Proposal for a Vector Tracking Architecture for a Dual Constellation L1/E1 GPS/Galileo Receiver

Abstract : In urban environments, standalone GNSS receiver architectures can be strongly affected to the point of not being able to provide a position accuracy suitable for use in vehicular applications. Specifically, the reception of GNSS signals is affected by the surrounding objects, such as high buildings, trees, lampposts and so on, which can block, shadow, reflect and diffract the received signal. As a result, two significant signal distortions are generated. On one hand, the reception of reflected or diffracted GNSS LOS echoes in addition to the direct LOS signal generates the phenomenon known as multipath. Multipath echoes represent one of the most detrimental positioning error sources in urban canyons. In fact, the reception of echoes distorts the ideal correlation function and leads to a degradation of the signal code and carrier estimations accuracy up to a loss of lock of the code and carrier tracking loops. Consequently, the pseudo-range and Doppler measurements are degraded. On the other hand, the total or partial obstruction of the GNSS LOS by the urban environment obstacles causes GNSS LOS blockage or GNSS LOS shadowing phenomena. The reception of Non-LOS (NLOS) signals introduces a bias on the pseudo-range measurements if only NLOS satellites are tracked. This bias can be very important as it is representative of the extra-path travelled by the NLOS signal compared to the theoretical LOS signal. The LOS shadowing can also decrease the LOS signal C/N0 and thus makes the signal more vulnerable to the multipath effect. Finally, the resulting degraded measurements cause the navigation processor to compute an inaccurate position solution or even to be unable to compute one in the case of few available measurements. Therefore, advanced GNSS signal processing techniques must be implemented in order to improve the navigation solution performance in urban environments. A promising approach for reducing the effect of multipath interference and NLOS reception is vector tracking (VT), first introduced in [1] where the signal tracking and navigation solution tasks are accomplished by the central navigation filter. In comparison to conventional or scalar tracking (ST), where each visible satellite channel is being tracked individually and independently, VT exploits the knowledge of the receiver’s position and velocity that can be further used to aid the receiver’s tracking performance. In [1], the navigation filter replaces the delay lock loops (DLLs) with an Extended Kalman filter (EKF), responsible for computing the navigation solution that drives the Numerical Control Oscillator (NCOs) of each tracking channel. Vector DLL (VDLL) tracking performance of the GPS L1 signal in weak signal-to-noise ratio (SNR) environment and robustness against signal interference and attenuation has been demonstrated in [2], [3] and [4]. The objective of this paper is to assess the performance of the Vector Delay Frequency Lock Loop (VDFLL) architecture in multipath and NLOS reception in a dense urban environment. The most evident advantages introduced by this method are robustness to momentary and individual satellite signal blockages, channel coupling and accumulative signal power facilitating the tracking of low C/N0 channels and an increased tracking performance in weak-signal or interference/jamming-affected environments. In this paper, a dual constellation GPS + Galileo single frequency L1/E1 VDFLL architecture is presented since this type of receiver can signi?cantly improve the availability of a navigation solution in urban canyons and heavily shadowed areas: an increased satellite in-view availability is directly translated in a higher measurement redundancy and improved position reliability. In the implementation proposed in this paper, the DLL and FLL discriminator outputs for both the L1 and E1 tracking channels will be accommodated in the measurement vector of the EKF navigation filter. The chosen EKF navigation filter system model is a (8x1) position and velocity state vector, containing the six position/velocity terms in ECEF coordinates and the receiver clock bias and drift terms. The code delay and Doppler frequency estimations, computed from the EKF navigation solution, will drive in the feedback loop of the code and carrier NCO updates for each GPS BPSK and Galileo BOC tracking channels. The innovation of this work consists in the design of a dual constellation US GPS + European Galileo single frequency band L1/E1 GNSS receiver, with an emphasis on the detailed mathematical formulation of EKF filter state and measurement model. In specific, great effort will be given to the description of the filter measurement innovation vector and the passage from the error to the absolute state configuration. The main objective of this paper is to perform a detailed comparison of the performance assessment between the conventional tracking, GPS-only- and dual GPS/Galileo VDFLL architectures in two different levels: - System level: expressed in terms of user’s position and velocity estimation accuracies, position and velocity errors bounding and resistance to degraded signal reception conditions; - Channel level: indicated by the code delay and carrier Doppler frequency accuracies. A realistic car trajectory in dynamic condition in urban scenario will be used. Moreover, several signal outages and significant power drops are simulated in different satellite channels in order to observe the tracking sensitivity of the proposed VDFLL architecture. 3D positioning root mean square error (RMSE) have been computed for the conventional tracking, GPS L1 VDFLL, and GPS/Galileo VDFLL architectures. Contrary to the conventional tracking, the L1/E1 VDFLL loop is able to recover the frequency and code-delay estimation at the end of the simulated outages.
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Communication dans un congrès
ION ITM 2016, International Technical Meeting, Jan 2016, Monterey, United States. ION
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https://hal-enac.archives-ouvertes.fr/hal-01271972
Contributeur : Laurence Porte <>
Soumis le : mardi 9 février 2016 - 21:10:28
Dernière modification le : mercredi 10 octobre 2018 - 10:02:02

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  • HAL Id : hal-01271972, version 1

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Enik Shytermeja, Axel Javier Garcia Peña, Olivier Julien. Proposal for a Vector Tracking Architecture for a Dual Constellation L1/E1 GPS/Galileo Receiver. ION ITM 2016, International Technical Meeting, Jan 2016, Monterey, United States. ION. 〈hal-01271972〉

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