Low earth orbit navigation in the tracking and data acquisition system era

Several alternatives are addressed for meeting spacecraft orbit and time determination requirements in the 1990s with the proposed follow-on to the Tracking and Data Relay Satellite System. Key objectives are to reduce ground requirements for routine two-way tracking, support onboard navigation, and satisfy projected user accuracy goals down to 10 m (position) and 1 /*s (time). This paper focuses primarily on three one-way range and Doppler tracking techniques based on 1) onboard processing of navigation beacon signals broadcast continuously, 2) onboard processing of scheduled tracking signals, and 3) ground-based processing of scheduled, usergenerated tracking signals. System configurations and requirements to support each method are compared and preliminary results of navigation performance evaluations are presented as a function of user orbit, relay satellite constellation, and other parameters. Comparison of results with the accuracy requirements in a 1990-2005 mission model indicate that essentially all can be satisfied with beacon tracking based on projected reductions in key error sources. The scheduled tracking alternatives are also applicable except in low-altitude orbits where performance is more sensitive to drag uncertainty, tracking frequency, and/or navigation data upload rates. ,