Field trials of 'HANS' in the Houston ship channel

There are bills before Congress that will mandate the use of Differential GPS (DGPS) in US ports and harbors; they only await the operational availability of the differential beacons nationwide, scheduled by the US Coast Guard for 1996. In confined waters GPS requires some form of graphic display - but ECDIS (the Electronic Chart Display and Information System for which specifications are being developed by the IMO and IHO) will not meet this need. ECDIS is based on the paper chart and the scale of the paper chart is inadequate for entry to many US ports; the pilot instead uses the range lights, beacons and buoys provided by the Coast Guard. For the past two decades computer systems, many with electronic displays, have been used to guide hopper dredges in the approaches to ports, taking their input from microwave positioning systems and enabling work to be continued twenty-four hours a day under all visibility conditions. HANS (Houston Area Navigation System) is the result of substituting DGPS in one such system; it displays the vessel's position and orientation relative to the channel with an accuracy that is limited only by that of the DGPS. In February, 1993, in partial consequence of a low visibility collision that closed the Houston Ship Channel, the Port of Houston Authority funded a trial of the HANS system. The system is installed in a lightering tanker, which makes three or four transits of the channel each week. The paper describes the system and the trial; it discusses the experience gained and the lessons learnt