The USGS elected to continue science data acquisition from the Enhanced Thematic Mapper Plus (ETM+) to assess the utility of imagery collected at the new lower orbit and earlier equatorial crossing time. Dr. Chris Crawford, USGS Landsat Project Scientist shared that “extending Landsat 7's science mission during 2022 provided an opportunity for the USGS to operate three Landsat satellites together and explore the value of Landsat 7 lower orbit imagery while underflying Landsat 8 and Landsat 9." 

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At its new orbit, Landsat 7 is no longer on a repeating ground track, meaning that acquisitions are not aligned and are continually shifting in relation to the World Reference System-2 (WRS-2) paths and rows used by Landsat 8 and Landsat 9. Due to this variability, pending acquisition files are the best source for knowing what will be acquired. These files are available up to three days in advance from the Landsat Acquisition Tool’s Pending Acquisitions tab. The ETM+ is currently acquiring about 450 images per day, and the current mean local crossing time at the equator is now earlier than 8:45 AM. This graphic displays the expected changes to the mean local crossing time over the next three years.   

The current USGS Landsat 7 extended operations plan is to continue acquiring science data through the end of September 2022, when the mean local crossing time will be around 8:35 AM. 

Data acquired during the extended science mission are processed to Landsat Collection 2 Level-1 calibrated data products, and then to Level-2 surface reflectance and surface temperature science products. No U.S. Landsat Analysis Ready Data or Level-3 science products will be generated at this time. Like other Landsat products, they are available for download on USGS data access portals. 

Note: The Product Identifier for each scene contains the acquisition date, so Landsat 7 scenes collected at the lower orbit can be identified by searching for Landsat 7 data acquired after May 4, 2022.   

Maintaining Data Quality and Calibration/Validation  

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Since May, the USGS/NASA Calibration and Validation Team has analyzed and quantified potential impacts of acquiring image data in a lower orbit. The response of the satellite’s ETM+ instrument to the internal calibrator has not revealed any significant changes to the reflective bands thus far. There may be a change in the thermal infrared spectral band radiometric response to the on-board blackbody of less than 1 percent, but additional data and analysis are required to confirm. At this time, no change has been made to the radiometric calibration. 

The lowering of Landsat 7 did not have a significant impact on the geometric quality of the data itself.  However, since Landsat 7 is no longer on the WRS-2 orbit, ETM+’s ground track and field of view for a given acquisition are not completely covered by the ground reference sites used for calibration.  On these occasions, it could impact the ability to perform scan mirror calibration with the same type of accuracy achieved for the nominal WRS-2 orbit. Early calibration and analysis show that if this occurs, degradation is expected to be small and confined to the sub-pixel scale.    

The USGS plans to maintain ETM+’s radiometric and geometric calibration while operating and acquiring Earth data during Landsat 7’s extended science mission. 

Please contact USGS Customer Services with any questions about this announcement. 

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