Landsat-7 ETM+ on-orbit reflective-band radiometric stability and absolute calibration

TLDR

Landsat‑7 ETM+ has been operating since 1999, and its reflective‑band calibration has relied on the Full Aperture Solar Calibrator, which indicates annual gain changes of 1–2 % for bands 1–4 and 8 and less than 0.5 % for bands 5 and 7. This attribution to diffuser degradation is supported by vicarious calibrations and observations of invariant hyperarid sites in the Sahara and Arabia. The instrument remains the most stable of all Landsat sensors, with weighted slopes of 0–0.4 %/yr for bands 1–4 and 8 and 0.4–0.5 %/yr for bands 5 and 7, and absolute calibration agreement within about 5 % despite some systematic differences.

Abstract

Launched in April 1999, the Landsat-7 Enhanced Thematic Mapper Plus (ETM+) instrument is in its sixth year of operation. The ETM+ instrument has been the most stable of any of the Landsat instruments. To date, the best onboard calibration source for the reflective bands has been the Full Aperture Solar Calibrator, a solar-diffuser-based system, which has indicated changes of between 1% to 2% per year in the ETM+ gain for bands 1-4 and 8 and less than 0.5%/year for bands 5 and 7. However, most of 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 vicarious calibrations and observations of "invariant sites", hyperarid sites of the Sahara and Arabia. Weighted average slopes determined from these datasets suggest changes of 0.0% to 0.4% per year for bands 1-4 and 8 and 0.4% to 0.5% per year for bands 5 and 7. Absolute calibration of the reflective bands of the ETM+ is consistent with vicarious observations and other sensors generally at the 5% level, though there appear to be some systematic differences.