Navigation Satellites for Geophysical Exploration

Abstract

ABSTRACT The Navy Navigation Satellite System was originally designed for military navigation by the Applied Physics Laboratory of Johns Hopkins University but is now released for civilian navigation. It consists of six beacon satellites in polar orbits for which the Navy maintains the accuracy of the transmitted navigation data. Several different passive receivers are available for these coded satellite signals. The navigation mathematics is an iterative process of least squares fitting of the satellite data. At sea discrete position fixes are produced instead of continuous navigation. On land several fixes may be combined to resolve altitude uncertainty. Two different physical interpretations of this data elucidate the geometric basis for the solutions and their accuracy relationships. The current state of the art in satellite navigation accuracy is only possible because intelligent techniques are being used to counteract the inherent error sources of the system. Special receiver design reduces interference problems. Mathematical modeling removes most of the effects of transmitting the satellite signals through the imperfectly known ionosphere and troposphere. Good dead reckoning alleviates the errors specifically identified with receiver antenna motion. Although satellite positioning can be used alone, integrating it into other navigation systems produces the best results. The other system produces dead reckoning for the fix computation while the fixes produce calibration factors for the first system. The redundant information between the components of the integrated system increases the accuracy of error smoothing and correction in both real time and postprocessing. In filling the nitch of sophisticated global position reference, the Navy Navigation Satellite System has proved a boon to the geophysical exploration industry. INTRODUCTION The navigation satellites currently being used in geophysical exploration are the Transit satellites making up the Navy Navigation Satellite System. This system was designed to provide world-wide position fixes in latitude and longitude at predictable intervals ten to twenty times a day for multiple users with minimal equipment. Accuracies of 110 feet are being obtained by some users. This paper covers the basic satellite system from hardware through position fix computation. Various error sources and their impacts on accuracy are examined for "stand alone" systems, "integrated, real-time marine navigation" systems, and postmission processing. HISTORY On 4 October 1957, the Russians launched the first Sputnik ushering in the space age. By early 1958 the Applied Physics Laboratory of the Johns Hopkins University had assessed the feasibility of a Doppler shift satellite navigation technique and proposed it to the Navy. December 1962 saw the prototype satellite launched, and by July 1964 the Navy Navigation Satellite System was operational.