| Spacecraft in libration orbits can use scalar satellite-to-satellite tracking (SST) data, such as crosslink range, to perform autonomous orbit determination. SST between two spacecraft provides information on the size, shape, and relative orientation of two orbits. A pair of Keplerian orbits of a certain size, shape, and relative orientation can have any absolute orientation about the center of mass of the primary body. This means that SST alone is not sufficient to autonomously determine the absolute orientation of the conic orbits of spacecraft near Earth. However, in the three-body problem, the gravitational influence of the third body can indirectly provide information about the direction to that third body and with it, the absolute orientation of the orbits. In other words, a halo orbit near the Moon is influenced very strongly by both the Earth and the Moon and has a unique size and shape. Because of the strong asymmetry of the three-body force field, a halo orbit with that size and shape can only have a single orientation with respect to the Earth and Moon. This means that a spacecraft in a halo orbit can track a second spacecraft using crosslink range measurements and determine the absolute positions and velocities of both spacecraft simultaneously without any Earth-based tracking or mathematical constraints. Simulations show that this technique works well in lunar halo orbits in the Circular Restricted Three-Body Problem. More realistic simulations show similar results in the Earth-Moon system using the DE403 planetary ephemeris, solar gravity, solar radiation pressure (SRP), and the LP100K lunar gravity field in an Extended Kalman Filter. The position errors for lunar spacecraft were on the order of 10 m RSS and about 100 m RSS for halo orbiters in various locations. Range biases, spacecraft reflectance, and maneuver errors were all successfully estimated, even in the presence of SRP and gravity modeling errors. Halo stationkeeping maneuvers were performed using real-time estimates, and a combination of frequent maneuvers and precise orbit determination resulted in stationkeeping budgets of only a few m/s per year. |
