Cillian O'Driscoll

Coherent, Noncoherent, and Differentially Coherent Combining Techniques for Acquisition of New Composite GNSS Signals

The growing demand of location, navigation and positioning services is boosting the development of new signals and modulations that will be adopted by new global navigation satellite systems (GNSS), such as the European Galileo, the Chinese Compass and the modernized GPS. A common feature of these new modulations is the presence of two channels, the data and pilot components, that separately carry the navigation message and the ranging information. Three different techniques, noncoherent combining, coherent combining with sign recovery and differentially coherent combining, are analyzed for the joint acquisition of data and pilot signals. For each acquisition strategy the probabilities of detection and false alarm are provided. In particular closed-form expressions for the probabilities of coherent channel combining and of the differentially coherent integration strategy are derived. To the best of our knowledge these expressions are new. Monte Carlo simulations are used to support theoretical analysis demonstrating the accuracy of the proposed models.

Composite GNSS Signal Acquisition over Multiple Code Periods

The new signals employed by modern global navigation satellite system (GNSS) will be characterized by a data/pilot structure and by the presence of secondary codes. These innovations require special acquisition techniques for efficiently recovering all the transmitted power and dealing with the problem of sign transitions. In this paper three integration strategies, noncoherent, semi-coherent, and differentially coherent integrations, are analyzed for the joint acquisition of data and pilot signals over multiple primary code periods. Each strategy is statistically characterized and false alarm and detection probabilities are provided. To the best of our knowledge the characterization and comparison of these algorithms are new. Monte Carlo simulations and real data analysis demonstrate the accuracy of the proposed model.

GNSS Jammers: Effects and countermeasures

GNSS jammers are small portable devices able to broadcast disruptive interference and overpower the much weaker GNSS signals. In this paper, the effects of GNSS jammers on GPS and Galileo receivers are thoroughly analyzed and the use of an adaptive notch filter is suggested as an effective countermeasure to jamming. Signals generated by a commercial jammer are broadcast in a large anechoic chamber along with the GPS and Galileo signals generated by a hardware simulator. The analysis is conducted in terms of C/N0 degradation and different jammer power levels are considered. The use of mitigation techniques, such as notch filtering, significantly improves the performance of GNSS receivers even in the presence of strong and fast-varying jamming signals. The presence of a pilot tone in the Galileo E1 signal enables pure PLL tracking and makes the processing of Galileo signals more robust to jamming.

Methodology for comparing two carrier phase tracking techniques

The carrier phase tracking loop is the primary focus of the current work. In particular, two carrier phase tracking techniques are compared, the standard phase tracking loop, i.e., the phase lock loop (PLL), and the extended Kalman filter (EKF) tracking loop. In order to compare these two different techniques and taking into consideration the different models adopted in each, it is important to bring them to one common ground. In order to accomplish this, the equivalent PLL for a given EKF has to be determined in terms of steady-state response to both thermal noise and signal dynamics. A novel method for experimentally calculating the equivalent bandwidth of the EKF is presented and used to evaluate the performance of the equivalent PLL. Results are shown for both the L1 and L5 signals. Even though the two loops are designed to track equivalent dynamics and to have equivalent carrier phase standard deviations, the EKF outperforms the equivalent PLL in terms of both the transient response and sensitivity.

A simplified expression for the probability of error for binary multichannel communications

We present a simplified expression for the probability of error, Pe, for binary multichannel communications. The original expression was derived by Proakis in 1968. More recently, Simon and Alouini presented an expression involving the generalised Marcum Q-function. In this letter we present a simplified form of Simon and Alouinis expression. The application of this result to the calculation of performance metrics for communications over fading channels is also discussed.

Design of a General Pseudolite Pulsing Scheme

Pseudolites, or pseudosatellites, are ground-based transmitters of global navigation satellite system (GNSS)-like signals. Pseudolite signal modulations often include a pulsing scheme that is adopted to reduce interference with nonparticipating GNSS receivers, mitigate the near-far problem, and allow time division multiple access. For these reasons, an effective pulsing scheme design is crucial for the proper functioning of a pseudolite based system. In this paper, the requirements that a pseudolite pulsing scheme has to satisfy are first identified, and a general pulsing scheme based on random permutations is proposed. The spectral and temporal characteristics of the proposed scheme are determined and thoroughly analyzed. From the analysis, it emerges that the proposed scheme meets all identified requirements, making the suggested solution an effective tool for pseudolite signal design. Finally, the proposed approach is compared with a number of existing pulsing schemes from the literature and found to provide significant benefits with respect to most existing schemes.

Pulsed pseudolite signal effects on non-participating GNSS receivers

Pseudolites are a technology with the potential of bridging the gap between outdoor and indoor navigation. Despite their potential, pseudolites can cause severe interference problems with non-participating receivers, i.e., devices not designed to exploit this technology. In this paper, the loss caused by pulsed pseudolite signals is determined as a function of the pulse duty cycle and the number of bits employed for signal quantization. Quantization, blanking and noise increase are identified as the main sources of signal degradation. Theoretical results are validated by simulations and experiments performed using commercial GPS receivers. The good agreement between theoretical and experimental results supports the validity of the proposed approach.

Performance of Sequential Probability Ratio Test for GPS Acquisition

Acquisition of signals in noise, in particular CDMA signals like the Global Positioning System (GPS) L1 C/A signal, can be carried out using fixed or variable dwell times. In this work, a sequential multiple dwell procedure for verifying acquisition is examined and compared to a fixed time single dwell strategy. The procedure under analysis is the sequential probability ratio test (SPRT) which has not been widely used for GPS applications, possibly due to its sensitivity to attenuation of the received signal relative to the design point. In this paper, it is shown that for the received signal to noise ratios (SNR) typically encountered in GPS, the SPRT can outperform the single dwell detector strategy in terms of mean acquisition time. In addition, it is shown that the single dwell (SD) detectors fixed dwell time approaches the worst case dwell time for the SPRT, as design point carrier to noise ratio decreases. Thus, for very weak signals, the SPRT can be a better choice of verification algorithm than the SD strategy, under certain constraints.

LightSquared effects on estimated C/N0, pseudoranges and positions

We present experimental results showing the impact of the proposed LightSquared (LS) Long-term Evolution (LTE) signals on both GPS and Galileo civil modulations in the L1/E1 band. The experiments were conducted in radiated mode in a large anechoic chamber. Three Galileo enabled receivers were chosen for the tests, and a state of the art GNSS signal generator was used to simulate both GPS and Galileo signals. The LTE signals were generated by an Agilent Programmable Signal Generator with a license to generate the signals according to the 3GPP LTE FDD standard. The interference impact was measured in terms of a Carrier-to-Noise power spectral density ratio (C/N0) degradation, in accordance with the methodology which the LS/GPS Technical Working Group (TWG) established by mandate of the FCC. A model for determining the impact of the LS signal on the considered GNSS signals is provided and is validated against experimental data. It is shown that the Galileo E1 Open Service (OS) signal is marginally more susceptible to this form of interference than the GPS L1 C/A signal due to its greater proximity to the lower edge of the L1 band. The impact of LS interference was further analyzed in terms of pseudorange and position errors. Despite its relevance for most GNSS users, this aspect was not considered by the TWG. Measurement and position domain analysis along with the study of the LS impact on the Galileo OS signals are the major contributions. The analysis confirms the results obtained by the TWG and shows that the receiver front-end plays a major role in protecting GNSS signals against RF interference. While it appears that, for now, the LS network will not be deployed, the approach taken and the results obtained herein can be readily adapted for any future terrestrial mobile network that may take the place of LS.

Compatibility analysis between LightSquared signals and L1/E1 GNSS reception

This paper presents experimental results showing the impact of the proposed LightSquared Long Term Evolution (LTE) signals on reception of both Global Positioning System (GPS) and Galileo civil signals in the L1/E1 band. A model for determining the impact of the interfering signal on the victim Global Navigation Satellite System (GNSS) receivers is also provided and this model is validated against the experimental data. It is shown that Galileo E1 Open Service (OS) receivers will be, in general, marginally more susceptible to this form of interference due to its greater proximity to the edge of the L1 band (as will be the new GPS III civil signal receivers in this band). While it appears that, for now, the LightSquared network will not be deployed, the approach taken and the results obtained herein can be readily adapted for any future signal that may take the place of LightSquared.