The use of satellite-based systems as electronic aids to navigation now provides highly accurate position location capability on a worldwide basis. The Navy Navigation Satellite System (NNSS) as depicted in Figure 1 has been used operationally by the U.S. Navy since 1964. The NNSS was released for commercial manufacture and exploitation on 29 July 1967. Since that time, commercial use has grown rapidly and shipboard satellite navigation equipment is currently in use on a worldwide basis, primarily in support of offshore oil exploration and related geophysical survey operations.
Commercial versions of satellite navigation shipboard equipment are presently being manufactured by ITT-Aerospace. This paper discusses current developments being applied to this equipment with emphasis on recent experience gained through utilization in offshore exploration applications.
The NNSS provides the user accurate position fixes on a worldwide, all-weather basis, in contrast to current earth-based radio aids to navigation, which are limited in coverage and in accuracy by the signal attenuation and dispersion associated with propagation along the earth's surface.
Figure 1 shows the relation of major elements making up the NNSS. The system essentially comprises one or more low altitude, polarorbit satellites, continuously transmitting timing and predicted orbital data to an unlimited number of passive (receiving only) users. These orbital parameters are updated every 12–16 hours on the basis of tracked orbital positions accumulated during the previous 12–16 hour period. Only one satellite is needed for position-fixing purposes; normally, several are maintained in orbit. Position fixes are provided whenever any satellite is visible to the user.
The satellite transmits a 2 minute data package simultaneously on 150 MHz and 400 MHz. Each 2 minute message segment transmitted by the satellite contains predicted orbital data which permits the user to compute the position of the satellite at any time during the pass. Four computed satellite positions, along with the accumulated doppler shift of the satellite signals measured over the intervening three 2 minute intervals, the latter being equivalent to change in the user's location, providing his altitude is known. In addition to altitude, the user must also supply as local inputs to the computation, his estimated velocity, as well as a rough estimate of his position, and of local time. The relation between locally supplied data and that derived from satellite transmissions is shown in Figure 2. The position fix thus derived is accurate to within about O. 1 nautical mile (nm) of the user's true location on a worldwide basis.
Reduction of the measured doppler frequencies and locally inserted data to an accurate position solution is most conveniently performed on a small scale general purpose digital computer. Figure 3 shows a complete shipboard installation, including separate general purpose digital computer. The use of a general purpose machine for computational purposes may not be optimum from the cost standpoint where the satellite navigation function is the sole purpose of the computer. In this instance, a special purpose data processor is used which eliminates the variety of input-output features and necessary programming flexibility required of the former.