Verification of ATMS Pointing by Analysis of Coastline Crossings

The first satellite of the Joint Polar Satellite System (JPSS) was launched by NASA on November 18, 2017. After an initial assessment by NASA teams, it was handed over to NOAA for use in weather forecasting and other operational applications and renamed NOAA-20. The Advanced Technology Microwave Sounder (ATMS), first developed by NASA for the Suomi-National Polar Partnership (S-NPP) mission, is one of the payloads. During the initial on-orbit assessment period, one of the tasks was to determine the actual pointing of this instrument and possibly make corrections to the geolocation software to account for any pointing bias. One of the special operations during this period consisted of putting ATMS into stare mode. Normally it scans from side to side, resulting in a wide swath of measurements as the spacecraft moves along its orbit, but in stare mode the scan mechanism is frozen at a fixed angle within the scan plane. By analyzing the stream of data from the instrument as it passes over sharp edges such as coastlines and other features with large radiometric contrasts between the two sides of the edges, it is possible to determine the three primary pointing angles of the instrument's boresight, i.e. pitch, roll and yaw, without having to wait for enough data to accumulate for conventional image analysis. By in turn comparing the actual angles with those computed from a geolocation model using orbital parameters, satellite attitude, instrument mounting angles, etc., it can be determined whether the geolocation processing should be corrected to account for previously unknown or uncertain errors. While most recent microwave sounders have incorporated stare mode, it is rarely used due to a perceived risk in stopping the scanner, but it was used to analyze coastline crossings of the Advanced Microwave Sounding Unit (AMSU) on the NASA Aqua satellite after it launched in 2002 and now of ATMS on the JPSS satellite. We present the results of the JPSS analysis carried out by a team at the Jet Propulsion Laboratory using the methodology established with Aqua.

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