The newly launched Landsat 9 is experiencing a once-in-a-mission lifetime event where its orbit is directly below Landsat 8 for an “underfly.” For several days in the middle of November, the pair of Earth-observing satellites are flying together, collecting data at the same time for the same sites and requiring simultaneous downlinking to global ground station antennas.
Underfly Positions Landsat 9 Below Landsat 8 for Simultaneous Scenes
The underfly isn’t just an interesting circumstance for Landsat 9. It is the best opportunity to cross-calibrate Landsat 9 data to Landsat 8, which is the currently accepted reference for calibration. This will help maintain the consistent gold standard of Landsat’s nearly 50-year archive of data as Landsat 9 moves into position to take the place of Landsat 7.
Cross-calibration for pairs of Landsat satellites typically occurs with each satellite observing, eight days apart, sites that experience very little change over decades, such as deserts, and then comparing the differences between the data.
“But none of that is coincident imagery. They’re never taking the images at the same time. This underfly gives us the opportunity to do the same type of analysis, over a large range of targets, when we get imagery at the exact same time,” said Cody Anderson, project manager of the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Cal/Val Center of Excellence (ECCOE), focused on calibration and validation of remote sensing data, including Landsat 9.
The newest Landsat was built by the National Aeronautics and Space Administration (NASA) and launched September 27, 2021. After a 100-day commissioning phase, which includes this and other calibration work, NASA will turn Landsat 9 over to USGS for operation. EROS maintains the archive of current and historical Landsat data.
Field Crews Collect Data Around the World
Since Landsat 8 and Landsat 9 have nearly identical sensors, “they should be giving the same answers over all the different targets,” Anderson said. The first underfly for the Landsat program was performed after the launch of Landsat 8 in 2013, but Landsat 7 and Landsat 8 have spectral band differences that don’t provide the exact same results. That limited the locations for comparison to those typically used for cross-calibration, like designated desert sites.
This underfly, in comparison, could take scenes from many other locations. The satellites will capture data simultaneously as field crews around the world collect data on the ground to cross-check the satellite measurements. These efforts involve partners with Geoscience Australia in multiple locations in Australia; the Indian Space Research Organization in several locations in India; the University of Lethbridge in Alberta, Canada; the Rochester Institute of Technology in New York; the University of Arizona at Ivanpah Playa in California; South Dakota State University in eastern South Dakota and western Minnesota; and a team from EROS throughout the southwestern U.S.
The ocean is another area of interest, Anderson said—both open ocean and right off the coast. “Traditionally, we’ve looked at water for our thermal calibration because water is thermally homogenous over a large area, unlike land,” he explained. It’s easy for the sensors to accurately measure surface temperature over large areas there, and the result should be the same for both satellites. The National Oceanic and Atmospheric Administration has a network of buoys off the coast with sensors that record surface temperature for direct comparison.
Measurements of land surface temperature can be difficult since they’re more variable, but that has become a significant part of the field collection work this time, Anderson said.
Heading to the Right Location and Information
The underfly will last for about five days, with Landsat 9 roughly 10 kilometers below, gradually moving directly under Landsat 8 on November 14 and then moving on, since it travels, relatively, a bit faster than Landsat 8 in the lower orbit.
Around Thanksgiving, Landsat 9 is expected to reach its spot opposite Landsat 8 in orbit and then undergo two ascent burns to push it up to the same altitude of 705 kilometers. Landsat 7 has drifted from its nominal spot in the past few years, so it won’t be in the way when Landsat 9 takes its place.
The slight altitude difference of Landsat 9 affects the system of paths and rows that Landsat uses for cataloging, archiving, and distributing data. Those are based on the Worldwide Reference System and on Landsat orbiting at the 705-kilometer altitude. So the Landsat 9 data collected before it reaches that altitude doesn’t quite follow the Worldwide Reference System. “It’s cataloged by the closest path-row during imaging, but it’s actually overlapping parts of two different paths,” Anderson said.
All of this effort—by mission planners juggling two satellites, ground stations handling twice as much data, ground crews, and calibration/validation experts—will culminate in the best final calibrations possible by the end of the commissioning phase in January. All the data collected during the commissioning phase also will be reprocessed. “It will be as scientifically accurate as the rest of the Landsat data archive is,” Anderson said.
At that point, scientists and land managers will have a new Landsat to count on for monitoring crops, water consumption, deforestation, wildfire aftermath, invasive species, and countless other issues that affect the health and well-being of our planet and its people.