Using GPS at sea to determine the range between a moving ship and a drifting buoy to centimeter-level accuracy

In many experiments at sea the measurement of distance is of great importance. This can be the distance between instruments mounted on a ship, where the ship's superstructure makes direct measurement difficult if not impossible, or, for example, the distance between the ship and a drifting buoy. In March 1997, the MOMAX shallow-water acoustics experiment was performed off the coast of New Jersey. Single-frequency GPS receivers were fitted both to a ship and to drifting buoys so that the range between them could be determined. The raw receiver output of pseudorange and carrier phase was recorded to disk for post-processing with a differential, moving-baseline kinematic software package. Two receiving antennas were fixed on the ship; thus variation in the calculated distance between them could be examined over time and various sea states. In addition, they provided a fixed baseline for the computation of the triangle closure error between them and a remote buoy. The derived distance between the two ship's antennas was 28.11 m, had a standard deviation of about one centimeter, showed a peak-to-peak scatter of a few centimeters, and was essentially independent of sea state. The triangle closure error between the two ship antennas and a buoy was generally better than two centimeters out to ranges of five kilometers. Single-frequency GPS is capable of obtaining centimeter-level range measurements at sea even when both the base station (ship) and the remote (buoys) are moving.