Weak GPS Signal Research Articles

A Novel Carrier Tracking Approach for GPS Signals Based on Gauss–Hermite Kalman Filter

In a conventional GPS receiver, the carrier tracking system is the key stage that keeps the receiver locked to the radio navigation parameters (RNPs) of the received signal. The most commonly used approaches to the tracking system are phase lock loop (PLL), frequency lock loop (FLL), and FLL-assisted PLL. The main limitation of the above approaches is that their performance deteriorates when working with weak signals and in harsh environments. In recent years, Kalman filter (KF)-based tracking loop architectures have gained increased attention due to their robust and better performance compared with conventional architectures. In this paper, a novel Gauss–Hermite Kalman filtering-based carrier tracking algorithm is proposed for static and moving receivers with weak GPS signals. The performance of the proposed algorithm is compared with two other approaches: extended Kalman filter (EKF) and unscented Kalman filter (UKF). Simulations were conducted using a software-defined GPS simulator and software device radio (SDR) modules. A comparative analysis of the tracking methods demonstrated that the proposed tracking method shows a better performance and improves the tracking sensitivity and capability under weak signal conditions as compared with EKF- and UKF-based tracking methods. In addition, the results show that the proposed approach improves the Doppler frequency measurement accuracy under dynamic operation conditions.

A Vehicle Location Algorithm Based on Community Discovery

The group behavior of the vehicle can accurately describe the movement law of the vehicle in the vehicle network. A vehicle node with strong GPS signal in the Internet of Vehicles can use its communication capability and its neighbor positional relationship to calculate the position of a vehicle with a weak GPS signal located within a certain distance range and having the same or similar driving destination. The algorithm divides the vehicles with the same or similar destination destinations into the same community based on the destination of the vehicle, and then calculates the position of the vehicle that needs to be located, and corrects the calculation result. The experimental results show that the proposed algorithm reduces the positioning error by 15%-35% compared with similar methods. Under different traffic conditions, the stability of the algorithm is improved by more than 25%.

Data Fusion of UWB and IMU Based on Unscented Kalman Filter for Indoor Localization of Quadrotor UAV

The indoor location technique plays a essential role during the application of quadrotor unmanned aerial vehicle (UAV). However, the control design problem for the quadrotor UAV is quite difficult in the indoor environment due to the weak GPS signal. Based on Ultra Wide Band (UWB), the related positioning issues can be solved of UAV through base station with known coordinate position and equipment with location tag, but it is difficult to meet the high-precision operation requirements. In this paper, an indoor positioning design method combined with the Inertial Measurement Unit (IMU) and UWB positioning technology is proposed, which can effectively suppress the error accumulation of the IMU and further improve the positioning accuracy. Moreover, the system architecture for a class of quadrotor UAV is designed. The multisensor fusion technology based on unscented Kalman filter (UKF) is used to avoid neglecting the high-order terms of the nonlinear observation equations of UWB and IMU, which can effectively improve the accuracy of solving the nonlinear equations. Finally, a hardware-in-the-loop simulation platform is designed to verify the effectiveness of the indoor positioning method and improve the positioning accuracy.

Acquisition of weak GPS signals using wavelet-based de-noising methods

Various factors cause GPS signals to be weakened in their path from satellites to receiver. This phenomenon leads to some problems in the normal stages of the positioning process, including acquisition stage. The wavelet transform is one of the tools that have been employed in several methods to improve receiver’s sensitivity in confronting weak GPS signals. Choosing an appropriate threshold plays a key role in the performance of this de-noising method. This study presents two methods for selecting an appropriate threshold. In the first method, the threshold is determined based on the statistical information extracted from the signal. On the other hand, the threshold in the second method is determined by type II fuzzy sets. The results show that employment of these two methods increases SNR gain as well as the range of acquirable SNR which makes signals acquisition with minimum post-correlation SNR of in a GPS receiver possible.

Design of Enhanced Rotation Locked Loop for Roll Angle Estimation of Rotating Vehicle in a Weak GPS Signal Environment.

In order to estimate the roll angle of a rotating vehicle, an enhanced rotation locked loop (RLL) algorithm is proposed in this paper. The RLL algorithm estimates the roll angle by using the property that the power of the GPS signal measured at the receiver of a rotating vehicle changes periodically. However, in case the received GPS power is decreased, the performance of the conventional RLL algorithm degrades, or it cannot estimate the roll angle anymore, therefore, for operating the RLL algorithm in a weak signal environment, this paper designs a method to increase the signal-to-noise ratio (SNR) by overlapping multiple GPS signals’ correlator outputs and a method to compensate the decreased response of a rotation discriminator at low-signal strength. Through computer simulations, the performance of the proposed algorithm is verified and it is shown that the roll angle can be estimated stably even at a weak signal environment down to 29 dB–Hz of C/N0.

Indoor localization via multi-view images and videos

Indoor localization systems are applicable to labyrinth-like environments where mobile and robotic operators require precise direction and location. Existing indoor localization systems require additional equipment, such as WiFi receivers and distributed beacons to capture the necessary information to accurately calculate direction and location. Image-based localization is mostly used in outdoor environments to compensate for the deficiency of weak GPS signals in large building areas. This paper presents a new indoor localization method that significantly reduces the dependency of external hardware resource by utilizing multiple-view configuration. The proposed algorithm initially extracts the self-similarity matrix features to represent the video that captures the user’s spatio-temporal information. We then perform the image and video retrieval based on the trained multi-task classifiers to determine scene location and orientation. Our method generates accurate location and orientation estimation.

INS-aided tracking with FFT frequency discriminator for weak GPS signal under dynamic environments

We present an approach using INS-aided GPS tracking loop with FFT frequency discriminator for tracking of dynamic weak GPS signals. In this approach, the FFT carrier frequency discriminator is used to deal with the attenuated GPS signal, and the INS is used to handle the platform dynamics. With a tactical level INS aiding, when the CN0 of the GPS signal is as low as 19 dB-Hz and the platform dynamic is as high as 100 g, the tracking results indicate that the Doppler tracking error is <4 Hz, and the code loop bandwidth can be decreased to 0.05 Hz. The positioning results show that the positioning error is <5 m and the velocity error is <0.5 m/s.

A Cascade Tracking Loop for Weak GPS Signals

It’s important to get accurate carrier phase and frequency information when using a standalone GPS receiver. In weak signal applications, to keep a stable tracking is hard to achieve because measuring error will be huge when the SNR is low. Different methods are used to improve the SNR before the detector in a tracking process, such as coherence integration. And this paper keeps eyes on a different viewpoint, on how to refine estimation results. A cascade structure is introduced for weak signal tracking. This structure is divided into two levels. In the first level, raw phase estimation and accurate frequency estimation is provided to achieve stable work in low CNR environment. In the second level, the raw phase estimation is refined to achieve accurate tracking requirement. This cascade structure can also work jointly with any other SNR-improving technology to get a better performance.

Weak GPS signal tracking using FFT discriminator in open loop receiver

An open loop tracking architecture, which tracks GPS signals under weak and challenging conditions, is analyzed. The in-phase and quadrature-phase integration pair is regarded as a single tone comp...

Novel Code Acquisition Scheme for Weak GPS Signal

In this paper, we propose a novel GPS C/A code paralleled acquisition scheme with adaptive threshold setting method. The proposed threshold setting criterion is based on the ratio between the peak and the second peak which determine the signal strength over the noise. For our scheme, the detection and false alarm probabilities are derived. When the length of the received data is 200ms, simulation results show that the proposed scheme improves the acquisition sensitivity by about 2 dB in comparison with the two conventional schemes under various Doppler frequency deviation conditions.

Urban and Indoor Weak Signal Tracking Using an Array Tracker with MVA and Nonlinear Filtering

We focus on the need for weak GPS signal tracking technique at a receiver powered on in urban or indoor environment; the tracking loop is unlocked and data bit edge position is unknown. A modified Viterbi algorithm (MVA) based on dynamic programming is developed and it is applied to GPS bit synchronization to improve bit edge position detection probability. Meanwhile, two combination carrier tracking schemes based on central difference Kalman filter (CDKF) and MVA module are designed for tracking very weak GPS signal. The testing results indicate that the methods can successfully detect bit edge position with high detection probability whether or not the tracking loop is locked. The tested combination tracking scheme is still able to work well when the signal quality deteriorates to 20 dB-Hz without additional large store space.

A Computationally Efficient Iterative MLE for GPS AOA Estimation

This paper presents a computationally efficient algorithm for GPS signal angle of arrival (AOA) estimation based on an iterative maximum likelihood estimator (iMLE) operating on an antenna array output. The iMLE has global convergence and therefore can estimate the true AOA MLE, regardless of the initial value, and has a simple mathematical form without any derivative or matrix decomposition computation. Simulations are performed for signals with different antenna array parameters, and the performances are compared with that of conventional MUSIC and CAPON methods. The results demonstrate that the iMLE can achieve better noise performance even with a small number of antenna elements. The paper also presents a tracking mode for the iMLE that further improves its performance for signals with low signal-to-noise ratio (SNR) and greatly extends its application into weak GPS signal or multipath environments.

Efficient Differential Coherent Accumulation Algorithm for Weak GPS Signal Bit Synchronization

An efficient differential coherent accumulation algorithm for weak GPS signal bit synchronization is presented. With this algorithm, every two successive 1ms coherent integration values are processed with the differential coherent first, whereafter these differential coherent values are accumulated according to 20 navigation data bit boundary candidates as the decision variable. The navigation data bit boundary candidate corresponding to the minimum of these decision variables is determined as the navigation data bit boundary position to achieve bit synchronization. Compared with previous differential coherent accumulation algorithm, this algorithm shows a theoretical reduction in computational load by approximately sixfold.

Improving cooperative trajectory mapping applications with encounter-based error correction

Cooperative trajectory mapping is an emerging technique that allows users to create such a map by using data that are collected from each other's mobile phones. This technique has been proposed for many applications, such as people localisation, public transportation tracking and traffic monitoring. Unlike the traditional Global Positioning System, cooperative trajectory mapping only requires information about the departure distance and moving direction from the previously reported position. This avoids problems such as the high-energy consumption of GPS and weak GPS signals in indoor conditions. However, the new technique also brings about other problems, such as measurement errors in cooperative trajectory mapping, which is when a measurement error causes the spatial relations among users to be wrong. We propose an encounter-based error correction algorithm to efficiently reduce measurement errors. Extensive simulation experiments are carried out to validate our solutions.

Carrier Tracking Performance Comparison of Standards and the Square-Root Kalman Filter

In order to solve the problem of loosing lock in weak GPS signal tracking, Kalman filter based carrier tracking method is presented.In this paper,two methods to track the GNSS carrier are compared,one is base on normal Kalman filter, another is based on square-root Kalman filter. The paper analyzes the under performance in the low carrier-to-noise ratio, and the expenditure of the actual project exists, but the high carrier to noise less discussed than the case will appear.The analyse and simulation result can be used to guide the engineering design of the GNSS receiver.

SAW based convolver for GPS signal acquisition module

Aiming at acquiring navigation signal with weak strength at large speed in challenging environment, SAW convolver is employed to implement partial correlation in weak strength and high dynamic GPS signal acquisition. After introducing principle of GPS signal acquisition, difficulty in acquiring GPS signal in special environments, for example indoor, urban street and woods, is analyzed in detail. The suitability of SAW device in acquiring challenging GPS signal is discussed about. A novel GPS receiver structure based on SAW device is proposed. In final, simulation is performed to testify effectiveness of this new structure.

Urban environment solutions to GPS signal near-far effect

Our aim at improving GPS receivers' availability in an urban environment, weak GPS signal acquisition becomes one key issue in satellite navigation signal processing. In special signal propagation environments, such as indoors, urban crayons, and woods, various factors such as signal propagation power loss, amplitude attenuation, and phase, time, and frequency delay distortions produce at the signal receiving end, what is now known in the literature as, the GPS signal near-far effect, which results in severe degradation of the GPS signal acquisition performance. To mitigate the near-far effect, we focus on four main factors, which are the signal strength attenuation; the Doppler frequency distortion; carrier phase uncertainty; and multi-path distribution because a proper optimization of these factors appears to improve the acquisition performance effectively.

Research of Weak GPS Signals Acquisition Algorithm Based on Duffing Oscillators Array

In this paper, an indoor GPS signals acquisition scheme based on duffing oscillators array is introduced. The main concept of this acquisition scheme is to utilize character of chaotic system, which is sensitive to periodic signals and immune to noise at the same time. Firstly, we decide whether weak GPS signals exist or not by means of the Lyapunov Exponents (LE) algorithm. Finally, the code phase is obtained by means of above discussion result by coherent integration technique. By implementing rigorous mathematical analysis, this paper proves that this acquisition scheme based on duffing oscillators array consumes less acquisition time, and is capable of detecting weak GPS signals. Simulation results prove also the advantage on detecting indoor GPS signals.

Carrier loop architectures for tracking weak GPS signals

The performance of various carrier recovery loop architectures (phase lock loop (PLL), Doppler-aided PLL, frequency lock loop (FLL), and Doppler-aided FLL) in tracking weak GPS signals are analyzed and experimentally validated. The effects of phase or frequency detector design, oscillator quality, coherent averaging time, and external Doppler aiding information on delaying loss of lock are quantified. It is shown that for PLLs the metric of total phase jitter is a reliable metric for assessing low C/N performance of the tracking loop provided the loop bandwidth is not too small (~> 5 Hz). For loop bandwidths that are not too small, total phase jitter accurately predicts carrier-to-noise ratio (C/N) at which loss of lock occurs. This predicted C/N is very close to the C/N predicted by bit error rate (BER). However, unlike BER, total phase jitter can be computed in real-time and an estimator for it is developed and experimentally validated. Total phase jitter is not a replacement for BER, since at low bandwidths it is less accurate than BER in that the receiver loses lock at a higher C/N than predicted by the estimator. Similarly, for FLLs operating at small loop bandwidths, it is found that normalized total frequency jitter is not a reliable metric for assessing loss of lock in weak signal or low C/N conditions. At small loop bandwidths, while total frequency jitter may indicate that a loop is still tracking, the Doppler estimates provided by the FLL will be biased.

Differential combining for acquiring weak GPS signals

By implementing rigorous mathematical analysis, this paper proves that differential combining (DFC) outperforms conventional non-coherent integration (NCH) in the detection of weak GPS signals. The processing loss induced by NCH can be decreased through the use of DFC by approximately 3 dB at low carrier-to-noise density ratios ( C/ N 0). This improvement can decreases the acquisition time, a substantial improvement in the context of weak GPS signals. Monte-Carlo simulation verifies the accuracy of the theoretical derivation. Statistical properties including probability of false alarm and probability of detection are essential to set a detection threshold, which identifies a decision variable level (and corresponding signal-to-noise ratio) beyond which desired performance of acquisition and tracking can be achieved. Conditional probability density functions (CPDFs) of the DFC-based decision variable are necessary to analyze detector performance in a statistical sense, but are too complicated to express in closed-form formula. This paper, based on the statistical expectation and variance of the decision variable, uses curve fitting to approximate CPDFs produced by Monte-Carlo simulation. The curve-fitting results, although not rigorously accurate, can provide a practical reference for this complicated problem. Analysis of the resultant CPDFs substantiates that, due to the lower processing loss, the DFC is superior to NCH in improving the sensitivity by 1.2–1.6 dB provided the false alarm is fixed to 0.1% and the detection threshold is set to 90%.