Navigation and Seamanship

6.4.3. Method of operation

Satellites moving around the earth have known orbits in terms of time and position with respect to earth. The GPS detects the presence of the satellites which are acting as moving reference points and calculates the position of the boat from the parameters namely the speed of the satellite, its altitude, azimuth angle and other data based on the relative movement of the satellite with reference to earth. The micro computer in the GPS makes the computation instantly and gives the position in terms of lat. and long. Other facilities of the equipment such as speed of the vessel, navigational route, destination point etc are also computed from the basic data. GPS provides highly precise, continuous worldwide, weather proof, position plus time and velocity information to GPS receivers – equipped vehicles vessels and aircraft GPS is the most accurate technology available for navigation today. Currently there is 24hours, worldwide two dimensional GPS coverage available which makes GPS fully functional for marine applications. The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.

The ground control segment consists of a master control centre and a number of widely separated monitoring stations. The ground control network tracks the satellites, precisely determines their orbits, and periodically uploads almanac ephemeris, and other system data to all satellites for retransmission to the user segment. The user segment is the collection of all GPS user receivers and their support equipment. More simply, the GPS Receiver’s position is determined by the geometric intersection of several simultaneously observed ranges (satellite to receiver distances) from satellites with known co-ordinates in space. The receiver measures the transmission time required for a satellite signal to reach the receiver.

The receiver processes satellite orbit data to determine the user’s three-dimensional position. A minimum of four observations are required to mathematically solve for four unknown receiver parameters (i.e., latitude, longitude, altitude, and clock offset).