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Landsat-7 will carry the enhanced thematic mapper plus (ETM+) as its payload. This instrument is a derivative of the thematic mapper (TM) instruments flown on the Landsat 4 and 5 spacecraft. Key changes to the instrument include a new 15 meter panchromatic band, an increased spatial resolution 60 meter thermal band and two new solar calibrators to improve the radiometric calibration of the reflective bands. The ETM+ is currently going through a series of radiometric performance tests to evaluate spectral responsivity, noise performance, linearity, radiometric stability, and absolute radiometric calibration in ambient and vacuum. To date, spectral responsivity, dynamic range, noise performance and absolute calibration tests have been conducted in ambient conditions for the reflective channels. System spectral responsivity, based on component level measurements, is similar to previous TM instruments. One notable difference is in band 5, where the ETM+ response cuts off near the nominal value of 1.75 micrometer versus the 1.78 micrometer of Landsat 4 and 5 TM's, providing a bandpass freer of atmospheric absorption. The gain setting on the reflective channels provided within-specification values of dynamic range and variations of less than 2% between detectors in a band for bands 1 - 5, and less than 4% for bands 7 and 8. Generally within-specification noise performance is observed on the instrument, with signal-to-noise ratios in the range of 150 - 300 at the upper end of the dynamic range. The current notable exception is the panchromatic band which shows significant coherent noise. In orbit, the ETM+ has three on board devices available for performing radiometric calibration: the internal calibrator (IC), the partial aperture solar calibrator (PASC) and the full aperture solar calibrator (FASC). The IC, which is similar to the internal calibrator on the thematic mappers, consists of two lamps, a blackbody and a shutter flag which transmits the light from the lamps and blackbody to the focal plane. The IC is calibrated prior to launch by reference to an external integrating sphere, using the ETM+ as a transfer instrument. The PASC provides a reduced aperture image of the sun on a daily basis. The FASC is a diffuser panel periodically deployed in front of the ETM+ aperture. BRDF measurements on a flight panel have recently been completed. The panel's reflectance varies little across the first four ETM+ bands, averaging about 0.91.
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