During the last two decades, significant developments have taken place in navigation and positioning techniques. These have occurred not only in the more classical radio and acoustic methods, but also in satellite and inertial techniques. Radio navigation is still the most commonly used surface navigation and positioning method. It spans a wide range of applications from the more localized high-accuracy positioning systems (e.g. Trisponder, Maxiran, Syledis) to worldwide navigation (e.g. Omega). The proliferation of systems reflects their relative merits and drawbacks, there being, in accuracy. Moreover, the different technique may not only be based on different co-ordinate systems, but also differ from one another by systematic biases. This can lead to ambiguities, especially in the determination of international and commercial concession boundaries. This is where a satellite0based navigation system comes into its own.
At present Transit (the Navy Navigation Satellite System) can provide positioning accuracies ranging from 50m for single-pass navigation down to 2–5 m for mutli-pass positioning. The Global Positioning System (GPS), which is due to replace Transit at the end of the current decade, is capable of quasi-instantaneous positioning to an accuracy of 10–20m, with higher accuracies obtainable from observations over a longer interval. The US Department of Defense estimates that by the beginning of the 21st century there will be the nearly half-a-million GPS receivers in operation throughout the world. Equally important, but covering a more limited objective than GPS, is the development of inertial navigation and surveying systems.
Radio, acoustic and inertial positioning methods are described in the next section. The two main satellite positioning systems, Transit and GPS, are reviewed in the following section. This is followed by a discussion of the various scientific and commercial applications, notably in surface navigation, surveying and geodesy and underwater positioning.
Radio positioning systems comprise a chain of radio beacons (transmitters or transponders) located at points of known geodetic co-ordinates, and one or more mobile units on vessels whose positions are required. The fixed beacons are usually on land, but may also be on fixed platforms out at sea. There are two main positioning methods in common use, the circular mode and the hyperbolic mode.
In circular mode, the mobile unit transmits either a pulsed or a continuous-wave radio signal (depending on the system), which the transponders receive and return in a suitably coded form. In the case of a pulsed signal (e.g. Syledis) the ranges to each transponder time differences between the transmitted pulse and each returned pulse. The position is the computed by trilateration. In the case of a continuous-wave signal (e.g. Argo) the phase difference between the transmitted signal and each return signal is measured. For each transponder, the loci of zero phase differences are a series of concentric circles centred on the transponder and separated by half the wavelength of the modulated signal. The phase measurements give the portion between two such circles but not the lane in which the mobile units is located (i.e. between which two circles).
