TIMED GPS navigation system (GNS): Design, implementation, and performance assessment

The APL-developed GPS Navigation System (GNS) satisfies derived TIMED spacecraft requirements by providing position, velocity, time, Earth-to-Sun vector, and defined orbital event notifications to onboard instruments, spacecraft control computers, and the Mission Operations Center. The GNS data products enable the mission's event-based commanding concept of operations. The system generates tables of both position-based event predictions and orbital element sets for each predicted ground station contact for up to 60 h in advance. The GNS is a Standard Positioning Service receiver system with access to the GPS civilian ranging coarse/acquisition code. It was designed to be a state-of-the-art spaceborne system optimized for autonomous on-orbit operations. The GNS on-orbit test program encompassed the typical "launch + 30 days" checkout period as well as periodic in-flight test sequences where the two GNS processors aboard TIMED are operated and compared against ground-based tracking systems. To date, both are functioning flawlessly and navigation products have surpassed mission accuracy requirements.