Pre-launch performance of the Landsat-7 enhanced Thematic Mapper plus

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. Rey changes to the instrument include a new 15 meter panchromatic band, a higher spatial resolution thermal band and two new solar calibrators to improve the radiometric calibration of the reflective bands. The ETM+ went through its baseline ambient radiometric and geometric calibration tests during the summer of 1997 and began environmental testing in the fall of 1997. The ETM+ suffered a power supply failure during the initial thermal vacuum testing during late 1997 to early 1998. Since the power supply repair was completed in May 1998, the ETM+ has been going through a series of radiometric performance tests to recharacterize noise, linearity, radiometric stability, aml absolute radiometric calibration in ambient and is currently preparing to enter thermal vacuum testing. Spectral and radiometric test results from the current and previous tests are presented and compared. 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 neat the nominal value of 1.75 mu m versus the 1.78 mu m of Landsat 4 and 5 TM's, providing a bandpass freer of atmospheric absorption. The pre-power supply repair noise results showed inspecification performance for all bands except the panchromatic band, which showed 104 kHz noise. The overall noise level in the panchromatic band was up to 11 counts (standard deviation). After the power supply repair, which involved replacement of 108 diodes in each power supply, the overall noise in the panchromatic band was between 2 and 3 counts and was largely random. During the ETM+ testing a radiometer monitors the output of the integrating sphere used to calibrate the instrument. Use of this Landsat Transfer Radiometer (LXR) has demonstrated ETM+ linearity and provided a precision of the radiometric calibration of the ETM+ to better than the 0.1% level. The ETM+ internal calibrator (IC) has shown an instability over time at up to the 15% level. The reason for the instability is currently being investigated.