Improving GNSS Bit Synchronization and Decoding Using Vector Tracking

T. Ren, M. Petovello, C. Basnayake

Abstract:	The studies on high sensitivity (HS) GNSS receivers design have established that extending the integration time is the most effective way to improve sensitivity. Furthermore extended coherent integration time is reported to facilitate the mitigation of two important positioning problems in challenging environments, which are multipath and cross-correlation false locks, and it completely avoids squaring loss. However, the preferred coherent integration time is limited by the GNSS navigation data bits modulated on the carrier (e.g., 20 ms period for GPS L1 C/A signal). Bit-aiding from either network broadcasting or the most recently stored navigation message can help to overcome the coherent integration period limitation, but in these cases the receiver will lose its autonomy with a corresponding increase in complexity and cost. In the interest of extending the coherent integration time but not requiring external information, the Maximum-Likelihood (ML) estimation method is preferred to estimate navigation message bit boundaries and bit values without assistance from external sources. However, ML bit synchronization and decoding algorithms require the carrier frequency and the ranging code phase to be well tracked. But in challenged environments, to maintain the carrier and code tracking in the individual channel (i.e., scalar tracking) is usually very difficult. The tracking loop bandwidth should be reduced in order to suppress noise in the degraded environment. This can easily result in losses-of-lock due to user dynamics. This feature motivates the utilization of other advanced tracking architectures for bit synchronization and decoding. The vector tracking algorithm can improve sensitivity by effectively sharing tracking information across different channels. Especially, if sufficient line-of-sight (LOS) signals exist, the tracking performance of obscured satellite signals noticeably improves. The mixture of LOS and non-LOS satellites generally exists in challenging environments such as dense foliage and urban canyons. The LOS signals are generally the satellites having higher elevation angles. On the contrary, non-LOS signals are usually the satellites with lower elevation angles. Once bit synchronization and decoding is achieved successfully in degraded communication channels, the coherent integration time is expected to be extended accordingly, thereby increasing the signal-to-noise ratio (SNR). After the post-coherent SNR passes a certain threshold, more satellites are expected to be included in the navigation filter to improve the dilution-of-precision (DOP) and hence the accuracy of the navigation solution in challenging environments. This paper presents the analysis on the utilization of vector tracking for the ML bit synchronization and decoding. The objective of this work is to determine the benefits of using vector tracking architecture in standalone mode to improve bit synchronization and decoding, and subsequently to determine if this improves the ability to extend coherent integration time and finally improves the navigation solution. More specifically, the improvement of the ML bit synchronization and decoding in vector tracking is quantified in terms of the successful synchronization rate, successful decoding rate, and decoding time as a function of the number of LOS satellites and position and velocity accuracies. This paper aims at land vehicle application, and the required apriori information, e.g., the initial position, user clock bias and drift can always be retrieved from the position records stored at last stop (i.e., when the vehicle was turned off), and the long term ephemeris is available by prediction. The impact from the accuracy of this apriori information (except ephemeris) will be researched in this work. Except for clock bias and drift, the other information (e.g., initial position, velocity) can be seen as not aging since last stop. The proposed method will be evaluated in the software receiver (GSNRx-wsTM) developed by the PLAN group, University of Calgary. Two sets of GNSS data have been collected already. One is the simulated data from the Spirent GNSS simulator which simulated signal-challenged conditions. The other is the field data collected with vehicular dynamics in dense foliage and urban canyon environments. The preliminary results show that the vector tracking can improve GNSS bit synchronization and decoding by 3 dB to 5 dB.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	121 - 134
Cite this article:	Ren, T., Petovello, M., Basnayake, C., "Improving GNSS Bit Synchronization and Decoding Using Vector Tracking," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 121-134.
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