Trade-off Analysis of Robust Carrier Phase Tracking Techniques in Challenging Environments

R. Sarnadas, T. Ferreira, J. Vilà-Valls, G. Seco-Granados, J.A. López-Salcedo, F.D. Nunes, F.M. G. Sousa, P. Crosta, F. Zanier, R. Prieto-Cerdeira

Abstract:	Carrier phase estimation techniques are typically used in GNSS applications for which the best solution performances are required. Such applications include precise ranging (e.g. using PPP to reach accuracies as low as a few centimetres), propagation characterization (e.g. in order to characterize atmospheric phenomena), data demodulation and robust tracking, being able to provide a navigation solution even in challenging environmental conditions. Nowadays, a rather benign propagation environment is needed to reliably exploit the benefits of the carrier phase observables, since carrier phase techniques are rather sensitive to fast fading phenomena (e.g. coming from ionospheric scintillations, high dynamics or fading) that can lead to the appearance of cycle slips or even loss of lock. Thus, the interest of techniques to provide robust carrier phase measurements in challenging environments becomes apparent since that would make it possible to extend advanced applications to a new set of scenarios, such as urban environments. This study aims at assessing the performance of innovative carrier phase tracking techniques in stressful environmental conditions in order to understand their limits, characterize their performance and analyse the improvement with respect to traditional techniques. For this purpose, three different families of techniques are explored for carrier phase tracking ranging: - Advanced closed-loop techniques A plethora of closed-loop techniques for robust carrier tracking has been proposed in the literature, such as collaborative inter-loop aiding techniques, adaptive bandwidth PLLs or Kalman filter(KF)-based architectures. Among all these approaches, KF-based solutions provide the best performance and robustness under harsh environments, since they are able to optimally adapt to the current working conditions through a systematic formulation and a flexible architecture. In the present contribution, a performance assessment between standard and adaptive KFs is provided, while at the same time, some innovative modifications to adaptive KF are proposed to cope with scintillation effects. - Open-loop techniques Due to their batch-processing architecture, they are able to cope with the presence of outliers that may cause loss of lock in traditional closed-loop receivers. This is indeed the underlying reason for the widespread deployment of open-loop techniques in high-sensitivity GNSS receivers, where propagation impairments abound. In this work, we focus on the study of innovative open-loop carrier tracking techniques such as those inspired in the wireless communications domain, and those derived from adaptive filtering, such as linear prediction-based carrier tracking. - Vector architectures For the vector architectures we consider two implementations: vector delay/frequency locked loops (VDFLL) and vector delay/phase locked loops (VDPLL). Both structures use an extended Kalman filter (EKF) to predict user’s position, velocity, and clock bias. However, while in the VDFLL, the code and frequency discriminators feed the EKF through the filter residuals, in the VDPLL only the code discriminator outputs are processed by the EKF. In this case, the phase loops are closed using a co-op hybrid solution: the individual loops track the satellite Doppler frequencies and the central loop follows mainly the frequency changes due to receiver’s clock bias and motion. Preliminary results show that the implementations have good performance in harsh scenarios which include ionospheric scintillation and fading. The performance of the previous techniques is assessed under harsh conditions, including: - ITU-R GISM model for the ionospheric scintillations: these are fast fading phenomena that can reach a high spatial and temporal variability and typically occur in equatorial regions. As ionospheric scintillations affect both the magnitude and the phase of the incoming signal, the phase may suffer disturbances, typically synchronized with a deep fade, that could lead to the occurrence of cycle slips (phase jumps), errors in the data demodulation and ultimately loss of lock. - 2-state narrowband LMS model for the urban and suburban environment, simulating channel model for both pedestrian and vehicular users based on a Loo model. This model has been claimed to provide results that are statistically relevant when compared to reality and, it will be used for carrier phase analysis purposes in fading environments. - High dynamics, by simulating accelerations and jerks to the user dynamics, which is where carrier tracking techniques are more prone to loss of lock. On top of the models described previously, AWGN is also added in order to push the techniques to work near the tracking threshold and to assess their sensitivity. Furthermore, the performances are compared to reference techniques such as the classical DLL/ PLL architecture for the scalar techniques or the VDFLL for the case of the vector techniques. Performance is assessed in terms of carrier phase tracking based on accuracy, robustness (e.g. loss of lock and cycle slips) and data demodulation errors. As a conclusion, the objective is not only to identify the limits and the potential improvements of the techniques with respect to the traditional architectures, but also to focus on the identification of the environments/ applications where each of them could be more promising. ACKNOWLEDGEMENTS The content of the present article reflects solely the authors’ view and by no means represents the official ESA or GALILEO views.
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:	3586 - 3599
Cite this article:	Sarnadas, R., Ferreira, T., Vilà-Valls, J., Seco-Granados, G., López-Salcedo, J.A., Nunes, F.D., Sousa, F.M. G., Crosta, P., Zanier, F., Prieto-Cerdeira, R., "Trade-off Analysis of Robust Carrier Phase Tracking Techniques in Challenging Environments," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3586-3599.
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