Mitigating the Squaring Loss in the WAAS C/A Code

Phillip Ward, Kenneth Barron

Abstract:	The Federal Avtatton Agency (FAA) Wide Area Augmentation System (WAJS) geostationary satellites will transmit L I C/A-codes compatible with the code division multrple access and carrier frequency characteristics of the Standard Positionmg Service (SPS) Global Positioning System (GPS) antennas and receivers. However, the designs of the GPS receiver code generators need to be changed IO repkate the FAA WAAS C/A-codes. Other differences in the WAAS signals also require GPS receiver design changes The WAAS receiver channels will each require a Viterbi decoder. The broadcast data containing the GPS integrity message, the differential corrections and the reconstructed ionospheric model for the FAA WAAS avionics user segment requires a 250 bit per second data rate (SO0 symbols per second modulation rate) to meet the response time requirements. The data modulatron rate on the conventional GPS satellite signals is only 50 bits per second. The higher data modulation rate on the WA&S Ll C/A-code signals increases the squaring loss, which increases the bit error rate in the data demodulation and decreases the tracking threshold of the receiver channel for this signal Also, the received WAAS stgnal IS weaker than the received GPS C/A-code signal, further compounding the problem. Convolutional coding of the WAAS data message in the satellite and Viterbi decoding in the receiver compensate for the increase in the symbol error rate due to the increased squaring loss and the weaker signal However, the convolutional coding technique provrdes no compensation for the loss in tracking threshold. Thus, the WAAS channel becomes the weak link of the modified avionics SPS GPS receiver. This paper describes a tracking loop technique which utilizes the WAAS convolutional coding technique to mitigate the increased squaring loss and the weaker signal effects on the tracking thresholds of the WAAS receiver channels.
Published in:	Proceedings of the 1996 National Technical Meeting of The Institute of Navigation January 22 - 24, 1996 Loews Santa Monica Hotel Santa Monica, CA
Pages:	827 - 841
Cite this article:	Ward, Phillip, Barron, Kenneth, "Mitigating the Squaring Loss in the WAAS C/A Code," Proceedings of the 1996 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1996, pp. 827-841.
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