The Use of the Tracking and Data Relay Satellite System for Near Lunar Navigation

Near lunar satellite, navigation poses many difficulties, due to the distance from the Earth, and especially due to the narrow baseline presented by the possible earth stations. These difficulties are particularly strong at the L1 and L2 Lagrange Points (EML1 and EML2), due to the low rate of divergence of position and velocity with time for different positions that are close to the Lagrange points. While the Tracking and Data Relay Satellite System (TDRSS), run by NASA's Goddard Space Flight Center, was developed to provide tracking and data relay services for near Earth missions, TDRSS can also provide tracking and data relay services at lunar distances. Due to the much higher EIRP of the steerable antennas and the wider baseline TDRSS provides, TDRSS has advantages over GNSS at lunar distances. The Tracking and Data Relay Satellites each have two five meter single access bi-directional antennas which can be independently aimed and used (with some restrictions) to transmit and receive signals at the same time. Because of the ability to steer the antennas so far off nadir and the much higher Effective Isotropic Radiated Power, the Tracking and Data Relay Satellites can provide usable signals to near lunar spacecraft at much larger angles than global navigation satellites. This is the major source of the larger baselines that the Tracking and Data Relay Satellites can provide compared to global navigation satellites, although the higher altitude of the Tracking and Data Relay satellites (geosynchronous altitude instead of MEO). These satellites can provide a much larger baseline than can Earth based systems, especially in the East-West direction. Even in the North-South direction, the Tracking and Data Relay Satellite System will typically provide considerably better baselines due to the Moon's orbital inclination and the orbital inclinations of the various TDRSS satellites.