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The geometric performance of the ETM+ is judged against three key requirements placed on the Landsat 7 system. They are:
Geodetic accuracy is monitored using calibration scenes containing ground control points. Scenes are first radiometrically and geometrically corrected. Control point locations are then measured on the processed imagery and compared to known ground locations. Any terrain effects are removed analytically in the comparison. The product's geodetic accuracy depends on the accuracy of four data inputs. These are:
The ephemeris is estimated post-pass using tracking data. Attitude is measured by on-board star trackers and gyros. The clock performance is monitored by the Landsat 7 mission operations center and the ETM+ alignment is determined by ground processing calibration. The ETM+ alignment is determined by measuring the orientation of the ETM+ payload relative to the L7 spacecraft attitude control system reference. Multiple scenes with ground control are used to measure the systematic biases attributable to ETM+ alignment. During the initial on-orbit calibration during first ninety days, seven different calibration scenes were used. Periodic geodetic accuracy testing showed a slow build-up of along-track bias after July, 1999. A sensor alignment calibration update was performed in June, of 2000. Twenty-four scenes acquired since July, 1999 (~1 per cycle) were used to perform the calibration. A separate independent set of eighteen scenes covering same time span were used to verify the results. The ETM+ alignment shows time-varying behavior and will continue to be monitored during the course of the mission. Current trending reveals post-calibration geodetic accuracy of systematic ETM+ products is approximately 80 meters (1 sigma) which is much better than 250 meter specification placed on the system. 13.1.3 Band-to-Band Registration Band-to-band registration assessment is performed periodically throughout the mission's life. The purpose of this assessment is to measure the relative alignment of the eight ETM+ spectral bands after processing to Level 1Gs for verification that the 0.17 pixel band-to-band registration requirement is being met. If not, the IAS remedies the band alignment by deriving new band center locations via band-to-band registration calibration and updating the CPF. Band registration is monitored using desert scenes as they provide the best cross-band correlation performance. The band center locations measured prelaunch were evaluated using on-orbit data were updated using calibration scenes during the in-orbit checkout period (first 90 days). The measured band registration accuracy was 0.06-0.08 pixels. However, the registration accuracy degraded after July, 1999. Measurements revealed that registration between the primary and cold focal planes in the line direction increased to 0.08-0.10 pixels Band calibration analysis showed a systematic shift in the band 5, 6, 7 locations after July, 1999. The primary focal plane band centers are very stable but the cold focal plane band centers more variable with a 3-4 microradian mean shift. The cold focal plane offsets coincide with change in ETM+ operating temperature range which is hotter than during the 90-day checkout. The band center calibration was updated for data acquired after July, 1999 although analysis revealed that band center estimates from individual scenes are still highly correlated with temperature telemetry. The current calibration is time-dependent pending development of a temperature dependent model. Nonetheless, registration performance was well within 0.17 pixel specification. 13.1.4 Multi-Temporal Image Registration The multi-temporal registration accuracy was determined using cloud-free scenes of the geometric calibration �super-sites�. The term "geometric super-site" is used to describe those pre-selected WRS path-row locations for which ground control, digital terrain data, and reference imagery have been collected. This supporting data set makes it possible to produce precision and terrain corrected ETM+ images, and to compare them to accurately geo-registered reference images. The required ground control, terrain models, and reference images were derived from digital orthophoto (DOQ) data. The one meter resolution DOQs were mosaicked and reduced to 15 meter resolution. Five cloud-free images of two separate calibration sites were used to measure registration accuracy. The image registration assessment was performed in two ways. First, the ETM+ images were compared against the DOQ reference images. This provides a measure of image distortion relative to an absolute reference. Second, two ETM+ scenes were cross correlated. This provides a measure ofimage distortion that changes scene-to-scene although systematic calibration distortions may cancel out. Coupled with the image registration analysis is the need to measure the ETM+ scan mirror performance to ensure the pre-launch profile is correct. The geometric calibration super-site scenes were also used for this purpose. A DOQ reference image was constructed to provide full-width coverage of a Landsat 7 scene so that measurements at all scan angles could be obtained. Mirror deviations as a function of scan angle were obtained by These were cross-correlating the ETM+ scene to the reference image. No apparent problem was observed with the along scan mirror profile. A minor adjustment to the cross scan profiles was made to model slightly non-linear scan line corrector behavior. Also, an adjustment to the prelaunch scan angle monitor start/stop angles was made to improve image-to-image registration accuracy. The scan mirror calibration update was made to the CPF in the fourth quarter of 1999. Image registration accuracy was measured before and after the scan mirror calibration. Results revealed that the required image registration accuracy was achieved using the baseline prelaunch scan mirror calibration parameters. Specifically,
The image registration also improved using the updated scan mirror calibration parameters. Analysis yielded the following results.
13.1.5 ETM+ Scan Mirror Stability Regular monitoring has revealed the ETM+ scan period is increasing with time due to growth in turnaround time, probably caused by bumper wear. However, the active scan time is showing increased variability, especially in the forward scans. The rate of growth of turnaround time, however, appears to be stable. The impact to ETM+ data is that scan gaps will gradually increase with time. Also, the scan mirror could, theoretically, lose synchronization with the calibration flag if scan time gets too long. This would effectively end the Landsat 7 mission. However, this mirror behavior is similar to that observed in Landsat 5 TM. The gimballed X-band antenna (GXA), when maneuvered in an across-track slew, sometimes disturbs the ETM+ scan mirror. This occurs when GXA the stepper motor frequencies correspond to scan mirror harmonics. The impact to ETM+ data is a wider than normal variation in scan line length. Most extreme examples exceed the maximum allowable scan length leading which leads to dropped scans. This phenomenon was not observed on earlier missions as pointable X-band antennae are new on Landsat 7. The scan mirror may lose synchronization or, in extreme cases, restart during imaging. Such an occurrence is correlated with regions of high electron flux at 705 km orbital altitude particularly in polar regions and within the South Atlantic anomaly. The correlation was confirmed by a July, 2000 solar flare which resulted in 14 anomalies in a single day. The impact to ETM+ Data is dropped scans and calibration flag incursions into the Earth imaging area. The scan mirror controller sees an �extra� timing pulse and thus loses synchronization with calibration flag. This phenomenon may have occurred on Landsat 5. 13.1.6 Scan Line Corrector (SLC) Anomaly On May 31, 2003 the scan line corrector (SLC), which compensates for the forward motion of the satellite, failed at approximately 21:45. Subsequent efforts to recover the SLC have not been successful, and the problem appears to be permanent. Landsat 7 Enhanced is still capable of acquiring useful image data with the SLC turned off, particularly within the central portion of any given scene. Various interpolation and compositing schemes are currently be investigated to expand the coverage of useful data. An interpolation example can be seen in Figure 13.13. Landsat 7 ETM+ will therefore continue to acquire image data in the "SLC-off" mode and has so since July 14, 2003. More information on the SLC-off anomaly can be found in Chapter 11.
Figure 13.13. Top image: Pre-SLC anomaly scene, middle of image. 13.1.7 Modulation Transfer Function (MTF) Characterization The MTF of the ETM+ is regularly measured for comparison to the pre-launch test results and for monitoring long-term instrument performance. The MTF characterization methodology involves the analysis of cloud free scenes over the Lake Ponchartrain bridge in Louisiana. The bridge is long, straight, double-spanned (two 10m spans, separated by 24.4m) is approximately aligned with the Landsat ground track (<1°), and offers high signal contrast to the waters below. The MTF characterization is performed on level 1R data. The 1R data is in 16-bit scaled radiance form with image artifacts removed. The data undergoes no geometric resampling. A 256x2048 image window covering the bridge is manually identifed and extracted. The image line numbers containing bridge crossovers are identified and removed from the extracted image. The corresponding windows for the 30-meter bands are then extracted. A bridge profile is then built by extracting the bridge segments from each data line. Each segment consists of 16 pixels centered about a 3-point moving average. The forward and reverse scans are separated and classified according to bridge segment phase. Phase is determined by correlating each bridge segment to bridge templates templates shifted at 0.125 pixel increments from �1 to +1 pixels (17 total). Each segment is assigned to a phase �bin� based on the offset of the best matching template. The range of 8 consecutive bins with the most segments is then identified. The segments within this bin range are then averaged resulting in 8 mean bin segments containing 16 samples each. The samples from the mean bins are interleaved and the end result is a 128 pixel oversampled bridge profile. An analytical bridge model was constructed in the frequency domain using the bridge span width, gap between spans and the intensities of the two spans and background water as model parameters. An optical transfer function (OTF) that borrows from previous Landsat 5 TM work was developed for ETM+. The OTF is a mathamatical statement that describes the relationship between the input and output of an imaging system. It provides a complete measure of system performance in that it includes the phase relationship as well as the amplitude degradation of the bridge as the frequency changes. The OTF is then used to compare the bridge models with the actual data. The forward and reverse scan bridge models are multiplied by the OTF model in the frequency domain. An inverse FFT is applied to convert the models to the space domain. The root sum square (RSS) differences between the models and the data for both forward and reverse scans are then computed. To minimize the RSS difference the bridge intensities were adjusted while keeping the span width and separation fixed. The RSS differences were also minimized by adjusting the variable OTF parameters while keeping the detector model fixed. Numerous iterations yielded a final OTF model.
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. 13.2.1 On-orbit Calibration Methods 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 8.1. 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.). 13.2.2 Ground Look Calibration (GLC) Methods 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.). 13.2.3 Radiometric Performance Results The combined calibration results for bands 1 -5, 7 and 8 are presented in Figures 13.17, 13.18, 13.19, 13.20, 13.21, 13.22, 13.23, 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 FASC�s 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. 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 13.24. 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 13.25). 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 13.26). On-orbit results indicate that the Landsat-7 ETM+ absolute radiometric calibration is stable to better than 1.5%/year in the reflective bands and 0.1%/year in the thermal band. The uncertainty in the calibration is estimated at <5% in the reflective and ~1% in the thermal regions. |
