CORRECTING FOR ROLLING SHUTTER DISTORTION IN DART OPTICAL NAVIGATION IMAGERY

A key process in optical navigation (opnav) is the use of observed star centers, combined with a known camera model, to derive the pointing of a given picture1. This process typically produces attitude accuracies on the order of a tenth or hundredths of a pixel. However, as CMOS detectors become increasingly common in opnav cameras, so to does the use of a rolling shutter, as is the case for the Double Asteroid Redirection Test (DART) Didymos Reconnaissance and Asteroid Camera for Opnav (DRACO). A rolling shutter means that as the spacecraft deadbands, each readline is exposed at a slightly different attitude, thus distorting the resulting star pattern, and potentially inducing error to the process of estimating picture attitude. This paper details these effects on DRACO imagery, and the techniques developed to counteract them.

光学导航（optical navigation，简称opnav）的关键流程之一，是依托观测得到的星点中心，结合已知相机模型，求解单幅图像的指向姿态。该流程通常可实现十分之一乃至百分之一像素级的姿态精度。然而，随着CMOS探测器在光学导航相机中的应用愈发普及，卷帘快门（rolling shutter）技术的应用也随之增多，双小行星重定向测试（Double Asteroid Redirection Test，简称DART）所搭载的迪迪莫斯侦察与光学导航小行星相机（Didymos Reconnaissance and Asteroid Camera for Opnav，简称DRACO）便是典型实例。卷帘快门的特性意味着，当航天器处于姿态死区时，每一行图像的曝光时刻对应的航天器姿态均存在细微差异，这会对生成的星点图案造成畸变，并可能在图像姿态估计流程中引入误差。本文详细阐述了该效应对DRACO成像的影响，以及为抵消此类误差所开发的相关技术。