Light Curve Analysis and Attitude Estimation of Space Objects Focusing on Glint

Grasping of dynamic states of space objects such as shape, attitude, and surface properties plays a significant role in removing orbital debris and in monitoring the health of operational satellites. Light Curve Inversion is a costeffective state estimation technique using light curves, a time history of magnitude obtained by ground-based observations. The problem of this method is, however, that the observer measure only scalar data regardless of plural parameters to be estimated so that estimation accuracy is dependent of the initial values of estimation procedure. This study proposes a high-accurate estimation method using only light curves focusing on their characteristic, especially the sudden change of magnitude, called “glint”. Constrained conditions governing glint detection are shown in this paper. They are essential to exclude meaningless values from initial value candidates by setting attitudes in glint detection as the initial state values. Moreover, they realize a high-accurate estimation regardless of the initial values by efficient update of states and covariance matrix with constrains in the estimation filter. 1 Background and Motivation Active Debris Removal (ADR) is one of the effective measures to maintain and/or reduce the number of orbital debris in Low Earth Orbit (LEO) [1]. Grasping of dynamic states of orbital debris such as their shape, attitude motion, and surface properties plays a significant role in completing ADR missions successfully. It is also beneficial to monitor operational satellites that may collide or break [2]. Light Curve Inversion is a dynamic state estimation technique using light curves, brightness of space objects as a function of time obtained by ground-based observations. Light curves, obtained with inexpensive and widely used devices such as telescopes and CCD cameras, are the more cost-effective information than the conventional ones. This advantage makes it possible to observe a target from multiple points, so that observers can conduct measurement under adverse weather conditions. The main causes of light twinkling observed as light curves are follows: 1. Positional relationship among the Sun, an observer, and a space object, 2. Projected area with respect to the observer direction, 3. Properties of sunlight reflecting surfaces. These are determined by dynamic states such as shape, attitude, and specular reflectance of the target to define the intensity of light reflected toward the observer. Inversely, observation data as light curves enable to estimate states in a dynamic system. Light Curve Inversion is historically used in the field of astronomy to determine the axis and shape of asteroids [3]. Studies that apply the technology to orbital debris and satellites in operation have been conducted in recent years. The previous study discussed the dynamic state estimation of a space object in GEO from its light curve using Unscented Kalman Filter (UKF) and Multiple-model Adaptive Estimation (MMAE) [4]. In Light Curve Inversion, on the other hand, the observer measure only scalar data of magnitude, regardless of plural parameters to be estimated. Thus, it is known that this estimation method requires the initial attitude and the initial angular velocity of the target that are very near to the true values to realize the high-accurate estimation. To make matters worse, the estimation accuracy decreases significantly considering the limiting magnitude of the telescopes. To avoid those difficulties, the previous studies determined the initial values by using images obtained with radar or adaptive optics [5]. However, imaging by radar is very expensive method due to its larger instruments. Adaptive optics, which is a more reasonable method than radar, have a risk of getting insufficient images for the state estimation because of ambiguity by atmospheric disturbance. 6091.pdf First Int'l. Orbital Debris Conf. (2019)