Weak Global Positioning System Signal Research Articles

Optimal GPS Acquisition Algorithm in Severe Ionospheric Scintillation Scene

The Global Positioning System (GPS) plays an important role in navigation and positioning services. When GPS signals propagate through a complex space environment, they are susceptible to interference of ionospheric scintillation. As one of the biggest interference sources on GPS navigation and positioning, ionospheric scintillation will lead to signal intensity decline and carrier phase fluctuation, making signal acquisition of the GPS receiver challenging. Thus, an acquisition algorithm based on differential coherent integration combining accumulation correlation and bit sign transition estimation is proposed. The coherent accumulation is applied to reduce computational loads and contribution by the Gaussian white noise in the signal. Moreover, the differential coherence integration is utilized to eliminate data blocks with bit transition, prolonging the coherence integration time and improving the data utilization rate. Experimental results show that under severe ionospheric scintillation condition, weak GPS signals can be acquired successfully after improving the acquisition algorithm, with the acquisition probability reaching 50% when the signal-to-interference ratio (SIR) drops to −34 dB. Comparing to the differential coherence integration, the complexity of the calculation reduces to only 21.75% effectively after the improvement. The execution time is less than half of the differential coherence integral.

Data Wipe-Off Technique for Tracking Weak GPS Signals

In this paper, the data wipe-off (DWO) algorithm is incorporated into the vector tracking loop of the Global Positioning System (GPS) receiver for improving signal tracking performance. The navigation data, which contains information that is necessary to perform navigation computations, are binary phase-shift keying (BPSK) modulated onto the GPS carrier phase with the bit duration of 20 ms (i.e., 50 bits per second). To continuously track the satellite’s signal in weak signal environment, the DWO algorithm on the basis of pre-detection method is adopted to detect data bit sign reversal every 20 ms. Tracking accuracy of a weak GPS signal is decreased by possible data bit sign reversal every 20 ms to the predetection integration time (PIT) or integration interval. To achieve better tracking performance in weak signal environment, the coherent integration interval can be extended. However, increase of the integration interval lead to decrease of the tracking accuracy by possible data bit sign reversal every 20 ms to the integration interval. When the integration interval of the correlator is extended over 20 ms in low C/No levels, the navigation DWO algorithm can be employed to avoid energy loss due to bit transitions. The method presented in this paper has an advantage to continuously estimate the navigation data bit and achieve improved tracking performance. Evaluation of the tracking performance based on the various integration intervals for the vector tracking loop of a GPS receiver will be presented.

External Vehicle Positioning System Using Multiple Fish-Eye Surveillance Cameras for Indoor Parking Lots

The autonomous positioning of a vehicle predominantly relies on the global positioning system (GPS). However, in indoor environments, such as tunnels and indoor parking lots, the accuracy of GPS-based positioning can be significantly reduced due to weak GPS signals. To this end, we develop an accurate indoor vehicle positioning system using multiple fish-eye surveillance cameras. Our system first extracts vehicle segments from the top-view image of each fish-eye camera. These segments are then integrated into a common undistorted coordinate system. The center of the vehicle is finally determined using our simple but effective box fitting method. Moreover, a 1/18 scale indoor parking lot is designed to evaluate the performance of the proposed system. Throughout our experiments, we obtained average positioning errors of 30 or 24 cm in the regions covered by a single camera or multiple cameras, respectively.

A new post-correlation anti-jamming technique for GPS receivers

Interference has destructive impacts on all signals, but due to the weakness of the global positioning system (GPS) signals, even low-power interference has a significant destructive effect. Mitigation of that effect is a challenging issue for GPS receivers. We investigate the performance of the acquisition in the presence of the most effective and common interferences in use, such as single-tone continuous wave interference (CWI), multi-tone CWI, and chirp jamming. Our purpose is not to mitigate or completely remove the interference signal, but by using a signal processing method after the correlation, a receiver can acquire weak signals in the presence of interference with a jammer-to-signal ratio up to 60 dB. The main idea is derived from image processing filters so that by applying proper filters to the values of the cross-ambiguity function in the search space, the interference signal contribution decreases without any need for prior knowledge of its specific characteristics. Accordingly, even weak GPS signals can be easily acquired, and the number of acquired satellites increases significantly compared to the usual anti-jamming methods. Moreover, signals can be acquired even at twice the normal decision threshold value.

Improving GPS Receivers Positioning in Weak Signal Environments Based on Fuzzy SSMF-FFT and Fuzzy Kalman Filter

With the advent of global positioning system (GPS) and the increasing expansion of technology, improving GPS receivers positioning has attracted great attention. When the signal received by these receivers is weak, receiver functioning becomes impaired. Due to the existing noise and the presence of Doppler shift in weak signal conditions, the signal acquisition section becomes problematic and in weak signal conditions or phase lock loop (PLL), the tracking section design of noise conditions gets difficult. In case of a lock loss on the signal, the user will not be able to calculate the Doppler frequency and the system will diverge. Therefore, a robust algorithm for the GPS receiver PLL is very vital. In this paper, the squared segmented matched filter-fast Fourier transform algorithm is used to improve the acquisition of weak GPS signals with an average SNR of 15 dB. By using the matched filter, the SNR is maximized and the code phase estimation will be more accurately. Also, the use of a segmented filter before the FFT reduces the number of FFT points and therefore, the computational complexity is reduced. To calculate the number of batches and obtain the best acquisition output, in the proposed algorithm, the system becomes fuzzy. In tracking section, fractional Fourier transform (FRFT) with the PLL based on fuzzy Kalman filter (FKF) is used to reinforce it against weak signal environments. The FRFT is used for estimating frequency and acceleration, and a third-order FKF is used for designing the PLL. As a result of these changes, the RMSE of positioning is improved more than 35%.

Weak GPS signal acquisition method based on DBZP

Double block zero padding (DBZP) is a widely used but costly method for weak global positioning system (GPS) signal acquisition in software receivers. To improve the computational efficiency, this paper proposes an algorithm based on the differential DBZP algorithm and the discrete cosine transform (DCT) domain filtering method. The proposed method involves using a differential correlator after the DBZP operation. Subsequently, DCT domain low pass filtering (LPF) and inverse DCT (IDCT) reconstruction are carried out to improve the signal to noise ratio (SNR). The theoretical analysis and simulation results show that the detection algorithm can effectively improve the SNR of the acquired signal and increase the probability of detection under the same false alarm probability.

A high-sensitivity GPS receiver carrier-tracking loop design for high-dynamic applications

In order to enhance the tracking performance of global positioning system (GPS) receivers for weak signal applications under high-dynamic conditions, a high-sensitivity and high-dynamic carrier-tracking loop is designed. The high-dynamic performance is achieved by aiding from a strapdown inertial navigation system (SINS). In weak signal conditions, a dynamic-division fast Fourier transform (FFT)-based tracking algorithm is proposed to improve the sensitivity of GPS receivers. To achieve the best performance, the tracking loop is designed to run either in the conventional SINS-aided phase lock loop mode (time domain) or in the frequency-domain-tracking mode according to the carrier-to-noise spectral density ratio detected in real time. In the frequency-domain-tracking mode, the proposed dynamic-division FFT algorithm is utilized to estimate and correct the error of the SINS aiding. Furthermore, the optimal values of the dynamic-division step and the FFT size are selected to maximize the signal-to-noise ratio gain. Simulation results demonstrate that the designed loop can significantly improve the tracking sensitivity and robustness for weak GPS signals without compromising the dynamic performance.

Iterative acquisition method based on Grey code for weak global positioning system signal

For weak global positioning system signal, long coherent integration can improve the acquisition performance effectively, but the unknown data bits restrict the coherent integration interval within a data bit. Data bits estimation can extend coherent integration interval over multiple data bits with more Doppler frequency search bins. To lighten the calculation burden of data bit combinations searching, an iterative acquisition algorithm is presented based on ordering data bit combinations by Grey code. Modified block-accumulating coherent integration over extended interval (BACIX) algorithm is also presented, which has linear complexity and better performance than BACIX algorithm.

Bit-flipping prediction acquisition algorithm of weak GPS signal

Long coherent integration duration is needed for weak Global Positioning System(GPS) signal acquisition.However,it is limited in 10 ms by the navigation data bit flipping,which is far from enough. To further improve the acquisition sensitivity,a bit-flipping prediction algorithm was proposed. Firstly,the 5 ms signal with possible involve bitflipping,which was detected through the comparison of the coherent integration results of the several blocks of data,was canceled. Then coherent integration was applied to its sub-block signal in rest 15 ms and the results did differential coherent integration to overcome its bit-flip and reduced the square loss of non-coherence. At the same time,the summation operation was done ahead of coherent integration to depress its computational complexity. The theoretical research and simulation results show that the acquisition sensitivity and acquisition efficiency have been improved,and the algorithm even can capture the weak signal with Signal-to-Noise Ratio(SNR) less than- 50 dB.

A scheme for weak GPS signal acquisition aided by SINS information

In order to enhance the acquisition performance of global positioning system (GPS) receivers in weak signal conditions, a high-sensitivity acquisition scheme aided by strapdown inertial navigation system (SINS) information is proposed. The carrier Doppler shift and Doppler rate are pre-estimated with SINS aiding and GPS ephemeris, so that the frequency search space is reduced, and the dynamic effect on the acquisition sensitivity is mitigated effectively. Meanwhile, to eliminate the signal-to-noise ratio gain attenuation caused by data bit transitions, an optimal estimation of the unknown data bits is implemented with the Viterbi algorithm. A differential correction method is then utilized to improve the acquisition accuracy of Doppler shift and therefore to meet the requirement of carrier-tracking loop initialization. Finally, the reacquisition experiments of weak GPS signals are implemented in short signal blockage situations. The simulation results show that the proposed scheme can significantly improve the acquisition accuracy and sensitivity and shorten the reacquisition time.

Robust Positioning Systems in the Presence of Outliers Under Weak GPS Signal Conditions

In this paper, two epoch-by-epoch robust positioning techniques for global positioning system (GPS) are proposed to deal with the problem of positioning in weak signal conditions in which the probability of outlier in signal acquisition is larger than zero. We propose to accept outliers into the positioning algorithm, however, in this case either robust estimation or outlier detection must be used to overcome the devastating effect of such outliers on traditional positioning algorithms. In order to improve the sensitivity of a GPS receiver, we propose to use novel methods that are able to deal with the problem of estimating the position of a receiver based on pseudo-ranging measurements that are contaminated by outliers. Simulations are carried out to demonstrate the robustness of the proposed techniques in terms of success rate of the algorithms in finding the correct solution, when there are a different number of outliers in ranging measurements from satellites.

Probability of outlier analysis in weak GPS signal acquisition

In this paper, global positioning system (GPS) signal acquisition is investigated under weak signal conditions, when a catastrophic deterioration in performance begins to occur causing outliers to happen in range estimation. The paper compares conventional detection techniques in GPS signal acquisition. The theoretical probability of outlier is derived for GPS Gold code and compared to the probability of outlier using Monte Carlo simulation. In addition, the theoretical probability of outlier for coherent detection technique is also derived. A novel binary hypothesis test is introduced which is used to generate a new set of curves to analyze the performance of detectors in weak signal conditions.

Computational Efficiency Improvement for Unaided Weak GPS Signal Acquisition

Acquisition of unaided weak Global Positioning System (GPS) signals requires long coherent integration time and thus all the possible navigation data bit combination paths have to be searched. In this paper, to improve the computational efficiency, the Improved Fast Modified Double-Block Zero Padding (IFMDBZP) algorithm using the Optimal Path Search Method (OPSM) is proposed instead of the FMDBZP algorithm using the All Paths Search Method (APSM). The proposed method consists of unlikely data bit combination path elimination by applying the Viterbi algorithm during each coherent integration step to improve the FMDBZP algorithm. The analyses show that the proposed OPSM can reduce the computation calculations and save memory space without suffering any loss compared to the APSM. And the longer the coherent integration time is, the more benefit one can gain from the proposed method. The simulation results also show that the IFMDPZP algorithm using the proposed OPSM has the same acquisition performance as the APSM.

GPS/TDOA Hybrid Location Algorithm Based on Federal Kalman Filter

Generally, accuracy of GPS(Global Positioning System) is higher than that of cellular network. But in tall building urban area or indoor, relatively weak GPS signal makes the positioning unstable and inaccurate. TDOA(Time Difference of Arrival) is one of the most widely used positioning methods in cellular mobile communication systems, but its accuracy is not high enough to meet the growing demand. In urban where there are more base stations to support big communication capacity than in rural, the mobile terminal positioning accuracy and stability is relatively better. In order to make up for these inherent deficiency of GPS or TDOA separate positioning, this paper deduces a GPS/TDOA hybrid positioning algorithm based on federated kalman filter by giving error model, state equation and measurement equation of each local kalman filters. Simulation results show that the algorithm effectively improves the data fusion reliability and positioning accuracy.

Performance Analysis of Vector Tracking Algorithms for Weak GPS Signals in High Dynamics

This paper explores the ability of vector tracking algorithms to track weak Global Positioning System (GPS) signals in high dynamic environments. Traditional GPS receivers use tracking loops to track the GPS signals. The signals from each satellite are processed independently. In contrast, vector-based methods do not use tracking loops. Instead, all the satellite signals are tracked by a lone Kalman filter. The Kalman filter combines the tasks of signal tracking and navigation into a single algorithm. Vector-based methods can perform better than traditional methods in environments with high dynamics and low signal power. A performance analysis of the vector tracking algorithms is included. The ability of the algorithms to operate as a function of carrier to noise power density ratio, user dynamics, and number of satellites being used is explored. The vector tracking methods are demonstrated using data from a high fidelity GPS simulator. The simulation results show the vector tracking algorithms operating at a carrier to noise power density ratio of 19 dB-Hz through 2 G, 4 G, and 8 G coordinated turns. The vector tracking algorithms are also shown operating through 2 G and 4 G turns at a carrier to noise power density ratio of 16 dB-Hz.

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.

Robust tracking of weak GPS signals in multipath and jamming environments

In this paper, we address the problem global positioning system (GPS) signals tracking in low signal-to-noise ratio (SNR) and multipath plus interference environments using a two-step approach: a block-averaging pre-processing (BAP) which converts the zero-mean interferences to colored Gaussian noise and a maximum likelihood (ML) based algorithm combined with a whitening transform. We apply the recently proposed BAP technique to improve the SNR. For code tracking with multipath mitigation, we exploit the fact that during a period with no data bit edges, the propagation delay causes only a circular shift to the C/A code block. This allows the decomposition of the averaged data vector into a constant C/A code component plus an undesired signal component. An efficient temporal whitening transform is derived from the sample covariance matrix and applied to suppress strong colored interferers, rendering the ML estimation problem of the multipath parameters tractable. A computationally efficient procedure for solving the complex ML optimization problem is considered using a finite difference maximization technique. By estimating the multipath signals and subtracting their contributions in a sequential scheme, the code synchronization is achieved. The resulting robust ML (RML) tracking procedure is more efficient in mitigating multipath and non-Gaussian interferences. The performance of the developed RML receiver is evaluated through computer simulations to show its superiority over that of narrow correlator and MEDLL approach.

New code delay compensation algorithm for weak GPS signal acquisition

Global positioning system (GPS) is a system combining code division multiple access (CDMA) and trilateration techniques to obtain precision position and timing information. The main goal of this paper is to present a high-sensitivity receiver for acquisition of weak GPS signals. A new peak-finding algorithm is developed to search the C/A code phase in the FFT-based correlation domain. This method can estimate the peak location accurately and provides a faster performance in software-based signal acquisition. Because the received frequency increases as the satellite advances the receiver and decreases as it recedes from the user in the Doppler circumstance, the code delay compensation (CDC) algorithm is proposed to enhance the capability of waveform search under indoor conditions. Using the carrier-aiding information, the CDC method can find the autocorrelation peak value effectively under both Doppler shift and weak signal environments. A step-by-step acquisition procedure is designed for indoor GPS receivers to detect the C/A code delayed phase effectively and reliable operation in a diversity of weak signal and Doppler environments. Simulation results demonstrate that our proposed scheme can achieve a significant improvement in processing gain over the traditional methods and acquire the GPS signal efficiently under the power levels of - 155 dBm in frequency domain processing.

A Novel Architecture for Ultra-Tight HSGPS-INS Integration

Global Positioning System (GPS) currently fulfills the positioning requirements of many applications under Line-Of-Sight (LOS) environments. However, many Location-Based Services (LBS) and navigation applications such as vehicular navigation and personal location require positioning capabilities in environments where LOS is not readily available, e.g., urban areas, indoors and dense forests. Such environments either block the signals completely or attenuate them to a power level that is 10-30 dB lower than the nominal signal power. This renders it impractical for a standard GPS receiver to acquire and maintain signal tracking, which causes discontinuous positioning in such environments. In order to address the issue of GPS tracking and positioning in degraded signal environments, a novel architecture for ultra-tight integration of a High Sensitivity GPS (HSGPS) receiver with an inertial navigation system (INS) is proposed herein. By enhancing receiver signal tracking loops through the use of optimal estimators and with external aiding, the capabilities of the receiver can be substantially improved. The proposed approach is distinct from the commonly used ultra-tightly coupled GPS/INS approaches and makes use of different tracking enhancement technologies used in typical HSGPS receivers, multichannel cooperated receivers and the current ultra-tightly coupled GPS/INS methods. Furthermore, the effects of inertial measurement unit (IMU) quality, receiver oscillator noise and coherent integration time on weak signal tracking are also analyzed. Simulated test results in both static and dynamic testes show that, the designed INS-aided GPS receiver can track the incoming weak GPS signals down to 15 dB-Hz without carrier phase locked, or 25 dB-Hz with carrier phase locked. When there are multiple strong GPS signals in view, the other weak signals can be tracked down to 15 dB-Hz with carrier phase locked.

Orbit Determination of Satellite on the Middle-High Earth Orbit Based on GPS

摘要: 利用GPS确定空间航天器轨道,尤其是GPS在中、高空间航天器定轨中的应用.针对中、高轨道中可见星少及GPS信号微弱等问题,提出了有效的微弱信号检测算法,提高了GPS接收机的接收能力.同时提出了利用伪加速度测量和轨道约束进行定轨解算的新方法,使得在HEO(高偏心轨道)中利用两颗可见星可以解算定位解,对GEO(静止地球轨道)只用一颗星就可求解,仿真结果表明该方法具有很好的效果. 关键词: GPS / GEO / HEO / 微弱信号检测 / 轨道约束