In September 2000, two Landsat 7 science team investigator groups reported a bias of 0.31 w/(m^2 sr um) in the calibration of Band 6 of the Enhanced Thematic Mapper Plus (ETM+). To compensate for this bias, the following occurred with the October 1, 2000 release of the CPFs:
- The Band 6 Thermal calibration processing in the Landsat Product Generation System (LPGS) and Image Assessment System (IAS) was revised
- The Calibration Parameter File (CPF) values were revised
- The calibration equation used by IAS/LPGS was modified
- The instrument component view coefficients were corrected
- The Band 6 biases were corrected
Users need to be aware of which changes were made and how these changes might affect the distributed data.
The software change that incorporated the Band 6 bias calibration fix was installed in the IAS system October 30, 2000, and in the LPGS system December 20, 2000. Because the National Landsat Archive Production System (NLAPS) relies on the CPF for Band 6 bias calculation, NLAPS was corrected with the October 1, 2000 release of the CPFs. Data processed after these dates are calibrated correctly. For data processed before these dates, subtract 0.31 W/(m^2 sr um) from the L1 image radiance to get calibrated radiance.
Note that the distinction between calibrated and non-calibrated data is based on processing date. This has no relation to the acquisition date (the date that the ETM+ acquired the image). Data acquired before 01-Oct-00 but processed after the calibration correction are calibrated correctly.
For users of other processing system Band 6 products, the effect depends on the calibration equation used for processing. (1) If the CPF gains and CPF biases are used for normal operational processing, this calibration correction occurs immediately on all images ordered. (2) If the original calibration equation used the shutter view factor (Vsh) (and IC biases are used), the new Vsh values have an effect, although the specific implementation of Vsh varies and has differing effects. Because only the IAS and LPGS were developed under the Landsat Project Science Office, other processing software is not under configuration and verification control. In addition, when changes are made to LPGS software, other processing systems are under no obligation to make similar changes to their software, so other systems may never implement new calibration methods.
Summary of Changes
Update to the CPF (view coefficients and biases):
A number of coefficients were determined during the prelaunch calibration of ETM+, including the view factors of the individual instrument components and the ratio between the internal and external gain. Based on the error in calibration, the prelaunch data are now considered suspect. Therefore, it was decided to empirically re-calculate one of the coefficients to correct the bias. Also during this time, the groups observed that the Vsh was not used correctly in the calibration software algorithms. This algorithm was corrected, and Vsh is the coefficient that was empirically adjusted to yield a 0.31 w/(m^2 sr um) lower bias.
Biases: Accompanying this change in Vsh was a change in the CPF biases. This is the simplest way to adjust for the bias error, but the LPGS processing system defaults to using the IC as the bias source. The LPGS only uses these biases for special requests. However, other processing systems may use them, so updating them was necessary. The biases were adjusted by determining the DN equivalent of the 0.31 w/(m^2 sr um) radiance change from the current gain, then adjusting the bias by that amount.
Update of the Calibration Equation
The processing software originally used a calibration equation that relied heavily on information collected through prelaunch tests. Because the prelaunch calibration results are believed to be the cause of this bias error, the calibration equation was simplified to its most basic form.
Lsc = (Qsc - Qsh) / RGi + (Vsh/R)Lsh - sum(ajLj)
In the ideal situation, new on-orbit data will be collected with the instrument sitting at a range of temperatures so the ajs, Vsh, and R can be re-derived. Until that opportunity becomes available, the solution is to re-solve for Vsh based on the ground look calibration that the science team members already perform; keeping the result of the equation the same as the current equation:
Vsh(CPF) = R ( Vsh(emp) + sum(aj) )
Vsh(CPF) is the new Vsh that will be put in the CPF.
Vsh(emp) is the Vsh solved for when the ground truth data is used
This new equation was implemented in IAS 4.3 (Oct 30, 2000), LPGS 4.1 (December 20, 2000), and NLAPS (October 1, 2000). Band 6 data processed on or after those dates are correct and need no adjustment. For data processed before those dates, adjust the calibrated radiances by subtracting 0.31 W/(m^2 sr um).