satellite navigation

satellite navigation, position finding by means of a space-based system of orbiting satellites. Now the most general way of navigating at sea, it has virtually displaced every other method of fixing the ship's position, whether hyperbolic, radio direction finding, or celestial navigation, one reason for this being the low cost of the equipment, others being its accuracy and ease of use. However, as the whole system is owned by the military, and can be withdrawn at any time, it is inherently unlikely that more traditional methods of navigation will be abandoned, and celestial navigation, and at least one ground-based radio aid such as Loran-C, will probably remain for the foreseeable future.

The Doppler shift is a well-documented phenomenon but its potential as a method of global positioning was only realized in the 1950s when radio signals picked up from the USSR's orbiting satellite Sputnik revealed a marked Doppler shift when received at fixed points on earth. From these transmissions American scientists were able to calculate Sputnik's orbit in no time. The corollary was that the Doppler shift of a satellite of known orbit could also be used to determine the position of the receiver.

The first satellite navigation system to become commercially available was the US Navy Navigation Satellite System known as Transit. Using the Doppler shift principle, it provided accurate position information to ships at sea up until the end of 1996 when the more advanced American military GPS, known as Navstar, took its place. The other satellite navigation system in operation is the Russian Glonass which has much in common with GPS. Plans for a European system entitled Galileo are in hand and the system is expected to be operational by 2008. GPS III is expected to replace the present GPS system by 2010.

The GPS system consists of a master ground control, ground monitoring stations, and a constellation of satellites the positions and orbits of which will be precisely known through monitoring and regular correction. In the current system 24 operational satellites, four in each of six orbital planes inclined at 55° to the equator, orbit the earth at an altitude of 20,200 kilometres (12,625 m.) with an approximately twelve-hour orbit period. (Because of the difference between solar and sidereal time each of the satellites will appear above any fixed point on the earth four minutes earlier each day.) The accuracy of GPS depends critically on precise timekeeping and each satellite carries four atomic clocks. The GPS receiver stores and continuously updates time information from the satellite and can calculate the delay in the transmission of signals of each satellite, which provides a measure of the distance between the satellite and receiver.

The current (2004) satellites radiate two codes, a Precision Code (P) and a Coarse Acquisition Code (C/A), both of which are now available for civil use. In general the accuracy of GPS fixes is taken to be of the order of 10–20 metres (33–65 ft), although in many cases it may well be considerably less. An important factor to remember is that the geodetic chart datum to which the receiver will normally be referenced is that known as WGS84, the World Geodetic System 1984, an ellipsoidal figure of the earth based on a centre equivalent to its centre of gravity. By no means the majority of nautical charts are yet based on this datum and most receivers have a facility for altering the datum to that of the chart in use. Significant errors in position may arise from failing to adjust the datum in this way. Most GPS receivers offer a bewildering range of navigational facilities such as speed over the ground, speed through the water, distance to go, time to go, course to steer, estimated time of arrival, cross-track error, and so on. Most of them are related to waypoints which form an essential feature of computerized navigation.

Differential GPS (DGNSS) is a system for improving the accuracy of GPS fixes. The main sources of error in satellite navigation are consistent over large geographical areas and the errors may be corrected by using reference stations, whose position is precisely known, to measure the errors of the signals from the satellites. Any corrections are transmitted to users' receivers which adjust their position measurements accordingly. Marine radio beacon stations with a range of 160–240 kilometres (100–150 mls.) are commonly used to transmit these corrections to maritime users. Application of the corrections typically will give an accuracy of 1–2 metres. The requirements for such high accuracy could include improved voyage planning, freedom of manoeuvre in restricted waters, etc. Sandford, W. H. , A Simple Guide to GPS for Marine Use (2001).

Mike Richey