DGPS/Radiobeacon Field Trials Comparing Type 1 and Type 9 Messaging

Abstract

A medium-frequency (MF) radio system is being deployed worldwide for the broadcast of differential corrections to GPS users. This system adds a digitally modulated subcarrier to transmissions from some of the marine radio beacons, which currently operate in the 283.5–325 kHz band. These “DGPS/radiobeacons” use groundwave propagation to carry the DGPS correction well beyond the “radio horizon” where the DGPS data is still valid. Improvements in the DGPS/radiobeacon receiver design or messaging protocol that reduce the required signal-to-noise ratio (SNR) are important because they can significantly increase the range or availability of the DGPS/radiobeacon service. Such an improvement is being pioneered by the U.S. Coast Guard. This innovation calls for the transmission of three satellite corrections per message (Type 9 messaging), rather than the usual transmission of all corrections in every message (Type 1 messaging). If noise damages a message, then only three corrections are lost, rather than the corrections for all satellites in view of the reference station. This paper contains an analysis and field results showing that the improvement made possible by Type 9 messaging is significant.