Worst-case GPS constellation for testing navigation at geosynchronous orbit for GOES-R

The Geostationary Operational Environmental Satellite - R Series (GOES-R) is the next generation NOAA weather satellite to be launched in 2015. GOES-R will use an L1 C/A GPS receiver (GPSR) to receive both GPS main beam and side lobe signals. The quality and availability of GPS signals at geosynchronous orbit (GEO) strongly impact navigation accuracy. For the GOES-R program, navigation accuracy requirements must be maintained during nominal operation of the spacecraft including station-keeping maneuvers. The GPSR solution 3-sigma accuracy requirement in position knowledge is 75 meters for the in-track and cross-track directions, and 100 meters for radial direction. Since maneuvers are not modeled in the onboard GPSR software, accuracy can degrade significantly during a maneuver. In order to verify that the GOES-R GPS navigation system can meet the stringent accuracy requirements during station- keeping maneuvers, a worst-case test scenario was developed for receiver testing. To find this scenario, we developed a simulation that models the GPS constellation and a GPS receiver and determines whether each GPS space vehicle (SV) can be tracked based on a high fidelity link budget model. Using this simulation, we modified the position of the GPS constellation relative to the Earth to find the scenario with the fewest number of trackable SVs during a North-South stationkeeping maneuver. The lowest visibility cases were found to be dependent on the right ascension, and occurred at 6 different shifts in right ascension. GPS receiver results from the Engineering Development Unit (EDU) are provided for both nominal and worst-case performance.