CHAPTER 13 SECTIONS > Geometric Performance | Radiometric Performance

13.2 Radiometric Performance

A significant improvement in the Landsat-7 system is the addition of the IAS Image as part of the ground processing system. The IAS has the role of monitoring the performance and calibration of the ETM+ instrument and providing updates to the calibration parameter file (CPF). The NASA/GSFC Landsat Project Science Office (LPSO) works with the IAS (located at EDC) in analyzing the calibration information and updating the algorithms used within the IAS. Additional funding from NASA supports vicarious radiometric calibration efforts at NASA/JPL, Rochester Institute of Technology, South Dakota State University and the University of Arizona.

Approximately every 6 months the scientists and analysts involved in characterizing ETM+ radiometric calibration meet and present their results. The results form the basis for updating the radiometric gain calibration parameters in the calibration parameter file. The most recent results are presented below.

The three on-board ETM+ calibration devices are the Full Aperture Solar Calibrator (FASC) which is a white painted diffuser panel, a Partial Aperture Solar Calibrator (PASC) which is a set of optics that allow the ETM+ to image the sun through small holes and an Internal Calibrator (IC), which consists of two lamps, a black body, a shutter and optics to transfer the energy from the sources to the focal plane. Details on the devices can be found in Chapter 8.

The FASC is deployed in front of the ETM+ aperture approximately monthly. Based on the orientation of the panel relative to the sun and instrument and the pre-launch measured reflectance of the panel, a calibration can be determined. The IC provides a signal to the ETM+ detectors once each scan line as well as a view of the black shutter. The shutter provides the dark reference for the reflective bands and a low temperature source for the thermal band. The lamps and black body provide the high radiance source for the bands. At the short wavelengths, the IC has shown both short term and long-term instabilities. Results will not be discussed for wavelengths less than 0.7 µm. Performance of the PASC has been anomalous and results are notincluded here. On-orbit performance of the calibrators can be found in a paper that covers the subject in great detail (Markham, B.L., et al.).

There are four investigations evaluating the ETM+ radiometric calibration using GLC or vicarious methods. Each of these investigations predicts the radiance at the sensor aperture using a combination of ground- and/or aircraft-based reflectance, radiance or temperature measurements, coupled with measured and/or modeled atmospheric parameters. Two investigations are looking primarily at the reflective band calibrations: those of Helder and Thome (Thome, K.J., et al.). The investigations of Palluconi and Schott are concentrating on the thermal band (Schott, J.R., et al.).

Figure 13.17 - ETM+ Band 2 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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Figure 13.16 - ETM+ Band 1 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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Figure 13.19 - ETM+ Band 4 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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Figure 13.18 - ETM+ Band 3 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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Figure 13.21 - ETM+ Band 7 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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Figure 13.20 - ETM+ Band 5 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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Figure 13.22 - ETM+ Band 8 Band Average High Gain
Calibration Results from On-board and Ground-Look
compared to Pre-launch values.
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The combined calibration results for bands 1 -5, 7 and 8 are presented in Figures 13.16, 13.17, 13.18, 13.19, 13.20, 13.21, 13.22, respectively. In each case the pre-launch calibration currently in the calibration parameter file is presented along with error bars representing <5%. The FASC results presented are based on the "best" portion of the FASC panel and have been adjusted based on an apparent 1° difference in the orientation of the panel from pre-launch measurements (Markhan, B. L., et al). The IC results have also been included, recognizing that part of the variability present is related to the IC itself. Note that in all bands, the vicarious results agree to within 5% of the FASC and pre-launch values and that the trends in the GLC results are not significant. The FASC results do show significant trends, but the trends are small (less that 1.5%/year). The FASC trends are believed to be largely due to changes in the FASCs reflectance and not representative of the instrument. Note, however, that there is some consistency between the FASC, GLC and IC trends, e.g., in band 7 all are increasing. If the consistency continues and the vicarious trends become significant, a calibration update will be performed.

Figure 13.23 - ETM+ Band 6 Band Average Low Gain
Mode Responsivity as Determined by Use of the
Internal Calibrator.
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In band 6 the IC is the only on-board calibration device. The response to the IC over the life of the mission is shown in Figure 13.23. The slope of the responsivity, though significant, shows a change of less than 0.06%/year. This system is remarkably stable, particularly relative to the Landsat 4 and 5 TM thermal bands. The ETM+ instrument also appears stable relative to the vicarious measurements, though a significant bias was detected (Figure 13.24). This bias was originally measured as 0.31 W/m2 sr mm and this correction was applied to the calibration parameter file on October 1, 2000. Updated measurements indicate the bias was closer to 0.29. The 0.31 correction was implemented by altering the shutter view coefficients in the calibration parameter file and changing the calibration equation. Although the coefficients were changed October 1, 2000, there was no effect on the U.S. Landsat Product Generation System (LPGS) product until December 20, 2000 when the software was revised. Depending on how data are calibrated in non-US processing systems, the calibration change may have been effective October 1, 2000 or may have been later. After correction for the bias, the calibrated product radiance has a scatter of about 1% around the vicarious results (Figure 13.25).

Figure 13.25 - ETM+ Band 6 In-situ Derived Top of
Atmosphere (TOA) Radiances Versus Image TOA after
bias correction for July 1999 to Present.
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Figure 13.24 - ETM+ Band 6 In-situ Derived Top of
Atmosphere (TOA) Radiances Versus Image TOA
before bias correction for July 1999 to Present.
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