P-Code Versus C/A-Code GPS For Range Tracking Applications

Carl Hoefener and Bob Van Wechel

Abstract:	There is an ongoing difference of opinion as to whether P-Code or C/A-Code GPS should be utilized for range tracking applications, particularly on training ranges. As in any difference of opinion, there are usually two sides to the argument. In this paper we will attempt to come to some logical conclusions regarding the subject. The first step of course is to identify the basic differences between C/A-Code and P-Code GPS. The major difference in the codes is in the chip rate. C/A-Code is transmitted at a 1.023 megahertz chip rate while P-Code is transmitted at a 10.23 megahertz chip rate. The other major difference is that while C/A-Code is transmitted on a single frequency (L 1), P-Code is transmitted on two frequencies (Ll & L2). The difference in chip rate contributes little to the fundamental increased accuracy of P-Code because the difference in power levels (C/A-code power is 3 dB higher than P-Code power) partially compensates for the chip rate difference. As the major error in GPS is caused by the ionospheric delay error from the satellite to the user, P-Code is much more accurate than C/A-Code because ionospheric corrections can be made by measuring the difference in transmission delay between the two frequencies (Ll & L2). For this reason, when a single GPS receiver is used as in navigation applications, P-Code outperforms C/A-Code by a large margin. However, in a confined test area such as a test and training range, where precision tracking is required, differential GPS can be applied. With differential GPS, the ionospheric delay uncer- tainties can be removed from a C/A-Code system, thus enabling it to achieve position accuracies almost as good as P-Code. Therefore, there tends to be a propensity to plan for the use of differential C/A- Code GPS on the test and training ranges of the future. This trend is caused because of the comparable accuracy of the two systems and the perception that C/A-Code GPS receivers are much less expensive than P-Code GPS receivers.
Published in:	Proceedings of the 1991 National Technical Meeting of The Institute of Navigation January 22 - 24, 1991 Sheraton San Marcos Hotel Phoenix, AZ
Pages:	119 - 121
Cite this article:	Hoefener, Carl, Van Wechel, Bob, "P-Code Versus C/A-Code GPS For Range Tracking Applications," Proceedings of the 1991 National Technical Meeting of The Institute of Navigation, Phoenix, AZ, January 1991, pp. 119-121.
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