view, and because lunar observations are potentially indifferent instrument configurations than during Earth observations (i.e., instrument temperaturesmay be different because the instrument radiators usually face the Earth), we do not consider thatlunar views will provide any enhancement to the absolute radiometric calibration of MISR. Inaddition, the Moon provides a single radiance level at the lower end of the MISR dynamic range;a full calibration of MISR requires measurement at multiple radiance levels. On the other hand, alunar CAM can provide an independent stability verification, thus enhancing our confidence thatthe strategy we have adopted is working, and the values we quote for radiometric uncertainty arevalid.8. Calibration integrity62