A Modified Frequency-locked Loop for Improved WAAS Carrier Tracking

Gary McGraw and Bernard Schnaufer

Abstract:	The proposed Wide Area Augmentation System (WAAS) downlink data consists of 250 bits in each second, 2-to-1 convolutionally encoded, yielding a 500 Hz BPSK symbol rate. This higher symbol rate requires the use of a Viterbi decoder to re- cover the 250 Hz data and may require modifica- tion in receiver carrier tracking loop designs as well. Frequency-locked loops (FLLs) are used in many GPS receivers instead of Costas loops because of the supe- rior dynamic tracking capabilities of FLLs. However, the method of data demodulation commonly used in FLLs has a tendency to yield bursts of data bit errors, negatiug the coding gain offered by the Viterbi de- coder. Loss of the WAAS integrity data, even if only for a few seconds, is clearly undesirable in many sit- uations. This paper discusses the problems that the WAAS data coding and modulation format present to current carrier loop designs. A modified FLL data demodulation process is proposed and simulation re- sults presented. The WAAS data error and cycle slip performance of this new FLL demodulation scheme is at least as good as a Costas loop, yet retains the other advantages of an FLL.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	1553 - 1562
Cite this article:	McGraw, Gary, Schnaufer, Bernard, "A Modified Frequency-locked Loop for Improved WAAS Carrier Tracking," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 1553-1562.
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