Proposed Evolution of the C/A Signal

Thomas A. Stansell, John W. Betz, Frank van Diggelen, Satoshi Kogure

Abstract:	The authors of this paper are grasping the third rail of GPS politics, which is to suggest a change in the C/A signal. Over several years, study groups and individuals have recommended either a sunset or a modernization of this venerable signal, e.g., (Betz 2010). Even so, there are no official plans to improve C/A, although in 2015 it has been 37 years since the first C/A signal was launched on 22 February 1978. The C/A signal was designed many years ago but technology and applications have changed drastically in the meantime. The authors of this paper are convinced the C/A signal should be changed but not ended. A detailed prescription for a modernized C/A signal is not provided. That should be left to official design and implementation teams. The paper does provide a definition of basic principles for the C/A evolution, aided by discussions of C/A advantages and disadvantages and of several ways C/A can be modernized, not only to overcome its disadvantages but also to provide significant performance enhancements. The paper will include the tradeoffs implicit in any changes, providing a foundation for the necessary debate that should follow. Regardless of the exact techniques, the authors recommend specific ways to: (a) maintain the advantage of rapid acquisition with modest use of memory, (b) reduce C/A on C/A interference, (c) eliminate the challenge of data bit synchronization, (d) provide a more robust L1 civilian pilot component, even more powerful than the L5 pilot, and (e) replace the limited and inflexible legacy NAV message with the flexible, robust, and more accurate CNAV-2 message. The authors also identify several options that can be explored for transitioning to a modernized C/A signal design, ranging from an evolutionary approach to a step change after many years of notification. To enable these options, changes to the C/A signal should now be designed into every new GPS satellite and prepared for in future blocks of the control segment. Changes to GPS take a long time, so it’s time to start planning so that at some point in the future (perhaps in 2035, when the C/A signal is 57 years old) an improved C/A signal can be available to provide even better performance than today.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1807 - 1825
Cite this article:	Stansell, Thomas A., Betz, John W., van Diggelen, Frank, Kogure, Satoshi, "Proposed Evolution of the C/A Signal," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1807-1825.
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