Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+)

Calibration Notices

January 28, 2010 – Calibration update to ETM+ thermal band

Effective January 1, 2010, the calibration of the ETM+ thermal band was modified to correct for a lifetime gain error detected by the vicarious calibration teams. Changes were made to the Calibration Parameter File (CPF) to correct a gain error that has been present since the launch of the instrument. This gain error causes the thermal band to predict too hot at cold temperatures and too cold at hot temperatures. For instance, a target with an at-satellite temperature of 0C will appear to be 0.8C based on the image data and a target with an at-satellite temperature of 27C will appear to be 26.3C. The error at 12C is approximately 0C. The vicarious calibration data were used to generate a correction to the calibration and the corrected coefficients were implemented in the processing system on Jan 1, 2010. All data processed by the USGS/EROS processing system after Jan 1, 2010 are correctly calibrated.

The vicarious calibration teams, the Rochester Institute of Technology and the NASA/Jet Propulsion Laboratory, have been collecting data under the satellite since the instrument launched in 1999. Early in the mission, they detected an offset error of 0.31 W/m2 sr um [Barsi, 2003]. This was corrected in the processing system in October 2000. Since then, the teams have continued to collect field data. The addition of a site on the Salton Sea, which can get as hot as 35C in the summer, expanded the temperature range over which the teams acquired ground truth. With these hot points, a gain error of 5.8% became statistically significant. The effect of this gain error is that the instrument was estimating that hot targets were too cold and cold targets were too hot. See the table below for estimates of the error on specific target temperatures.

The error was corrected by adjusting the internal-to-external gain ratio in the Calibration Parameter File (CPF). Other parameters needed to be adjusted for non-default processing conditions, but for most users, the internal-to-external gain ratio is the only parameter that will affect the product. Users should note that with the current distribution system, the downloadable product may not have been processed with the latest CPF. Please check the "Product Creation Time" in the metadata file (MTL) that comes with the downloaded product to ensure that the product is processed after January 1, 2010.

This update will be modified when the correction is published in an official paper.

Table 1. Error estimates for at-satellite radiances. The error varies with target temperature with the satellite estimating too hot for cold targets and too cold for hot targets.

Target temperature	Approximate Radiance [W/m2 sr um]	Estimated Calibration Error [K]
273K / 0C	6.0	0.8 too hot
285K / 12C	7.5	0
300K / 27C	9.0	0.7 too cold

J.A. Barsi, J.R. Schott, F.D. Palluconi, D.L. Helder, S.J. Hook, B.L. Markham, G. Chander, E.M. O'Donnell, "Landsat TM and ETM+ thermal band calibration," Canadian Journal of Remote Sensing, 29(2), 141-153 (2003)

April, 2007 - Updated ETM+ CPF format for Bumper Mode

As of April 1, 2007, the Landsat 7 ETM+ Calibration Parameter File (CPF) format was updated to include several new parameters and groups, in support of the L7 ETM+ change to bumper mode operations.

For a full description of the contents and format of the L7 ETM+ CPF file, see the CPF Definition Document - .pdf (1.00 MB).

January 2, 2003 – Calibration update to thermal band 6

Previously, 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) )

where:

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).

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