Time Synchronization Calibration for MERSI Autonomous Geolocation

Autonomous geolocation is the direct georeferencing of remote sensing imagery without ground control points (GCPs), which is a critical problem for direct broadcast satellites. For the Medium Resolution Spectral Imager (MERSI) with a 250 m ground sample distance, the attitude and orbit determination system meets sub-pixel geolocation accuracy requirements. However, latitude-related geolocation errors were observed from global GCPs of FY-3D MERSI-Ⅱ, and attitude jitter errors were observed in FY-3F MERSI-Ⅲ. In this study, three time synchronization errors of four dynamic components were investigated for spaceborne integrated sensors, the time delay between Global Navigation Satellite System (GNSS) receivers, Earth time, and navigation system, when the time of the whiskbroom scanner was the reference. The time synchronization calibration method was developed to calibrate MERSI-II and MERSI-III with two orbits. Given the correlation between the time synchronization and spatial parameters, the scanning compensation angles were used to select the optimal parameters. MERSI-Ⅱ exhibits a 0.590 s delay with the GNSS receiver, whereas MERSI-Ⅲ shows 0.27 s and 0.175 s delays with the GNSS receiver and the navigation system, respectively. After correcting the time synchronization of MERSI-II, the median geolocation error in the scan direction at 80° N decreased from 1.5 to 0.5 instantaneous field of view (IFOV). The global root mean square errors (RMSEs) was 0.493 IFOV (109.02 m) in the image plane. After correcting the time synchronization of MERSI-III, the global RMSEs was 0.374 IFOV (93.5 m) in the image plane.