Wavelet packets based denoising method for measurement domain repeat-time multipath filtering in GPS static high-precision positioning

Repeatable satellite orbits can be used for multipath mitigation in GPS-based deformation monitoring and other high-precision GPS applications that involve continuous observation with static antennas. Multipath signals at a static station repeat when the GPS constellation repeats given the same site environment. Repeat-time multipath filtering techniques need noise reduction methods to remove the white noise in carrier phase measurement residuals in order to retrieve the carrier phase multipath corrections for the next day. We propose a generic and robust three-level wavelet packets based denoising method for repeat-time-based carrier phase multipath filtering in relative positioning; the method does not need tuning to work with different data sets. The proposed denoising method is tested rigorously and compared with two other denoising methods. Three rooftop data sets collected at the University of Nottingham Ningbo China and two data sets collected at three Southern California Integrated GPS Network high-rate stations are used in the performance assessment. Test results of the wavelet packets denoising method are compared with the results of the resistor–capacitor (RC) low-pass filter and the single-level discrete wavelet transform (DWT) denoising method. Multipath mitigation efficiency in carrier phase measurement domain is shown by spectrum analysis of two selected satellites in two data sets. The positioning performance of the repeat-time-based multipath filtering techniques is assessed. The results show that the performance of the three noise reduction techniques is about 1–46 % improvement on positioning accuracy when compared with no multipath filtering. The statistical results show that the wavelet packets based denoising method is always better than the RC filter by 2–4 %, and better than the DWT method by 6–15 %. These results suggest that the proposed wavelet packets based denoising method is better than both the DWT method and the relatively simple RC low-pass filter for noise reduction in multipath filtering. However, the wavelet packets based denoising method is not significantly better than the RC filter

An Efficient Method for GPS Multipath Mitigation Using the Teager-Kaiser-Operator-based MEDLL

An efficient method for GPS multipath mitigation is proposed. The motivation for this proposed method is to integrate the Teager-Kaiser Operator (TKO) with the Multipath Estimating Delay Lock Loop (MEDLL) module to mitigate the GPS multipath efficiently. The general implementation process of the proposed method is that we first utilize the TKO to operate on the received signal’s Auto-Correlation Function (ACF) to get an initial estimate of the multipaths. Then we transfer the initial estimated results to the MEDLL module for a further estimation. Finally, with a few iterations which are less than those of the original MEDLL algorithm, we can get a more accurate estimate of the Line-Of-Sight (LOS) signal, and thus the goal of the GPS multipath mitigation is achieved. The simulation results show that compared to the original MEDLL algorithm, the proposed method can reduce the computation load and the hardware and/or software consumption of the MEDLL module, meanwhile, without decreasing the algorithm accuracy

Multipath Propagation, Mitigation and Monitoring in the Light of Galileo and the Modernized GPS

Among the numerous potential sources of GNSS signal degradation, multipath takes on a prominent position. Unlike other errors like ionospheric or tropospheric path delays which can be modeled or significantly reduced by differential techniques, multipath influences cannot be mitigated by such approaches. Although a lot of multipath mitigation techniques have been proposed and developed in the past among them many receiver internal approaches using special signal processing algorithms multipath (especially multipath with small geometric path delays) still remains a major error source. This is why multipath has been a major design driver for the definition of the Galileo signal structure carried out in the past years and the subsequent signal optimization activities. This thesis tries to provide a broad and comprehensive insight into various aspects of multipath propagation, mitigation and monitoring (without claiming to be exhaustive). It contains an overview of the most important aspects of multipath propagation, including the discussion of different types of multipath signals (e.g. specular vs. diffuse multipath, satellite vs. receiver multipath or hardware-induced multipath), typical characteristics such as periodic signal variations whose frequency depends on the satellite-antenna-reflector geometry and the impact on the signal tracking process within a GNSS receiver. A large part of this thesis is dedicated to aspects of multipath mitigation, first providing a summary of the most common multipath mitigation techniques with a special focus on receiver-internal approaches such as the narrow correlation technique, double-delta correlator implementations, the Early-Late Slope (ELS) technique or Early/Early tracking implementations. However, other mitigation approaches such as using arrays of closely spaced antennas or multipath-limiting antennas are discussed as well. Some of these techniques are used for subsequent multipath performance analyses considering signals of the (modernized) GPS and Galileo. These analyses base on a new methodology to estimate typical and meaningful multipath errors making use of multipath error envelopes that are scaled in a suitable way to account for different multipath environments. It will be shown that typical (mean) multipath errors can be derived from these scaled envelopes by computation of the envelopes running average and that these mean multipath errors are of the same order as multipath errors obtained from complex statistical channel models. Another part of this thesis covers various aspects of multipath detection and monitoring. First, current techniques for multipath detection and monitoring are described and discussed with respect to their benefits and drawbacks or their real-time capability. Among the considered approaches are techniques like code minus carrier monitoring, SNR monitoring, the use of differenced observations or spectral and wavelet analysis. Following this introductory overview, a completely new approach for real-time multipath monitoring by processing multi-correlator observations will be introduced. Previously being used primarily for the detection of Evil Waveforms (signal failures that originate from a malfunction of the satellites signal generation and transmission hardware), the same basic observations (linear combinations of correlator outputs) can be used for the development of a multi-correlator-based real-time multipath monitoring system. The objective is to provide the user with instant information whether or not a signal is affected by multipath. The proposed monitoring scheme has been implemented in the form of a Matlab-based software called RTMM (Real-Time Multipath Monitor) which has been used to verify the monitoring approach and to determine its sensitivity.Die Qualität eines Satellitensignals wird durch eine Vielzahl potenzieller Fehlerquellen negativ beeinflusst. Neben atmosphärischen Einflüssen tragen Mehrwegeeinflüsse einen wesentlichen Anteil zum Gesamtfehlerbudget der Satellitennavigation bei. Während eine ganze Reihe von Fehlereinflüssen durch geeignete Modellierung oder differenzielle Verfahren deutlich reduziert werden können, ist dies durch die räumliche Dekorrelation der Mehrwegeeffekte nicht möglich. Obwohl in der Vergangenheit eine Vielzahl von Verfahren zur Mehrwegereduzierung vorgeschlagen und entwickelt wurden, stellen Mehrwegesignale noch immer eine wesentliche, stets zu berücksichtigende Fehlerquelle dar. Aus diesem Grund spielten die zu erwartenden Mehrwegefehler auch eine sehr wichtige Rolle im Zuge der Definition sowie der Optimierung der Galileo-Signalstruktur und können somit als wesentliches Design-Kriterium angesehen werden. Die vorliegende Arbeit gibt einen umfassenden Einblick in verschiedene Aspekte der Mehrwegeausbreitung, -reduzierung sowie der Detektion und der Überwachung auftretender Mehrwegeeffekte. Die Arbeit beschreibt zunächst die wichtigsten Aspekte der Mehrwegeausbreitung, wobei beispielsweise unterschiedliche Arten von Reflexionen oder unterschiedliche Entstehungsarten ebenso diskutiert werden wie typische Auswirkungen von Mehrwegesignalen wie die Entstehung periodischer Signalvariationen. Solche Signalvariationen sind in starkem Maße abhängig von der durch die Satellitenposition, dem Antennenstandpunkt und der Lage des Reflexionspunktes definierten Geometrie. Die Frequenz dieser Signalvariationen wird für unterschiedliche geometrische Verhältnisse berechnet. Zudem werden der Einfluss bzw. die Auswirkungen einer Mehrwegeausbreitung auf den Signalverarbeitungsprozess in einem GNSS Empfänger aufgezeigt. Einen weiteren Schwerpunkt dieser Arbeit bilden die derzeit gebräuchlichen Methoden zur Reduzierung von Mehrwegeeinflüssen. Dabei werden zunächst die wichtigsten empfängerinternen Ansätze vorgestellt. Aber auch Methoden wie die Verwendung von Antennenarrays oder spezieller Antennen bleiben nicht unberücksichtigt. Einige dieser Methoden bilden im Folgenden die Grundlage für die Bestimmung von typischen Mehrwegefehlern. Dazu wird eine neuartige Methodik vorgestellt, um aus Hüllkurven des Mehrwegefehlers aussagekräftige mittlere Mehrwegefehler zu bestimmen. Hierzu werden die Hüllkurven mit Hilfe einiger aus statistischen Kanalmodellen abgeleiteter Parameter in geeigneter Weise skaliert, um unterschiedlichen Mehrwegeumgebungen Rechnung zu tragen. Es wird gezeigt, dass die mit Hilfe dieser relativ einfachen und effizienten Methode ermittelten Mehrwegefehler in derselben Größenordnung liegen wie die aus komplexen statistischen Kanalmodellen ermittelten Fehler. Einen weiteren Themenkomplex stellen Methoden zur Detektion und zum Monitoring von Mehrwegeeinflüssen dar. Dabei werden zunächst derzeit verwendete Ansätze vorgestellt und hinsichtlich ihrer Vor- und Nachteile sowie hinsichtlich ihrer Echtzeitfähigkeit diskutiert. In Anschluss daran wird ein neuartiger Ansatz zur Detektion und zum Monitoring von Mehrwegesignalen in Echtzeit vorgestellt, der auf der Auswertung von Multikorrelatorbeobachtungen basiert. Ziel dieser Entwicklung ist es, einen potenziellen Nutzer sofort darüber informieren zu können, wenn ein Signal mit Mehrwegefehlern behaftet ist. Der vorgeschlagene Ansatz wurde in Form einer Matlab-basierten implementiert, welche im Folgenden zur Verifizierung und zur Bestimmung der Empfindlichkeit des Verfahrens verwendet wird

Effects of Site-Dependent Errors on the Accuracy of C/A Code DGPS Positioning

Several differential GPS processing techniques can be used; for instance, single differencing and double differencing, which are popular in practice. Irrespective of the DGPS processing technique used, the ultimate accuracy of the user-location depends on the existence of non-common or site-dependent errors, which occur at the points of observation and the reference. Of these, the most common and dominant site-dependent error is the multipath. Therefore, this research evaluates the effects of site-dependent errors on C/A code differential GPS correction accuracies by providing special emphasis on the multipath error. For the analyses, four segments of about 24-hour continuous static C/A code based DGPS observations were conducted at three precisely known ground stations and four different multipath environments were introduced by placing three different types of artificial signal reflectors at one of the observation stations. By using the known GPS receiver-reflector configuration, pseudo-range multipath was precisely calculated for each observation segment. C/A code DGPS positioning accuracies before and after multipath mitigation were presented by evaluating the effect of the most dominant site-dependent error, i.e., multipath, on C/A code DGPS correction accuracies

Review on Sparse-Based Multipath Estimation and Mitigation: Intense Solution to Counteract the Effects in Software GPS Receivers

Multipath is the major concern in GPS receivers that fade the actual GPS signal causes positioning error up to 10 m so special care need to be taken to mitigate the multipath effects. Numerous methods like hardware based antenna arrays technique, receiver based narrow correlator receiver, double -delta discriminator, Adaptive Multipath Estimator, Wavelet Transformation and Particle filter, Kalman filter based post receiver methods etc. used to resolve the problem. But some of the methods can only reduce code multipath error but not effective in eliminating carrier multipath error. Most of these techniques are based on the assumption that the Line-of-Sight (LOS) signal is stronger than the Non-Line of-Sight (NLOS) signals. However, in the scenarios where the LOS signal is weaker than the composite multipath signal, this approach may result in a bias in code tracking. In this chapter, different types of multipath mitigation and its limitation are described. The recent development in sparse signal processing based blind channel estimation is investigated to compensate the multipath error. The Rayleigh and Rician fading model with different multipath parameters are simulated to test the urban scenario. The inverse problem of finding the GPS signal is addressed based on the deconvolution approach. To solve linear inverse problems, the suitable kind of appropriate objective function has been formulated to find the signal of interest. By exploiting this methods, the signal is observed and the carrier and code tracking loop parameters are computed with minimal error

GNSS Vulnerabilities and Existing Solutions:A Review of the Literature

This literature review paper focuses on existing vulnerabilities associated with global navigation satellite systems (GNSSs). With respect to the civilian/non encrypted GNSSs, they are employed for proving positioning, navigation and timing (PNT) solutions across a wide range of industries. Some of these include electric power grids, stock exchange systems, cellular communications, agriculture, unmanned aerial systems and intelligent transportation systems. In this survey paper, physical degradations, existing threats and solutions adopted in academia and industry are presented. In regards to GNSS threats, jamming and spoofing attacks as well as detection techniques adopted in the literature are surveyed and summarized. Also discussed are multipath propagation in GNSS and non line-of-sight (NLoS) detection techniques. The review also identifies and discusses open research areas and techniques which can be investigated for the purpose of enhancing the robustness of GNSS

Hybrid Wavelet and Principal Component Analyses Approach for Extracting Dynamic Motion Characteristics from Displacement Series Derived from Multipath-Affected High-Rate GNSS Observations

Nowadays, the high rate GNSS (Global Navigation Satellite Systems) positioning methods are widely used as a complementary tool to other geotechnical sensors, such as accelerometers, seismometers, and inertial measurement units (IMU), to evaluate dynamic displacement responses of engineering structures. However, the most common problem in structural health monitoring (SHM) using GNSS is the presence of surrounding structures that cause multipath errors in GNSS observations. Skyscrapers and high-rise buildings in metropolitan cities are generally close to each other, and long-span bridges have towers, main cable, and suspender cables. Therefore, multipath error in GNSS observations, which is typically added to the measurement noise, is inevitable while monitoring such flexible engineering structures. Unlike other errors like atmospheric errors, which are mostly reduced or modeled out, multipath errors are the largest remaining unmanaged error sources. The high noise levels of high-rate GNSS solutions limit their structural monitoring application for detecting load-induced semi-static and dynamic displacements. This study investigates the estimation of accurate dynamic characteristics (frequency and amplitude) of structural or seismic motions derived from multipath-affected high-rate GNSS observations. To this end, a novel hybrid model using both wavelet-based multiscale principal component analysis (MSPCA) and wavelet transform (MSPCAW) is designed to extract the amplitude and frequency of both GNSS relative- and PPP- (Precise Point Positioning) derived displacement motions. To evaluate the method, a shaking table with a GNSS receiver attached to it, collecting 10 Hz data, was set up close to a building. The table was used to generate various amplitudes and frequencies of harmonic motions. In addition, 50-Hz linear variable differential transformer (LVDT) observations were collected to verify the MSMPCAW model by comparing their results. The results showed that the MSPCAW could be efficiently used to extract the dynamic characteristics of noisy dynamic movements under seismic loads. Furthermore, the dynamic behavior of seismic motions can be extracted accurately using GNSS-PPP, and its dominant frequency equals that extracted by LVDT and relative GNSS positioning method. Its accuracy in determining the amplitude approaches 91.5% relative to the LVDT observations

Analysis of the dynamic response of a long span bridge using GPS/accelerometer/anemometer under typhoon loading

Large flexible engineering structures, such as long span bridges or tall buildings, are susceptible to quasistatic and dynamic deformations caused by different loadings, thus accurate displacement measurements are desirable to assess the integrity and reliability of the structure. In this study, an integrated system that includes Global Positioning System (GPS), accelerometer and anemometer was developed to obtain the responses of a long span bridge to the extreme wind loadings. Spectral analysis based on the Fast Fourier Transform (FFT) algorithm was first carried out to detect the dominant frequencies of the middle pylon. Then the noisy GPS displacement measurements and accelerometer data are de-noised using the Vondrak filter, and the low frequency disturbance was separated from GPS displacement time series. A least-squares based displacement reconstruction scheme using noise-mitigated accelerations was employed, and the Tikhonov regularization scheme with optimal selected regularization factor was used to alleviate the ill-posedness. At last, an adaptive recursive least squares (RLS) filter was adopted to separate the slow-varying movements, and the total displacement with enhanced measurement accuracy was obtained from the combined quasi-static and high-frequency dynamic displacements. A field monitoring data set collected on the Erqi Yangtze River Bridge, a three-tower cable-stayed bridge located in Wuhan, China, was used to validate the effectiveness of the proposed integration processing scheme. The GPS/accelerometer/anemometer installed on the center supporting tower was used to characterize the interaction between the responses and the ambient wind loadings. The results demonstrate the proposed technique can significantly improve the measurement accuracy of pylon displacement under strong winds. The deformation accuracy with the amplitude of several millimeters can be successfully detected,and the spectrum of the pylon response obtained from both GPS data and accelerometer data reveals the identified first dominant frequency of the middle pylon is 0.172 Hz

Use of the Wavelet Transform for Interference Detection and Mitigation in Global Navigation Satellite Systems

Radio frequency interference detection and mitigation are becoming of paramount importance due to the increasing number of services and applications based on the position obtained by means of Global Navigation Satellite Systems. A way to cope with such threats is the implementation in the receiver of advanced signal processing algorithm able to raise proper warning or improve the receiver performance. In this paper, we propose a method based on the Wavelet Transform able to split the useful signal from the interfering component in a transformed domain. The wavelet packet decomposition and proper statistical thresholds allow the algorithm to show very good performance in case of multiple pulse interference as well as in the case of narrowband interference, two scenarios in which traditional countermeasures might not be effective