Performance Evaluation of High Sensitivity GNSS Techniques in Indoor, Urban and Space Environments

Abstract

Conventional GNSS receivers have problems in weak signal environments making it difficult to provide GNSS position fixes, and this constitutes one of the bottlenecks in the extension of location services in indoor and dense urban conditions. Besides the signal attenuation, the main limitations faced in such environments are due to the complex propagation of the GNSS signals (multipath) and due to the different attenuation of the signals coming from different satellites, which may cause cross-correlation peaks to be on the order or even higher than the true autocorrelation one, also known as near-far (NF) problem. These limitations have led to four main groups of strategies with all sorts of combinations of them in the form of hybridized receivers: - High-sensitivity GNSS receivers, - Positioning using sensors (inertial, pressure, optical…), - Positioning using telecom/wireless networks, - GNSS pseudolites. High-sensitivity GNSS receivers are the stand-alone approach to cope with the weak signal problem, these receivers are designed to exploit the little energy that reaches them, which in general terms has been translated into the use of open-loop or snapshot architectures that dwell on the incoming signal for extremely long periods (compared to conventional GNSS receivers). By means of long non-coherent integration, they can achieve low sensitivity, and avoid the effects of bit transitions and clock drifts [1]. The idea of High-sensitivity GNSS receivers is neither new nor original. In most cases previous studies have focused on weak signal acquisition. A thorough review of the indoor challenges and techniques for weak signal acquisition can be found in [2]. However, optimal weak signal tracking and transition from acquisition to tracking has not been sufficiently addressed, some examples can be found in [3] and [4], but in this work the focus was not put into high-sensitivity techniques. In [5] while tracking techniques have been considered, only the urban case (CN0>20 dBHz) with a focus on multipath estimation has been studied. Further studies on high-sensitivity tracking techniques and characterization of the loops in the vicinity of their threshold are needed. Regarding the multipath problem several strategies can be found in literature to mitigate its effects [6], [7]. Some novel works have shown that it can be possible to use this effect, usually considered adverse, to improve the receiver positioning in weak signal environments [8]. With respect to the near-far problem, inherent protection by the use of spreading codes with cross-correlation margins on the order of 24-28 dB may not be sufficient in the indoor case. Some techniques mainly based in the existing background in multiuser detection techniques for CDMA wireless communications are reviewed in [2]. However, research onto low-complexity near-far mitigation techniques is still an open area and needs further study. Longer spreading codes in modernized GNSS signals (including Galileo) provide better protection against near-far but this also brings more complex processing. Trading-off the advantages gained by the increased protection versus the added complexity is also an open research line. Moreover, the usage of high-sensitivity techniques is not only restricted to the urban/indoor case other use cases such as space applications (HEO/LEO orbits) need these type of receiver architectures. Multipath, the propagation channel, and user dynamics are vastly different but the signal attenuation can be similar to the indoor case. Therefore modified techniques based on the extensive works already done for the indoor/urban case need to be studied and tailored to this use case.