NASA and USGS work jointly to maintain and refine as necessary, the radiometric and geometric calibrations of the Landsat 5 TM and Landsat 7 ETM+. As part of this effort, NASA funds several external teams that provide independent analyses and perform vicarious radiometric calibrations. Twice yearly, these investigators meet with USGS and NASA personnel to review the current calibrations of these instruments and compare the results from independent methods, including the vicarious calibration results.On November 29-30, 2006 this “TM+ calibration working group” met at NASA GSFC. Some key results/recommendations presented at the current meeting included:
- The Landsat-7 ETM+ geodetic error (RMSE) continues to be better than 20 meters in the cross track and along track directions consistent with the trend since the shut down of the noisy gyro in March 2005.
- The transition to bumper mode is anticipated for the summer of 2007 when the required synchronization time for SAM mode reaches 4 minutes.
- The Landsat 7 ETM+ reflective band radiometric calibration gains continue to be stable to an uncertainty of several tenths of a percent per year. No changes were recommended.
- The Landsat 7 ETM+ thermal band radiometric calibration continues to be accurate to within the ability to assess using vicarious calibration, circa ±1%, across the temperature range of about 0° to 20° C. Recent data acquisitions of warm targets have not verified previous indications of a calibration error at higher temperatures, so no change in the gain calibration is currently planned.
- The change in the Landsat 5 satellite to fixed solar array operations, included a pitch maneuver to achieve greater solar irradiance on the solar arrays during non-imaging operations, has increased the uncertainty in the spacecraft attitude knowledge. The processing logic for attitude has been modified to correct for artificial steps introduced into the attitude information.
- An update to the Landsat-5 TM reflective bands calibration was presented. This update is a result of analyses conducted using North African test sites with the data made available from the European Space Agency. The update represents the calibration trend as an exponential decay in bands 1-3, albeit with a longer time constant and different magnitude than the current model. Band 4, 5 and 7 are modeled as a flat line with bands 5 and 7 adjusted for the cold focal plane icing effects. The curves continue to be tied to the cross calibration with Landsat-7 ETM+ in June 1999. The update is planned to occur in early 2007; the post calibration scaling factors (LMAX’s) will likely be adjusted as part of the calibration update.
- An update to the bias of the Landsat-5 TM band 6 calibration was recommended at the last calibration meeting. Testing of 0.089 W/m2 sr mm update was performed and indicated some anomalies in one scene. This scene also had banding problems of unknown origin. The recommendation of the group was to test on more scenes and if the rest agreed to proceed with implementation.
Contributor: Brian Markham