Session A3b: Next-Generation Satellite Navigation Technology

Modernizing the C/A Code Signal
P. A. Dafesh, G. K. Khadge, J. W. Zheng and J. A. Maynard, The Aerospace Corporation
Location: Beacon A

The since the initial design of the Course Acquisition (C/A) code signal , it was recognized that the 1023 chip length of the C/A code was a compromise between ease of acquisition and short code cross-correlation performance [1,2].
The higher cross-correlation levels and 1 kHz spectral lines of C/A code have motivated many investigations on the so-called C/A on C/A effect, which has been characterized as significant or negligible, dependent on specific receiver assumptions and use cases including: coherent integration length during track, tracking loop bandwidth, coherent integration length during acquisition and relative Doppler conditions [3,4,5,6,7].
The C/A-on-C/A effect and lack of a pilot channel for enhanced carrier tracking and acquisition has further motivated some authors to seek replacements of the C/A code that are not backward compatible with most fundamental C/A code receiver operations, including code tracking and bit sync [11, 12]. The proposed C/A code evolutions have ranged from combining C/A code with an 20 chip overlay codes having periods of 20 ms or greater, to replacing C/A code with a longer data-less spreading code signal that serves only to enhance L1C’s pilot component [12]. Such approaches are equivalent to “sunsetting” the C/A code signal, so that its only purpose is as an enhancement to L1C. Longer overlay codes, that are multiples of the C/A code bit period, have also been proposed. Unfortunately, these codes have limited benefit in terms of resolving C/A code spectral lines and do not provide immediate bit sync, as do the 20 chip overlay codes[12].
Despite C/A code’s shortcomings, it has been successfully used in a wide range of applications and is stated to be “near optimal for consumer applications” as evidenced by the fact that every MGNSS receiver today includes C/A code. In some cases, the sensitivity and TTFF performance of C/A code in MGNSS receivers meets or exceeds that of modernized signals like B1C or Galileo’s E1 signal by leveraging decades old techniques to overcome C/A code’s shortcomings [8,9,10 ].
Rather than replacing C/A code outright, this paper proposes a backward compatible approach to modernizing the C/A code signal that can be implemented using algorithms and codes already resident in today’s mGNSS receivers. This allows for the addition of advanced features to C/A code, such as a pilot channel that provides a coherent tracking channel with high reliability bit sync, and frame sync, while retaining backward compatibility with legacy receivers in all modes of operation, and simultaneously improving cross-correlation correlation properties as compared to legacy C/A code.
The work will provide the design, performance simulation, and measurements a new backward compatible approach to modernizing C/A code that simultaneously reduced C/A on C/A effects, improving spectral line behavior and providing C/A code with many enhancements of modernized signals. The paper will compare the modernized C/A approach with alternative approaches such as enhanced C/A (eC/A) in terms of performance, backward compatibility, mutual interference, and spectral line behavior [11,12]. An incremental approach to implementing the modernized C/A code will also be described.

1. C. R, Cahn, M. M. Goutmann, G. P. Haefner, “System 621B Signal Definition Study,” Technical Report Space and Missile Systems Office TR-72-248 Vol. 1, October 1972.
2. B Bradford Parkinson, et. al, “`Global Positioning System: Theory and Applications,” First edition, pg. 105, 1996.
3. A. J. Van Dierendonck, Robert Erlandson and Gary McGraw, Robert Coker “Determination of C/A Code Self-Interference Using Cross-Correlation Simulations and Receiver Bench Tests,” ION GPS 2002.”
4. A.J. Van Dierendonck, R.J. Erlandson, Karl Shallberg, “The Inadequacy of the Spectral Separation Coefficient and Aggregate Gain Factor for Quantifying the Effects of GPS C/A Code Self Interference,“ Proceedings of ION GNSS+, 2013.
5. Robert Golshan, Tiange Fan, Alberto Arredondo and Tom Stansell, “Implications of C/A-on-C/A Interference on Carrier Tracking Loop Performance,” ION GNSS+ 2014.
6. A. Cerruti, J. Betz J and J. Rushanan, “Further investigations into C/A on- C/A interference, “ Proceedings of 2014 International Technical Meeting of the Institute of Navigation, January2014
7. Christopher J. Hegarty, “A simple model for GPS C/A-code self-interference,” NAVIGATION. 2020;67:319–331.
8. Van Diggelen, Frank, “Who’s Your Daddy?”, Inside GNSS, March/April 2014.
9. Van Diggelen, Frank, “The Smartphone Revolution,” GPS World, Dec. 1 2009.
10. See uBlox Max-M10 datasheet, table 3, MAX-M10S typical performance in single-GNSS modes, https://content.u-blox.com/sites/default/files/MAX-M10S_DataSheet_UBX-20035208.pdf
11. John W. Betz, “On the Power Spectral Density of GNSS Signals, with Applications,” ION 2010 International Technical Meeting January 25-27, 2010.
12. Thomas A. Stansell, John W. Betz, Frank Van Diggelen, Satoshi Kogure, “Proposed Evolution of the C/A Signal, ION, GNSS+ 2015.