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Landsat-7 ETM+ Radiometric Stability and Absolute Calibration
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Launched in April 1999， the Landsat-7 ETM+ instrument is in its fourth year of operation。 The quality of the acquired calibrated imagery continues to be high， especially with respect to its three most important radiometric performance parameters: reflective band instrument stability to better than +-1%， reflective band absolute calibration to better than +-5%， and thermal band absolute calibration to better than +- 0。6 K。 The ETM+ instrument has been the most stable of any of the Landsat instruments， in both the reflective and thermal channels。 To date， the best on-board calibration source for the reflective bands has been the Full Aperture Solar Calibrator， which has indicated changes of at most -1。8% to -2。0% (95% C。I。) change per year in the ETM+ gain (band 4)。 However， this change is believed to be caused by changes in the solar diffuser panel， as opposed to a change in the instrument's gain。 This belief is based partially on ground observations， which bound the changes in gain in band 4 at -0。7% to +1。5%。 Also， ETM+ stability is indicated by the monitoring of desert targets。 These image-based results for four Saharan and Arabian sites， for a collection of 35 scenes over the three years since launch， bound the gain change at -0。7% to +0。5% in band 4。 Thermal calibration from ground observations revealed an offset error of +0。31 W/m~2 sr urn soon after launch。 This offset was corrected within the U。 S。 ground processing system at EROS Data Center on 21-Dec-00， and since then， the band 6 on-board calibration has indicated changes of at most +0。02% to +0。04% (95% C。I。) per year。 The latest ground observations have detected no remaining offset error with an RMS error of +- 0。6 K。 The stability and absolute calibration of the Landsat-7 ETM+ sensor make it an ideal candidate to be used as a reference source for radiometric cross-calibrating to other land remote sensing satellite systems。
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