Since 1968 positioning and navigation via satellite has been available for the offshore community by means of the TRANSIT, or Navy Navigation Satellite System (NNSS) series of artificial earth satellites. As experience with the NNSS has grown and as techniques for processing the signals have improved offshore users have become more confident in the satellite system's ability to provide relatively high positioning accuracy, at least on a periodic basis. At this point, however, it is logical to ask what lies ahead since, for example, many users may be conducting long-range planning efforts involving deep water, far-from-shore projects.
The purpose of this report is to briefly examine the overall concepts of positioning via satellite and to summarize progress to date, including a look at some recent advances in the state-of-the-art in NNSS navigation. Then a brief look into the future at the forthcoming Global Positioning System (GPS) which is currently under active development by a joint U.S. military services group, and which is scheduled for full development by the late 1980's.
Positioning via satellite is an extension of techniques used in almost all commonly-used terrestrial radiolocation systems. The principle difference is of course' that the "transmitter" is continuously moving along an orbital path which is largely predictable by means of certain mathematical equations. As long as the user can obtain the satellite position at every instant of measurement, then he can apply more or less standard arithmetic techniques to convert the measurements into useful positioning information.
This is illustrated in Fig. 2-1 which shows a single satellite ('S') in polar orbit and a user 'u' that makes a measurement 'z' to the satellite. For electromagnetic systems, the measurement 'z' can be proportional to either doppler shift (as is the case with the NNSS) or range itself (as is the case with the upcoming GPS). This proportion ability derives from the fact that electromagnetic waves propagate with a relatively fixed velocity through the atmosphere, so that motion of the satellite (plus user) results in a changing propagation time (or a changing phase shift for satellite-transmitted signals of constant frequency).
The important requirement is that the measurement can be accurately modeled (or "predicted") mathematically by means of a relationship involving both the satellite and user positions. Then, given a series of measurements (either simultaneous or in time sequence and knowledge of the satellite position (usually transmitted from the satellite) it is a relatively straightforward matter to infer the user position using one of a variety of techniques, some of which are discussed later in this report.