Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 240

code signal, and the fact that [2] assures that the same Y code signal is broadcast on
both L1 and L2, whereas the semicodeless techniques further exploit a deduced
relationship between the Y code and P code.
Since they operate without full knowledge of the Y code signal, the codeless and
semicodeless designs operate at significantly reduced SNRs, which requires the
tracking loop bandwidths to be extremely narrow. This, in turn, reduces their ability
to operate in a high dynamic environment without aiding. Fortunately, robust aid-
ing is generally available from tracking loops operating upon the C/A code signal.
Typical codeless and semicodeless receiver designs use L1 C/A carrier tracking loops
to effectively remove the LOS dynamics from the L1 and L2 Y code signals, and then
extract the L1-L2 differential measurements by some variation of a signal squaring
technique that does not require full knowledge of the replica code. Codeless tech-
niques effectively treat the Y code PRN as 10.23-Mbps data, which can be removed
through squaring or by cross-correlation of the L1 and L2 signals. Semicodeless
techniques exploit some known features of the Y code (e.g., [25]). In addition to the
signal-to-noise disadvantage mentioned earlier, codeless receivers suffer from other
robustness problems. Although the parallel C/A-code processing provides access to
the GPS navigation message, codeless processing of L2 does not allow decoding of
the navigation data for the purpose of verifying that the desired SV is being tracked.
Also, two SVs with the same Doppler will interfere with each other in the codeless
mode; therefore, the scheme fails for this temporary tracking condition. Modern
semicodeless receivers, on the other hand, provide relatively robust tracking of the
L2 Y code signal with assistance from the L1 C/A code. These concepts will become
obsolete when the modernized GPS civil signals become available.

References

[1] Ward, P. W., “An Inside View of Pseudorange and Delta Pseudorange Measurements in a

Digital NAVSTAR GPS Receiver,” Proc. of ITC/USA/'81 International Telemetering Con-
ference, GPS-Military and Civil Applications , San Diego, CA, October 1981.

[2] IS-GPS-200, NAVSTAR GPS Space Segment/Navigation User Interfaces (Public Release

Version) , ARINC Research Corporation, El Segundo, CA, December 7, 2004.

[3] Ward, P. W., “An Advanced NAVSTAR GPS Multiplex Receiver,” Proc. of IEEE PLANS

'80, Position Location and Navigation Symposium, December 1980.

[4] Kohli, S., “Application of Massively Parallel Signal Processing Architectures to GPS/Inertial

Systems,” IEEE PLANS '92 Position Location and Navigation Symposium , April 1992.

[5] Townsend, B., et al., “Performance Evaluation of the Multipath Estimating Delay Lock

Loop,” Proc. of ION National Technical Meeting, January 1995, pp. 277-283.

[6] Przyjemski, J., E. Balboni, and J. Dowdle, “GPS Anti-Jam Enhancement Techniques,” Proc.

of ION 49th Annual Meeting , June 1993, pp. 41-50.

[7] Ward, P., “Performance Comparisons Between FLL, PLL and a Novel FLL-Assisted-PLL

Carrier Tracking Loop Under RF Interference Conditions,” Proceedings of the 11th Inter-
national Technical Meeting of The Satellite Division of The Institute of Navigation , Nash-
ville, TN, September 1998, pp. 783-795.

[8] Stephens, S. A., and J. C. Thomas, “Controlled-Root Formulation for Digital Phase-Locked

Loops,” IEEE Trans. on Aerospace and Electronic Systems , January 1995.

[9] Ward, P., “Using a GPS Receiver Monte Carlo Simulator to Predict RF Interference Perfor-

mance,” Proc. of 10th International Technical Meeting of The Satellite Division of The
Institute of Navigation , Kansas City, MO, September 1997, pp. 1473-1482.