Chung-Liang Chang

An adaptive multipath mitigation filter for GNSS applications

Global navigation satellite system (GNSS) is designed to serve both civilian and military applications. However, the GNSS performance suffers from several errors, such as ionosphere delay, troposphere delay, ephemeris error, and receiver noise and multipath. Among these errors, the multipath is one of the most unpredictable error sources in high-accuracy navigation. This paper applies a modified adaptive filter to reduce code and carrier multipath errors in GPS. The filter employs a tap-delay line with an Adaline network to estimate the direction and the delayed-signal parameters. Then, the multipath effect is mitigated by subtracting the estimated multipath effects from the processed correlation function. The hardware complexity of the method is also compared with other existing methods. Simulation results show that the proposed method using field data has a significant reduction in multipath error especially in short-delay multipath scenarios.

Performance analysis of narrowband interference mitigation and near-far resistance scheme for GNSS receivers

This paper presents a method to mitigate the interferences against pseudolite-type and continuous-wave interferences in a global navigation satellite system (GNSS) environment. Ground-based pseudolites can transmit GPS-liked signals and augment GNSS constellations to enhance the accuracy, integrity, and availability. Unfortunately, a nearby pseudolite may induce a strong interference to GPS receivers. As a result, the receiver may fail to acquire and track signals from satellites. In order to account for such a problem, a mitigation scheme that combines the use of frequency domain excision method and direct reconstruction method is proposed to effectively nullify continuous-wave and pseudolite-type interferences, so that the GPS signals can still be tracked in the presence of strong interferences. Experiment results show that with the removal of interferences, the measured narrow-to-wide power ratio for all satellites can be enhanced. A sensitivity analysis is then conducted to assess the performance of the proposed algorithm in the presence of errors in the estimation of the signal power, Doppler frequency and code delay. Finally, the problem of combined interferences of pseudolites and continuous-wave sources is investigated both theoretically and experimentally. It is shown that the method is applicable to mitigate narrowband continuous-wave interferences and pseudolite-type interferences.

A new pre-processing approach against array uncertainty for GNSS

Adaptive antenna processing has been widely applied to GNSS interference mitigation. Nevertheless, a great variety of uncertainties exist in array signal processing, such as array gain, phase, antenna location and mutual coupling, etc. These uncertainties in array system are what concern designers in optimizing their algorithms. The proposed approach is presented as a solution by appending an adaptive filter and employing a mixed least mean squares (LMS)/ Hinfin-optimal algorithm as a pre-processor to the beamformer. The simulation results demonstrate that the algorithm can offer improved capabilities for anti-jamming, isolate dispersed interferences as well as jammers and eliminate system uncertainties.

Using fuzzy logic controller with adaptive detection scheme for fast acquisition of satellite navigation signals

Abstract The paper presents a fuzzy logic controller (FLC) with adaptive threshold control scheme to regulate frequency search step size so as to enhance performance in global navigation satellite system (GNSS) signal acquisition. Conventional frequency search can be achieved utilizing step‐by‐step frequency bin search or through fast Fourier transform (FFT). Nevertheless, too large a frequency search step size and short length of FFT result in large loss of correlation, whereas too small frequency search step size and long length of FFT increase signal acquisition time. To shorten signal acquisition time without influencing the accuracy of signal parameters, the FLC is employed to regulate the frequency search step size in the frequency domain on the basis of an input variable: code correlation value and its rate of change. To enhance the robustness of signal acquisition performance, the signal detection threshold is adaptively adjusted and combined with FLC. Besides, the basis value of membership function in FLC is adjustable. Hence, the input and output parameters of membership functions (MFs) vary with the adjustment of the detection threshold. Simulations and experimental results demonstrate that the proposed method enhances acquisition performance with regards to execution time and precision of parameters in GNSS signals.

Self-tuning synthesis filter against mutual coupling and interferences for gnss and its implementation on embedded board

Traditional spatial-temporal adaptive signal processing techniques are often applied to conduct narrowband and wideband interferences. However, its mitigation performance degrades greatly due to mutual coupling. To solve this problem, this paper aims to utilize a spatial-temporal self-tuning synthesis filter capable of mutual coupling compensation and interference mitigation. The spatial filter and temporal filter are to compensate for the effect of mutual coupling and interference mitigation, respectively. Self-tuning mechanism is to adopt least square (LS) and minimum variable distortionless response- (MVDR-) based method to adjust spatial and temporal weights of antenna array. The experiment platform is established by the embedded development board. Simulation and experiment results demonstrate that the proposed method can effectively compensate for mutual coupling, mitigate the cochannel interference up to 30 dB, and enhance the acquisition performance of receivers in global navigation satellite system (GNSS).

Low-Complexity Spatial-Temporal Filtering Method via Compressive Sensing for Interference Mitigation in a GNSS Receiver

A compressive sensing based array processing method is proposed to lower the complexity, and computation load of array system and to maintain the robust antijam performance in global navigation satellite system (GNSS) receiver. Firstly, the spatial and temporal compressed matrices are multiplied with array signal, which results in a small size array system. Secondly, the 2-dimensional (2D) minimum variance distortionless response (MVDR) beamformer is employed in proposed system to mitigate the narrowband and wideband interference simultaneously. The iterative process is performed to find optimal spatial and temporal gain vector by MVDR approach, which enhances the steering gain of direction of arrival (DOA) of interest. Meanwhile, the null gain is set at DOA of interference. Finally, the simulated navigation signal is generated offline by the graphic user interface tool and employed in the proposed algorithm. The theoretical analysis results using the proposed algorithm are verified based on simulated results.

A Novel Pre-Processing Scheme to Enhance GNSS Signal Detection in the Presence of Blanking

In air navigation, the rotation of aircraft results in the discontinuous tracking of GNSS signals. As the platform rotates, the GNSS signals are subject to blanking effects. To solve this problem, a ring-type antenna array is used to prevent signal discontinuity and a hypothesis-test based detection scheme is developed so that the correct antenna combination can be formed to provide uninterrupted reception of GNSS signals in the presence of blanking, noise, and interferences. A fixed threshold detection scheme is first developed by assuming that the statistics of the noise are known. It is shown that the scheme is capable of differentiating signal from noise at each antenna element. To account for the interference effect, a multiple hypothesis test scheme, together with an adaptive selection rule, is further developed. Through this detection and selection process, it is shown, through simulations, that the desired GNSS signals can be extracted and successfully tracked in the presence of blanking and co-channel interference.

A Dynamic Cooperative Scheme with Multiple Antennas for Indoor Mobile Robot Localization

This paper proposes a spatial cooperative diversity and decision-making technique to enhance signal detection and indoor mobile robot positioning performance of a global positioning satellite system (GNSS) receiver. Though the adaptive antenna array technique in early research could effectively promote antijamming freedom, overcome time-varying system, and mitigate narrowband and wideband interferences, factors such as the decrease of signal magnitude caused by obstacles (especially in the indoor environment), multipath, and blanking effect caused by the change in antenna direction with the motion of mobile robot can degrade the detection and interference mitigation performance of GNSS receivers. This paper aims to develop a dynamic cooperative scheme to proceed with the switch, selection, combination, and optimization among antennas. In addition, a signal processing experimental platform is also established to receive actually indoor GNSS signals for verification. The proposed scheme is capable of effectively promoting the postcorrelation signal-to-noise ratio (SNR) capability of a GNSS receiver under the indoor environment.

The effect of intermittent signal on the performance of code tracking loop in GNSS receivers

This paper analyzes the code tracking performance in the presence of signal blanking in Global Navigation Satellite System(GNSS). The blanking effect is usually caused by buildings that obscure the signal in either a periodic or random manner. In some cases, ideal blanking is used to remove random or periodic interference. Nevertheless, the effect of temporary discontinuity of signal often leads to the tracking and position error. To analyze this problem, three types of blanking model are considered: no blanking, periodic blanking, and random blanking of the signals input into the code tracking loop. The mean time to lose lock (MTLL) is to assess the performance of code tracking system under signal blanking. Finally, the effect of steady-state tracking errors on the performance of tracking loop resulting from blanking environment is also discussed.

Smart Agricultural Machine with a Computer Vision-Based Weeding and Variable-Rate Irrigation Scheme

This paper proposes a scheme that combines computer vision and multi-tasking processes to develop a small-scale smart agricultural machine that can automatically weed and perform variable rate irrigation within a cultivated field. Image processing methods such as HSV (hue (H), saturation (S), value (V)) color conversion, estimation of thresholds during the image binary segmentation process, and morphology operator procedures are used to confirm the position of the plant and weeds, and those results are used to perform weeding and watering operations. Furthermore, the data on the wet distribution area of surface soil (WDAS) and the moisture content of the deep soil is provided to a fuzzy logic controller, which drives pumps to perform variable rate irrigation and to achieve water savings. The proposed system has been implemented in small machines and the experimental results show that the system can classify plant and weeds in real time with an average classification rate of 90% or higher. This allows the machine to do weeding and watering while maintaining the moisture content of the deep soil at 80 ± 10% and an average weeding rate of 90%.