The Global Positioning System: A Shared National Asset (1995)
Chapter: Ground-Based Integrity Improvements
corrections, instead of corrections generated autonomously on the satellites, could be used to update the navigation message every hour. In order to operate in this manner, the data rate of the Block IIR UHF communication crosslinks may have to be modified. The exact improvement to the combined clock and ephemeris error is not known, because a complete analysis was not conducted. However, more frequent uploads of integrated space-based and ground-based clock and ephemeris information should result in errors no greater than 1.2 meters (1s).
The planned Block IIR operation should be reexamined and compared to the accuracy advantages gained by incorporating inter-satellite ranging data in the ground-based Kalman Filter and uploading data at some optimal time interval, such as every hour, to all GPS satellites.
Satellite Health Monitoring to Improve System Reliability and Availability
Since the Block IIR satellites will have a UHF communications crosslink capability, satellite health monitoring could be implemented that could improve overall system reliability and availability. For example, if a satellite detected an anomalous on-board health reading, but was not in contact with a ground station, it could relay the information through the crosslinks, enabling another satellite that was in contact with a ground station at that instant to download the information. The MCS in turn, could upload commands to the failing satellite via the crosslinks. This would improve the reliability of each individual satellite by minimizing out-of-service time, thus improving the percentage of time that a full 24-satellite constellation would be available to users.
Block IIR satellite communication crosslinks should be used to the extent possible with the existing crosslink data rate to support on-board satellite health monitoring for improved reliability and availability and in order to permit a more rapid response time by the operational control segment.
Ground-Based Integrity Improvements
The Block IIR communications crosslinks also could be used to improve GPS signal integrity for all users. For example, if an anomalous pseudorange signal was detected at a monitoring station, the MCS could upload a command to the satellite broadcasting the anomalous signal by relaying this command through the crosslinks. The faulty satellite could be commanded to broadcast a code that could not be tracked by a user's receiver, and would therefore, be dropped from the users' positioning solution.
To use the crosslinks to improve GPS integrity for PPS and SPS users, the receivers at the monitor stations must be upgraded to monitor the C/A-code. The data rates on the crosslinks must be able to support commands sent from the MCS.
