Fernando M. G. Sousa

Gating Functions for Multipath Mitigation in GNSS BOC Signals

A new multipath mitigation technique is proposed for binary offset carrier (BOC) signals in global navigation satellite systems (GNSS) using the concept of gating function originally conceived for the GPS coarse-acquisition (C/A) code. Specially-tailored pulses are utilized to diminish the number of false-lock points of the code discriminator response and to improve the multipath mitigation capability. The code loop includes only four real correlators (two extra correlators are required for the simplified bump-jumping algorithm with BOC(n,n) signals). Results obtained with BOC(n,n) and BOC(2n,n) signals show that this technique eliminates the multipath ranging errors for reflected rays with relative delays typically above twenty percent of the spreading code chip duration, thus comparing favorably with the conventional receiver correlation techniques.

Strobe Pulse Design for Multipath Mitigation in BOC GNSS Receivers

Binary offset carrier (BOC) modulations have been considered for the new GNSS signals since they achieve better tracking performance than PSK in the presence of channel noise and multipath. Besides, the concept of delay lock-loop based on symmetrical strobe pulses can be extended to BOC signals with significant advantage in the close-in multipath region. Herein, a new approach to the design of asymmetrical strobe pulses for BOC signals is proposed. A target code discriminator response with desirable characteristics is defined allowing to determine the strobe pulse as the solution of an integral equation. The resulting pulse provides good multipath mitigation capability, extended code tracking range, and lack of false code lock points.

A DSP based long distance echo canceller using short length centered adaptive filters

This paper describes an implementation of a long distance echo canceller which copes with double talking situations and exceeds the CCITT G.165 recommendation. The proposed solution is based on short length adaptive filters centered on the positions of the most significant echoes, which are tracked by time-delay estimators. To deal with double talking situations a speech detector is employed. The resulting algorithm enables long-distance echo cancellation with low computational requirements. It reaches greater echo return loss enhancement and shows faster convergence speed as compared with results reported in recent literature.

Nonlinear Filtering in GNSS Pseudorange Dynamics Estimation Combining Code Delay and Carrier Phase

In Global Navigation Satellite System (GNSS) receivers, code delay measurements yield unambiguous but noisy pseudorange estimates. On the other hand, the pseudorange obtained with carrier phase measurements is almost noiseless, but is affected by an ambiguity term multiple of the carrier wavelength. In this paper, we propose a structure for code and carrier phase estimation and a novel technique to merge those estimates. The overall architecture consists of 1) a bank of correlators, each one feeding an extended Kalman filter, 2) an interpolation algorithm to produce the best code delay and carrier phase estimates, and 3) a nonlinear filter that combines these estimates to reduce progressively the phase ambiguity, yielding precise pseudorange estimates in high dynamics scenarios and in the presence of weak signals. The proposed approach is able to accommodate the majority of the new GNSS signals (modernized GPS and Galileo), produces smaller mean square errors than the conventional code/phase merge technique (Hatch filter), is robust to transients that may affect the phase estimation during the reception of weak signals, and is free from false code-locks.

Practical simulation of GNSS signals in the presence of ionospheric scintillation

We propose a low complexity technique to generate amplitude correlated time-series with Nakagami-m distribution and phase correlated Gaussian-distributed time-series, which is useful in the simulation of ionospheric scintillation effects during the transmission of GNSS signals. The method requires only the knowledge of parameters S4 (scintillation index) and σΦ (phase standard deviation) besides the definition of models for the amplitude and phase power spectra. The Zhang algorithm is used to produce Nakagami-distributed signals from a set of Gaussian autoregressive processes.

Performance analysis of a VDFLL GNSS receiver architecture under ionospheric scintillation and multipath conditions

A noncoherent vector delay/frequency-locked loop (VDFLL) architecture for GNSS receivers is proposed. A bank of code and frequency discriminators feeds a central extended Kalman filter that estimates the receivers position and velocity, besides the clock error. The VDFLL architecture performance is compared with the one of the classic scalar receiver, both for scintillation and multipath scenarios, in terms of position errors. We show that the proposed solution is superior to the conventional scalar receivers, which tend to lose lock rapidly, due to the sudden drops of the received signal power.

Characterization and performance analysis of a VDFLL GNSS receiver architecture

A non-coherent vector delay/frequency-locked loop architecture for GNSS receivers is proposed. Two dynamics models are considered: PV (position and velocity) and PVA (position, velocity, and acceleration). In contrast with other vector architectures, the proposed approach does not require the estimation of signals amplitudes. Only coarse estimates of the carrier-to-noise ratios are necessary.

Near-far effect mitigation for GNSS software receivers using subspace projection

This paper proposes an improved implementation of the subspace projection method aiming to minimize the near-far effect that occurs in Global Navigation Satellite System receivers. Our technique does not demand phase estimation of the strong signals, thus being suitable for non-coherent receivers. Simulation results demonstrate the effectiveness and the limitations of the proposed mitigation technique.

GNSS hybridization for indoor positioning

We describe a hybridized receiver architecture tightly coupled with different sensors, namely MEMS accelerometers and gyroscopes, Wi-Fi engine and a barometer, allowing significant increase of positioning performance for indoor GNSS signal reception. This is possible due to the increase of integration times, use of higher sensitivity GNSS receivers and by resorting to external aiding sensors tightly coupled with the receiver. Expected overall navigation performance increase is described and the benefits of using such external sensors are quantified. Indeed, hybridization of GNSS receivers with other sensors is expected to be a key technology that will enable new services and applications for the use of GNSS for navigation in indoor and densely urbanized environments.

Performance comparison of a VDFLL versus VDLL and scalar GNSS receiver architectures in harsh scenarios

We analyze the advantages and drawbacks of a vector delay/frequency-locked loop (VDFLL) architecture regarding the conventional scalar and the vector delay-locked loop (VDLL) architectures for GNSS receivers in harsh scenarios that include ionospheric scintillation, multipath, and high dynamics motion. The VDFLL is constituted by a bank of code and frequency discriminators feeding a central extended Kaiman filter (EKF) that estimates the receivers position, velocity, and clock bias. Both code and frequency loops are closed vectorially through the EKF. The VDLL closes the code loop vectorially and the phase loops through individual PLLs while the scalar receiver closes both loops by means of individual independent PLLs and DLLs.