Abstract: A preferred embodiment permits the safe navigation of a channel or harbor by a ship. A preferred embodiment includes a ship-borne system having a first global positioning system (GPS) receiver and a second fixed reference station with a GPS receiver. Both GPS receivers have GPS signal receiving antennas. Earth orbiting satellites of the GPS maintain communication at both locations via the two receive antennas. A broadcast radio transmitter at the reference station transmits a received GPS signal to the ship-based GPS antenna for receipt by the ship's separate radio receiver and processing by a computer. Using known navigation chart information on the water depths within a body of water, channel or harbor, known marine surveyor data, including the phase center of the ship-borne GPS receive antenna, and the two separately received GPS signals, the ship's keel clearance referenced to a known chart datum is calculated at one second intervals.

Abstract: A method for measuring depths of a waterway, including the steps of determining nautical chart vertical reference values along a selected waterway, building a data base of the reference values and storing the data base in a computer accessible from the vessel, providing a radio receiver onboard the vessel and Global Positioning System (GPS) equipment accessible from the vessel, and operating the computer, radio receiver, and GPS equipment to continuously obtain positions of the vessel horizontally, and computations as to depth of the vessel above a pertinent vertical reference value.