The Nationwide DGPS (NDGPS) is a GPS augmentation system consisting of a network of radiobeacons located throughout the United States. The system broadcasts pseudorange corrections to users hundreds of kilometers away enabling users to obtain improved accuracy and integrity compared with standalone GPS. The Federal Railroad Administration (FRA) has investigated ways to increase the safety and the efficiency of the national rail system by improving the NDGPS.
The focus of this research was on developing a method to broadcast additional data on the NDGPS data link to support high performance users. The work concentrated on minimizing the amount of data needed to be transmitted to the remote user over a low bandwidth data link while still supporting a double difference carrier phase solution by the user. This research was a part of Ohio University's NDGPS-HP prototype architecture development. The system is intended to support not only a sub-meter accuracy, but also provide integrity, continuity and availability to the user. Currently the NDGPS data link broadcasts messages defined in the RTCM Recommended Standards for DGNSS SC-104 Version 2.3. This research investigates the latest standards (i.e., RTCM SC-104 Version 3) and ways to optimize them for use with a low bit rate data link (e.g. 200 bps) for high performance (HP) applications. A message set, labeled RTK-HP, was developed to meet the requirements of the NDGPS-HP system. The message set consists of variable length GPS measurement messages that were designed to take advantage of slow time-varying errors common to NDGPS reference stations.
A latency simulator was developed to easily analyze the effects of different message sizes, bandwidths, and transmit frequency on the latency. Several scenarios were considered to characterize the latency of the RTK-HP messages over 200 bps and 500 bps data links, which are capable with future versions of the NDGPS architecture. The results of the simulation showed that mean latencies of 5-6 seconds and maximum latencies of 8-10 seconds were achieved with the RTK-HP protocol over a 200 bps data link, while a mean latency of 2 seconds and a max latency of 3.4 seconds were achieved over a 500 bps data link. Both data rates provide an improvement over the SC-104 Version 3.0 protocol which has a mean latency of 8.5 seconds and a maximum latency of 12.7 seconds.
A limited prototype NDGPS-HP system was developed to serve as a test bed. The system consisted of hardware in the loop testing performed in a lab setting. The system was tested by sending RTCM SC-104 Version 3.0 messages from the reference station PC to a user PC. The messages received by the user were compared to those sent by the reference station and were found to agree within 1 cm for the pseudorange and 0.15 cm for the carrier phase.