James T. Curran

Improving the Design of Frequency Lock Loops for GNSS Receivers

An analysis of the design and performance of the discrete-update GNSS frequency lock loop (FLL) is presented. Expressions for the design and the steady-state performance of one first-order and two second-order FLL loop filters are developed. Transient performance of the FLL in the presence of thermal noise is examined and the relative performance of four carrier frequency discriminators is studied. It is shown that careful discriminator choice can yield a 10 dB performance improvement relative to naive designs.

Reducing Front-End Bandwidth May Improve Digital GNSS Receiver Performance

A novel evaluation of filtering and quantization losses for weak DS-CDMA receivers is presented. Using this method, joint optimization of filter center frequency and bandwidth is conducted for one-, two- and three-bit quantizers. It is demonstrated that a joint loss-analysis of these effects is necessary to optimize front-end design.

Front-end filtering and quantisation effects on GNSS signal processing

Traditionally, the effects of presampling filtering and of quantisation on the processing of GNSS signals have been dealt with in isolation. Analysis of the losses incurred during the quantisation process has almost invariably been based on the assumption that the signals at IF are distorted by additive white Gaussian noise. This paper, in contrast, considers the joint effect of filtering and quantisation, illustrates the need to consider these losses jointly and presents novel expressions for the total loss incurred.

Indoor GPS positioning using a slowly moving antenna and long coherent integration

A method to enhance the indoor positioning accuracy by alleviating the effect of multipath distortion is presented. Using this method the line-of-sight (LOS) and non-line-of-sight (NLOS) signals are separated in the frequency domain and multipath induced errors in the pseudorange measurements are decorrelated at a faster rate than under static antenna conditions, resulting in improved positioning accuracy with longer coherent integrations.

Improving GNSS carrier-phase tracking via oscillator g-sensitivity compensation

In this paper the influence of oscillator g sensitivity on carrier-phase tracking performance in a global navigation satellite-system receiver is examined. It is illustrated that the local oscillator can be a limiting factor in the tracking threshold when the receiver is subject to pedestrian or vehicular dynamics. A novel feed-forward compensation technique is presented, which draws upon the inertial measurement unit resources of the receiver to alleviate these detrimental oscillator-based effects. Performance evaluation considered live global navigation satellite-system signals, a range of consumer-grade oscillators, and both high-quality and Microelectromechanical systems (MEMS)-grade inertial measurement units, in both pedestrian and vehicular operating environments. The results show that the process-noise spectral density corresponding to the signal acceleration state of the Kalman-filter-based tracker can be reduced significantly with compensation. It is also shown that three-dimensional velocity error reduces from 0.17 to 0.06 m/s with compensation.

Ionospheric scintillation threats to GNSS in polar regions: The DemoGRAPE case study in Antarctica

This paper addresses the design and implementation of an Ionospheric Scintillation Monitoring Receiver based on the Software Defined Radio paradigm. The monitoring platform exploits a digital data grabber and a GNSS fully software receiver, which grants a high level of flexibility for the processing strategy and the storage of raw samples of the signals in case of meaningful scintillation events. Such an implementation approach yields valuable advantages in critical areas, such as polar regions, where resources are limited and installation or maintenance and replacement of GNSS receivers may be critical. The paper describes the successful installations of the platforms in two Antarctic stations, providing results at the same quality level of professional GNSS receivers used for ionospheric scintillation monitoring.

Coding Aspects of Secure GNSS Receivers

This paper presents an overview of the coding aspects of a GNSS receiver. Coding allows detection and correction of channel-induced errors at the receiver, here the focus is on the mitigation of threats from malicious interferences. Although the effects of interference at different stages of GNSS baseband processing has been deeply analyzed in the literature, little attention was devoted to its impact on the navigation message decoding stage. Theis paper provides an introduction to the various coding schemes employed by current GNSS signals, discussing their performance in the presence of noise in terms of block-error rate. Additionally, the benefits of soft-decoding schemes for navigation message decoding are highlighted when jamming interferences are present. The proposed scheme requires estimating the noise plus interference power, yielding to enhanced decoding performances under severe jamming conditions. Finally, cryptographic schemes as a means of providing anti-spoofing for geosecurity location-based services, and their potential vulnerability are discussed, with particular emphasis on the dependence on the dependence of the scheme on successful navigation message decoding.

A Ray-Tracing Technique to Characterize GPS Multipath in the Frequency Domain

Multipath propagation is one of the major sources of error in GPS measurements. In this research, a ray-tracing technique is proposed to study the frequency domain characteristics of multipath propagation. The Doppler frequency difference, also known as multipath phase rate and fading frequency, between direct (line-of-sight, LOS) and reflected (non-line-of-sight, NLOS) signals is studied as a function of satellite elevation and azimuth, as well as distance between the reflector and the static receiver. The accuracy of the method is verified with measured Doppler differences from real data collected in a downtown environment. The use of ray-tracing derived predicted Doppler differences in a receiver, as a means of alleviating the multipath induced errors in the measurement, is presented and discussed.

Enhancing Weak-Signal Carrier Phase Tracking in GNSS Receivers

Examining the performance of the GNSS PLL, this paper presents novel results describing the statistical properties of four popular phase estimators under both strong- and weak-signal conditions when subject to thermal noise, deterministic dynamics, and typical pedestrian motion. Design routines are developed which employ these results to enhance weak-signal performance of the PLL in terms of transient response, steady-state errors, and cycle-slips. By examining both single and data-pilot signals, it is shown that appropriate design and tuning of the PLL can significantly enhance tracking performance, in particular when used for pedestrian applications.

An SNR Dependent Model for the CDMA FLL

The tracking bandwidth and variance of the code-division multiple-access (CDMA) frequency-lock loop (FLL) are considered in this correspondence. It is shown that the behavior of the discriminator can vary significantly with varying SNRc with corresponding variations in loop performance. A model is presented which reflects this signal-to-noise ration (SNR)-sensitivity and, thus, facilitates accurate loop design.