Dispelling Some Popular Myths About GPS Receivers for Military Applications

Phil Ward

Abstract:	There is a popular myth in the Global Positioning System (GPS) military user community that only precision code (P-Code) receivers will support military applications and that only coarse acquisition code (C/ A-Code) receivers are used to support civilian applications. The characterization of the GPS precise positioning service (PPS) often has been described erroneously as synonymous with the use of a P-Code GPS receiver, while standard positioning service (SPS) often has been described erroneously as synonymous with the use of a C/ A-Code GPS receiver. Related to the PPS/SPS myth is the false belief that when the GPS Control Segment activates GPS antispoofing (AS) (denial of access) and/or selective availability (SA) (denial of accuracy) in the GPS Space Segment, these encrypted functions are correctable with a keyed P-Code GPS receiver, but cannot be corrected with a keyed C/ A-Code receiver. There are some who erroneously fear that a C/ A-Code receiver can be denied access. There is also the misconception that P-Code GPS receivers are intrinsically more accurate, operate at higher dynamics, and are more immune to jamming than C/ A-Code GPS receivers, without qualification. This paper dispels these misconceptions by counterexamples. It summarizes the effectiveness criteria for military applications wherein a PPS C/ A-Code GPS receiver should be used in lieu of a PPS P-Code GPS receiver if there is also the expected cost advantage. For example, the Tl 420 five-channel, C/ A-Code GPS receiver is a modem, low-size/weight/power, militarized PPS receiver that has been demonstrated as highly cost effective for numerous military applications. In contrast, the Tl 41 00 GPS receiver is a commercial receiver that continuously tracks both the Ll and L2 P-Code signals of up to four GPS satellites for civilian SPS applications. The Tl 420 has demonstrated accuracy performance in the field that meets or exceeds the accuracy usually specified for military P-Code PPS receivers. It also performs without loss of lock and without external aiding in sustained acceleration dynamics exceeding 4g and at velocities up to 11,000 meters/second. The theory is shown for the surprising fact that a broadband jam­mer expending its energy uniformly over the 20-MHz bandwidth of the P-Code Ll signal will succeed in jamming the Ll signal of an unaided five channel P-Code receiver at a jammer power level that is 3 dB lower than the level required to jam out the Ll signal of an unaided five-channel C/ A-Code receiver at the same range from the jammer. Other examples of popular GPS myths are described and dispelled with field performance demonstration results.
Published in:	Proceedings of the 2nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1989) September 27 - 29, 1989 The Broadmoor Hotel Colorado Spring, CO
Pages:	281 - 290
Cite this article:	Ward, Phil, "Dispelling Some Popular Myths About GPS Receivers for Military Applications," Proceedings of the 2nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1989), Colorado Spring, CO, September 1989, pp. 281-290.
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