Precise measurements of ship´s attitude using satellite navigation

Assessing the error in marine gyrocompasses has proved difficult, but with the advent of multiple receiver - multiple antenna GPS satellite navigation systems it is now possible to measure heading, pitch and roll to better than 0.1"; such a GPS system (Ashtech GPS 3DF) was installed on RRS Discovery in August 1992 and was used intensively in the Southern Ocean from November 1992 to May 1993. The system comprises a 24 channel GPS receiver split into four six channel banks, with each bank connected to an antenna. The four antennas were arranged as a rectangle with the long sides (ca. 970 cm) fore/aft and the short sides (ca. 650 cm) athwartships. Heading data from the GPS receiver were logged on a dedicated PC and also by the standard RVS shipboard computer system. Analysis of heading data from the GPS 3DF system has shown the limitations of the ship\´s gyrocompass. The authors summarise findings to date, and include new data on the measurements of pitch and roll using GPS 3DF. One major limitation of GPS 3DF is that it is not presently available on a continuous basis. Even when the GPS satellite constellation is complete, and position finding may be available continuously, attitude determination requires more stringent signal and satellite visibility criteria. The authors discuss receiver settings that affect data availability and comment on the quality of the resulting data