Noise Floor Independent Delay Lock Loop Discriminator

Gang Xie and Xiang Yuan

Abstract:	In a typical Global Positioning System (GPS) receiver, a carrier tracking loop and a code tracking loop are used to track the carrier and C/A code of a received GPS satellite signal, respectively. The code tracking loop is often implemented in a form of Delay Lock Loop (DLL), where a discriminator estimates its code phase tracking error, i.e., the phase difference between the received C/A code and the receiver’s duplicated C/A code. There are several well-known DLL discriminator functions, of which the normalized Early-Minus-Late (E-L) envelope discriminator function is fairly fashionable. However, this E-L discriminator requires the estimation of noise floor in noncoherent integration results. Though there are several approaches of estimating the noise floor, the estimation cannot be accomplished very accurately. If the E-L discriminator cannot be given an accurate estimate of noise floor, the DLL will deliver inaccurate code phase measurements accordingly, which finally result in receiver positioning errors. Meanwhile, inaccurate DLL discriminator outputs will also eventually cause the receiver to lose track on signals. For weak GPS signal tracking, this DLL discriminator problem can be more critical and should be solved. In order to overcome the above difficulties in the E-L discriminator, this paper proposes an improved normalized E-L envelope discriminator which is independent of noise floor. In other words, this noisefloor independent (NFI) E-L discriminator itself does not require the estimation of noise floor, and certainly its performance does not depend on the accuracy of noise floor estimation. All simulation and bench tests demonstrate that the NFI E-L discriminator can improve the accuracy of both code phase measurements and GPS position fixes. Meanwhile, it can also help the receiver to more robustly track weak signals.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	191 - 196
Cite this article:	Xie, Gang, Yuan, Xiang, "Noise Floor Independent Delay Lock Loop Discriminator," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 191-196.
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