POLARIS - A GPS-NAVIGATED OCEAN ACOUSTIC CURRENT PROFILER

Results from an initial feasibility study to test whether Global Posit ioning System (GPS) navigation can be combined with more traditional a coustic methods to measure ocean current profiles are presented. A typ ical acoustic current profiler such as PEGASUS measures currents by ra nging on two acoustic sources (one-way beacons or two-way transponders ) as it falls or rises through the water column. These sources must be previously deployed on the bottom, and their positions accurately det ermined via the attending research vessel. As discussed below, this pr ocedure introduces a number of complications. In particular, any unres olved errors in the source deployment will remain as systematic errors in the resulting velocity data. The method described here replaces th e bottom-mounted sources with hydrophones drifting near the ocean surf ace. The positions of these hydrophones are computed every few seconds using GPS. This feasibility test shows that a combined GPS-acoustic s ystem can approach accuracy levels found in the standard method. Furth ermore, systematic errors can be significantly reduced. Random errors are estimated to be approximately +/-1-2 cm s(-1) dependent on station geometry.