Landsat-5 Thematic Mapper (TM) has been imaging the Earth since March 1984 and Landsat-7 Enhanced Thematic
Mapper Plus (ETM+) was added to the series of Landsat instruments in April 1999. The stability and calibration of the
ETM+ has been monitored extensively since launch. Though not monitored for many years, TM now has a similar
system in place to monitor stability and calibration. University teams have been evaluating the on-board calibration of
the instruments through ground-based measurements since 1999. This paper considers the calibration efforts for the
thermal band, Band 6, of both the Landsat-5 and Landsat-7 instruments.
Initial calibration results for the Landsat-7 ETM+ thermal band found a bias error which was corrected through changes
in the processing systems in late 2000. Recent results are suggesting a calibration error in gain, apparent with high
temperature targets. For typical earth temperature targets, from about 5-20C, the gain error is small enough to be within
the noise of the vicarious calibration process. However, for very high temperature targets (greater then 35C), Landsat-7
appears to be predicting several degrees too low. Questions remain on whether the change happened suddenly or is
varying slowly, so the team will wait for another collection season before making any updates to the calibration.
The calibration efforts for Landsat-5 TM considers only data collected since 1999, though there are efforts underway to
extend the calibration history prior to the Landsat-7 launch. The latest data suggests that the Landsat-5 thermal band has
a bias error of about 0.65K too low since 1999. Studies early in the life of Landsat-5 show that the instrument was
calibrated within the error of the calibration process. It is impossible to tell, at this point, when or how the change in
bias may have occurred. A correction will be calculated and implemented in the US processing system in 2006 for data
acquired since April 1999.