Recommendations on Differential GNSS

The use of differential navigation techniques combined with GLONASS and GPS, collectively referred to as Global Navigation Satellite Service (GNSS), presents maritime authorities with an opportunity to provide highly precise navigation service for harbor entrance and approach areas with a reduced dependence on any one system. The use of 48 satellites (24 GPS and 24 GLONASS) also provides improved availability in situations of partial satellite signal blockage due to terrain, structures, and large vessels. Receiver autonomous integrity monitoring techniques also become highly reliable with this increased number of signals for comparison. Differential techniques for GPS are well understood and have been standardized across the industry. The Radio Technical Commission for Maritime Services (RTCM) Special Committee 104 has developed a proposed format for messages containing differential GLONASS information. In addition to the data formats to transmit GLONASS corrections a differential GNSS service must resolve the technical issues in combining the separate systems. When GLONASS and GPS measurements are to be combined there are two technical issues that must be resolved. First, the difference in the WGS 84 and PE 90 datum must be adjusted, and second, how to handle the difference between GPS time and GLONASS time. This paper presents the results of this USCG R&D study to determine the best method of integration and recommendations on implementing DGNSS service.