J.A. Garcia-Molina

Cloud GNSS receivers: New advanced applications made possible

The widespread deployment of GNSS (Global Navigation Satellite Systems) is pushing the current receiver technology to its limits due to the stringent demands for providing seamless, ubiquitous and secure/reliable positioning. This fact is further aggravated by the advent of new applications where the miniaturized size, low power consumption and limited computational capabilities of user terminals pose serious concerns to the implementation of even the most basic GNSS signal processing tasks (e.g. as in Smart-City or IoT applications). This work presents a paradigm shift for the implementation of next-generation GNSS receivers by taking advantage of Cloud computing platforms, thus leading to the concept of Cloud GNSS receiver.

Unambiguous tracking of high-order BOC signals in urban environments: Channel considerations

The usage of BOC modulations in GNSS provides a better code tracking accuracy than BPSK modulation at the cost of new correlation peaks appearing in the autocorrelation function. For high-order BOC modulations side peaks might be only few meters away from the main correlation peak to be tracked, so there is a risk to lock into one of those peaks, inducing therefore a bias in the pseudorange estimation. The probability of false lock can be in particular important in harsh channel conditions, where the unambiguous tracking can be difficult due to the low C/No and the presence of multipath, which impact can be relevant during fading periods. This paper presents typical channel conditions that should be considered in the design and assessment of unambiguous tracking techniques of high-order BOC signals, focusing in urban environments where the receiver might need to track the signal even at low C/No. A preliminary assessment of some state-of-the-art techniques in urban environments is presented, showing the limitations of some of the techniques, which have been designed considering very mild propagation conditions typically present in open sky environments. The multi-correlator approach is then presented as a possible way to overcome the limitations of techniques based on legacy GNSS receiver architectures. In this case the unambiguous tracking problem is presented as the minimization of a non-linear Least Squares cost function subject to restrictions and some design alternatives are discussed.

Performance analysis of hybrid GNSS and lte localization in urban scenarios

Severe performance degradation of Global Navigation Satellite Systems (GNSS) is produced in urban scenarios, mainly due to dense multipath and non-line-of-sight (NLoS) conditions. Thus, the integration of GNSS with additional positioning systems, such as the location methods in Long Term Evolution (LTE) cellular systems, may cope with these challenging scenarios. This work proposes a generic evaluation model to assess the performance of hybrid GNSS and LTE positioning in representative urban environments. This assessment considers field GNSS observables and simulated LTE time-of-arrival (ToA) measurements. The evaluation results show the need to enhance hybrid positioning solutions within future cellular standards.

Synchronisation of low-cost open source SDRS for navigation applications

This paper describes a new method for the synchronisation of multiple low-cost open source software-defined radios (SDR). This solution enables the use of low-cost SDRs in interesting navigation applications, such as hybrid positioning algorithms, interference localisation, and cooperative positioning among others. Time synchronisation is achieved thanks to a time pulse that can be generated either by one of the SDRs or by an external source, such as a GNSS receiver providing 1PPS signal. Experimental results show that the proposed method effectively reduces the synchronisation offset between multiple SDRs, to less than one sampling period.

Land mobile multipath channel reduction effects on a real GNSS receiver

One of the current limitations to evaluate the impact of a realistic multipath environment over a GNSS receiver is the high number of multipath rays that are needed to model the channel in a HW RF constellation simulator. That makes necessary the application of reduction techniques to decrease the number of channels per satellite to be simulated. The initial evaluation of two reduction techniques based on the solution of optimization problems is presented for an urban vehicle scenario, focusing the study on the evaluation of the multipath effect over code pseudorange measurements.

GNSS receiver performance assessment with a realistic aeronautical channel model

A multipath model for the aeronautical channel, which was derived from a channel measurement campaign, has been implemented into hardware simulators. The way of implementation is described in detail and simulation results obtained with a GNSS receiver and showing the impact of multipath are presented.

Mitigation of false locks in the acquisition of high-order BOC signals in HS-GNSS receivers

This paper addresses the problem of acquiring weak high-order binary offset carrier (BOC) signals in the context of high-sensitivity global navigation satellite system (HS-GNSS) receivers. The use of high-order BOC modulations provides an improvement over conventional BPSK modulation in terms of positioning accuracy, at the cost of secondary peaks appearing in the correlation function. The acquisition of the main peak is a critical problem because the presence of secondary peaks often introduces a bias in the time-delay estimation. This problem is even more complicated to address when the signal is received with a low carrier-to-noise ratio. In this situation, we must resort to Post-Detection integration (PDI) techniques to acquire the received signal, and after that an estimator can be applied to identify the main correlation peak. The contributions of this work are twofold. Firstly, we analyze the performance of PDI techniques such as the Non-coherent PDI and the Differential PDI to acquire weak high-order BOC signals in static and dynamic channels. Secondly, after detecting the signal by using a PDI technique, we propose to apply the Maximum Likelihood (ML) and the Least Square (LS) estimators to estimate the time-delay of the main peak.

Computational performance of a cloud GNSS receiver using multi-thread parallelization

The proliferation of GNSS-based (Global Navigation Satellite Systems) services and applications providing ubiquitous, seamless and secure/reliable positioning is driving the use of high-performance devices. This implies the requirement of a higher computational capability in miniaturized size and low power consumption devices such as smartphones or Smart- City sensors. In this context, a possible alternative is to carry out the computational tasks outside the device, making use of the scalable, secure and nearly unlimited resources of Cloud infrastructure. This work presents the implementation of a Cloud-based GNSS receiver by taking advantage of one of the available Cloud infrastructures, such as Amazon Web Services (AWS). We will provide a review of the most relevant features of AWS for GNSS signal processing, as a case of study for stimulating the use of Cloud infrastructures within the GNSS community, while paving the way for the development of nextgeneration GNSS receivers. Furthermore, parallel computing will be studied to improve the Cloud GNSS receiver performance.

Subcarrier slip detection for high-order BOC signals

The most recent generation of Global Navigation Satellite Systems (GNSS) are implementing Binary Offset Carrier (BOC) modulation. The advantage of BOC signals is that the main lobe of the correlation is very narrow, but on the other hand, they present side peaks, i.e. multiple stable tracking points. For high-order BOC signals, the amplitude of the side peaks can be similar to the amplitude of the main one or even exceed it under specific scenarios. Some techniques to mitigate the code ambiguity exploit the fact that BOC signals can be understood as the sum of two BPSK signals. Therefore, BOC signals are composed by three components: the code, the subcarrier and the carrier. Code gives unambiguous but noisy measures, whereas that the subcarrier tracking gives ambiguous but precise measurements. The technique presented in this paper solves the ambiguity combining these two delays: the same principle as the carrier smoothing techniques is exploited. Additional, a new subcarrier slip detector is presented in order to detect the subcarrier slips and reset the code smoothing. This paper is focused on high-order BOC signals as the BOC(15,2.5) present in the European Galileo System. The performance of the new algorithm has been evaluated though several simulations using the DLR channel model and other synthetic scenarios.