F. van Graas

Closed-Loop Sequential Signal Processing and Open-Loop Batch Processing Approaches for GNSS Receiver Design

Global navigation satellite system (GNSS) receiver design is considered in terms of closed-loop and open-loop receiver architectures that utilize sequential and batch processing techniques. The paper uses the Global Positioning System (GPS) as a case study to demonstrate that an open-loop approach that combines batch and sequential signal processing improves GNSS signal tracking characteristics as compared to the traditionally applied closed-loop sequential receiver design for a number of important application areas. Particularly, for flight test scenarios considered, it is demonstrated that open-loop batch/sequential processing improves the GPS tracking margin by 8 dB as compared to the closed-loop sequential tracking for the case of deep GPS/Inertial Navigation System (INS) integration mode that performs code and carrier phase tracking and data bit recovery.

Decoding Navigation Data Messages from Weak GPS Signals

A new method for decoding navigation data bits from very weak Global Positioning System (GPS) signals is presented. Coherent signal integration is applied over a time period that includes multiple navigation data bits. A search is performed for a combination of bits that maximizes the signal energy during the coherent integration.

Real-time acquisition and tracking for GPS receivers

Current GPS receivers spend much time in base-band processing, performing acquisition and tracking. This is due to the large number of required operations in the software-based signal processing. This paper presents a novel signal acquisition and tracking method that reduces the number of operations, simplifies hardware implementation and decreases the acquisition time. The implementation of this method in an FPGA provides very fast processing of incoming GPS samples that satisfies real-time requirements.

Use of Deeply Integrated GPS/INS Architecture and Laser Scanners for the Identification of Multipath Reflections in Urban Environments

This paper investigates the constructive use of multipath reflections of Global Positioning System (GPS) signals for navigation in urban environments. Urban navigation applications are generally characterized by a significant presence of multipath signals. In order to maintain reliable and accurate navigation capabilities, it is critical to distinguish between direct signal and multipath. At the same time, multipath reflections can be exploited as additional measurements for those cases where the number of direct path satellites is insufficient to compute the navigation solution. The paper develops a method for the identification of multipath reflections in received satellite signals: i.e., multipath is separated from direct signal and a line-of-site between the GPS receiver and a multipath reflecting object is determined. Once multipath reflections are identified, they can be used constructively for navigation. The method presented in the paper exploits an open-loop batch-processing GPS receiver, laser scanner and inertial navigation system (INS) to identify multipath reflections in received satellite signals. Experimental GPS, inertial and laser scanner data collected in real urban environments are applied to demonstrate identification of multipath reflections.

Utilizing multipath reflections in deeply integrated GPS/INS architecture for navigation in urban environments

This paper investigates the constructive use of multipath reflections of Global Positioning System (GPS) signals for navigation in urban environments. Urban navigation applications are generally characterized by a significant presence of multipath signals. In order to maintain reliable and accurate navigation capabilities, it is critical to distinguish between direct signal and multipath. At the same time, multipath reflections can be exploited as additional measurements for those cases where the number of direct path satellites is insufficient to compute the navigation solution. The paper develops a method for the identification of multipath reflections in received satellite signals: i.e. multipath is separated from direct signal and a line-of-site between the GPS receiver and a multipath reflecting object is determined. Once multipath reflections are identified, they can be used constructively for navigation. The method presented in the paper exploits an open loop batch-processing GPS receiver, laser scanner and inertial navigation system (INS) to identify multipath reflections in received satellite signals. Experimental GPS, inertial and laser scanner data collected in real urban environments are applied to demonstrate identification of multipath reflections.

Ground-multi path mitigation via polarization steering of GPS signal

Multipath (MP) is the dominant error source in Global Positioning System (GPS) code-based position solutions requiring high accuracy. A technique is introduced here to mitigate error due to ground-reflected MP signals. The technique uses two orthogonal dipoles to capture the direct GPS signal and the ground-reflected GPS signal. Adjusting the amplitude and phase of the received voltage between the two dipoles can reduce the impact of MP error. Theoretical derivations of this technique are performed for a GPS signal upon reflection from dry soil, seawater, and fresh water. The theoretical results are verified with a real world experiment on the aforementioned surfaces. GPS pseudo-range (PR) and carrier-to-noise ratio (C/No) measurements for specific satellites are used to verify the predicted theoretical results.

Beam Steering in Global Positioning System Receivers using Synthetic Phased Arrays

This correspondence introduces a concept of synthetically generated phased arrays to the area of Global Positioning System (GPS). GPS antennas generated at different spatial locations are combined into a synthetic phased array to improve the beam sharpening performance without increasing the antennas physical size. A fast Fourier transforms (FFT)-based method is applied for simultaneous steering of the synthetic arrays beams in multiple directions. The synthetic beam-steering approach discussed can be applied for improving GPS robustness to RF interference and for simultaneous tracking of multiple signal sources (i.e., direct signal and multipath reflections). Simulation results and experimental test results are presented to demonstrate the efficacy of synthetic beam steering techniques for GPS antennas.

Ranging airport pseudolite for local area augmentation

This paper discusses the integration of an airport pseudolite (APL) into a local area augmented differential GPS based precision approach system. A prototype architecture is described that is being used to develop requirements for the local area augmentation system. Key features of this prototype system are presented along with its current performance. Key features discussed include the use of a multipath limiting antenna, APL signal structure factors, a unique APL automatic gain control, and GPS blanking technique to maximize APL tracking performance, while minimizing the electromagnetic interference to nominal DGPS performance.

Mitigation of GPS Cross-Correlation Errors using Semi-Codeless Tracking

Cross-correlation properties of the Global Positioning System (GPS) coarse acquisition (CA)-code can significantly degrade measurement performance for high precision applications. The use of semi-codeless P-code measurements of the Link 1 (L1) carrier signal for the mitigation of CA-code cross-correlation errors is investigated. The trade-off between cross-correlation error mitigation and squaring loss (SL) of the semi-codeless tracking is discussed. Test results are used to demonstrate the efficacy of using P-code measurements for the cross-correlation error mitigation.

Bias detection and its confidence assessment in Global Positioning System signals

The Federal Aviation Administration plans to migrate to a Global Positioning System (GPS) based navigation system for aeronautical use. As with any safety-critical application, any system failure mode, which might lead to misleading information (MI) for the user, needs to be detected at a high level of confidence. However, the failure detection schemes cannot be too conservative in order not to unreasonably decrease the systems continuity and/or availability. This work focuses on the detection of GPS signal-in-space anomalies. Since the received GPS signal is altered by noise, any signal parameter determination translates into a statistical estimation process. An estimation procedure to analyze the received satellite signal for possible biases is introduced in this paper. Since the statistical behavior of an anomalous signal is unknown, a distribution-independent detection scheme is presented. Further, the algorithm also provides a confidence assessment on the statistical inference. The derived bias detection procedure is corroborated with simulation results.