Issues Associated with GPS Availability for Certain Marine Applications

Vis Sankaran and Alan Zorn

Abstract:	The current orbital configuration of the 24 Block II satellites was chosen because it offers the best overall availability for world-wide uses: air, land, and marine. Certain factors are present, however, which affect GPS availability for a number of marine applications. First, a large elevation mask angle is frequently required for some submarine missions. Second, a track window period which requires the same satellites to be visible for the duration of a given period is sometimes required for submarine and some surface ship applications. Third, operating on or just under the ocean surface has the advantage that an altitude measurement is available to augment GPS. The first two factors reduce GPS availability, while the third enhances it. This paper addresses GPS availability issues associated with certain marine applications. Satellite visibility and GPS availability are quantified while considering the three above factors. It is shown that large elevation mask angles and track window periods significantly reduce satellite visibility and GPS availability, while altitude aiding enhances availability to the extent that it can neutralize the other two effects. Without altitude aiding, availability is a more serious problem in middle latitudes than it is at high or low latitudes. The reason for this is found to be simply the fact that satellites are less visible at middle latitudes, by about an hour per day. Although the primary focus of this paper is on marine applications, some of the results may be extended to other applications on land or in the air which have mission features in common with those considered here.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	1413 - 1419
Cite this article:	Sankaran, Vis, Zorn, Alan, "Issues Associated with GPS Availability for Certain Marine Applications," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 1413-1419.
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