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 degrees 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 13-17 pixels and better than 1.0 pixel achieved before and after in-orbit geometric calibration.