Landsat 9动态：Landsat 9正在Landsat 8星下伴飞
作为Landsat 9卫星校准过程的组成部分，Landsat 9号卫星正在Landsat 8号卫星下方飞行。
该次结伴飞行能够提供给校准专家全面的测量数据对Landsat 9进行比较，Landsat 9的管理和运行将于2022年初移交给美国地质调查局(USGS)。
Landsat 8卫星轨道高度705千米，Landsat 9号正在Landsat 8号下方10千米左右的轨道运行，两者将对地表同一区域进行拍摄成像。
潜行飞行将持续约5天，Landsat 9号在Landsat 8下方约10公里处，于11月14日逐渐在Landsat 8的正下方移动，然后继续移动，因为它在较低轨道上的移动速度相对要比Landsat 8号快一点。
感恩节前后Landsat 9号预计将到达其轨道上与Landsat 8号相对的位置，然后进行两次上升燃烧，将其推至705公里的同一高度。在过去的几年里，Landsat 7号已经偏离了它的标称位置，所以当Landsat 9号取代它的位置时，它不会成为障碍。
Landsat 9 (OLI-2)
Band 1 Visible (0.43 - 0.45 µm) 30-m
Band 2 Visible (0.450 - 0.51 µm) 30-m
Band 3 Visible (0.53 - 0.59 µm) 30-m
Band 4 Red (0.64 - 0.67 µm) 30-m
Band 5 Near-Infrared (0.85 - 0.88 µm) 30-m
Band 6 SWIR 1(1.57 - 1.65 µm) 30-m
Band 7 SWIR 2 (2.11 - 2.29 µm) 30-m
Band 8 Panchromatic (PAN) (0.50 - 0.68 µm) 15-m
Band 9 Cirrus (1.36 - 1.38 µm) 30-m
Landsat 9 Thermal Infrared Sensor 2 (TIRS-2)
Band 10 TIRS 1 (10.6 - 11.19 µm) 100-m
Band 11 TIRS 2 (11.5 - 12.51 µm) 100-m
原文：
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.
Landsat 系列卫星各传感器波长。
往期：