Geometric Calibration of the Whiskbroom DaQi-1 WSI Based on an Equivalent Linear-Array CCD

The wide swath imager (WSI) equipped on the DaQi-1 satellite uses a 45° rotary scanning mirror, a K-mirror, and several linear-array charge coupled devices (CCDs) to collect ground images in a whiskbroom mode. In the imaging process, the imaging ray of a ground point is reflected several times by the rotary scanning mirror and the K-mirror before it can arrive at the focal plane. Moreover, the relative geometric relationships among the rotary scanning mirror, the K-mirror, and the linear-array CCDs are constantly changed. Geometric calibration and sensor orientation of the DaQi-1 WSI thereby become very complicated. In this study, the mechanical rotation of the rotary scanning mirror and the K-mirror is equated to the rotation of an equivalent linear-array CCD (ELACCD), and a feasible ELACCD-based in-orbit geometric calibration method for the DaQi-1 WSI is presented. In the presented method, a specific ELACCD-based physical sensor model (PSM) is first established. Then, imager parameters to be calibrated are grouped into three categories, and an ELACCD-based in-orbit geometric calibration model is established. Finally, three categories of calibration parameters are precisely estimated in sequence. Five datasets of DaQi-1 WSI images were tested. The experimental results showed that the established ELACCD-based PSM could precisely describe the imaging process of the DaQi-1 WSI. The presented ELACCD-based geometric calibration method could significantly improve the sensor orientation accuracies of WSI images. The orientation accuracies of the tested images were respectively 13 to 17 pixels and better than 1.0 pixel achieved before and after in-orbit geometric calibration.