Cross-correlation Performance Comparison of L1 & L2C GPS Codes for Weak Signal Acquisition

The GPS constellation now contains six fully-operational Block IIR-M satellites. Each of these modernized satellites transmits the second GPS civil signal L2C. One of the key objectives of the L2C signal is to offer high cross-correlation protection, allowing weak signal acquisition in the presence of strong GPS signals in environments such as inside buildings and in urban canyons. The L2C signal achieves this by using longer spreading codes. The L2C signal is composed of CM and CL codes multiplexed together on a chip-by-chip basis. The CM code is 20 milliseconds long while the CL code has a length of 1.5 seconds [1]. The worst-case crosscorrelation performance of CM and CL codes is about 27 dB and 45 dB respectively while that of C/A code is about 21 dB [2, 3]. While these figures immediately suggest L2C as the preferred choice for weak signal acquisition, they are based on the fundamental code period (e.g. 1 millisecond for the C/A code). However, the crosscorrelation noise in a GPS receiver also depends on the size of signal observation interval. Hence, for crosscorrelation performance comparison, the signals must be observed over the same duration. This leads to an interesting situation, where the cross-correlation characteristics of signals are significantly affected by the relative Doppler offset (between the local and incoming satellite signals). This paper presents a detailed comparison of the cross-correlation performance of L1 and L2C signals underdiverse conditions. The paper reveals that a GPS receiverobserving the L2C signal is likely to suffer significantlymore cross-correlation noise (hence requiring moreacquisition effort) than when observing the L1 signal overthe same duration. It is concluded that while the newmodernized GPS civil signal L2C is specifically designedto replace the L1 signal, in weak signal environments([4]), in fact the L1 signal could be the preferred optionfor signal acquisition in such environments.REFERENCES [1] NAVSTAR Global Positioning System InterfaceSpecification IS-GPS-200 revision D, 7 March 2006. [2] A.J.V. Dierendonck, “GPS Receivers”, GlobalPositioning System, Theory and Applications Vol. 1,Table 11, Page 105, 1996. [3] Tran, M., “Performance Evaluations of the New GPSL5 and L2 Civil (L2C) Signals”, Journal of Instituteof Navigation, Vol. 51, No 3, Fall 2004 pp 199-212. [4] Fontana LCDR Richard D., Wai Cheung, Paul M.Novak, Thomas A. Stansell, Jr. (2001), “The New L2Civil Signal”www.navcen.uscg.gov/gps/modernization/TheNewL2CivilSignal.pdf