Analysis of Code Cross Correlation Noise in GPS Receivers Operating in Augmented GPS Systems

摘要

A number of proposed augmentations or dual systems to GPS have been proposed using the GPS C/A codes at the same L1 and L2 frequencies as GPS. These systems rely on the code division multiple access (CDMA) capacity of the C/A to permit use of same frequency by multiple emitters. In evaluating the impact of these sources on the GPS receiver tracking performance it is implicitly assumed that the interference due to partial correlation among various C/A codes (Gold codes) used is negligible compared to the receiver thermal noise level, i.e., the various codes are assumed to be nearly orthogonal. This paper presents an in depth analysis of this interference problem for the case of an non-coherent delay lock loop. A closed form expression is derived for the interference power spectral density in terms of discrete cross correlation functions between the C/A codes of the reference code being tracked and the C/A codes of the interfering satellites, and relative Doppler and range delays among various satellites.rnThe analysis of the paper explicitly shows the dependence of the interference power on various factors including relative Doppler and code delays among various signals. Generally, lower relative Doppler and higher relative delay (but much higher than chip period) correspond to higher interference level. Higher differential delay results in increased interference contribution due to 50 bps data modulation. Several examples show that the interference level can vary over a wide range depending upon various parameters. For arnpair of interfering satellites with low relative carrier Doppler (～50 -100 Hz) , the interference can be - 40 dB-Hz or higher relative to carrier power level which is comparable to the receiver noise level. Such a scenario may occur in relatively small but significant fractions of time and area on earth. This paper, as an example, analyzes some variations of a proposed GPS augmentation to determine the added CDMA noise by such an increase of the satellites in view for any GPS user. A conclusion of this paper is that the number of signal sources in the augmented system must be limited so that the interference level does not exceed the specified limits for any class of services provided by GPS.