Line Of Sight Error Research Articles

Ultra-Wideband Ranging Error Mitigation with Novel Channel Impulse Response Feature Parameters and Two-Step Non-Line-of-Sight Identification.

The effective identification and mitigation of non-line-of-sight (NLOS) ranging errors are essential for achieving high-precision positioning and navigation with ultra-wideband (UWB) technology in harsh indoor environments. In this paper, an efficient UWB ranging-error mitigation strategy that uses novel channel impulse response parameters based on the results of a two-step NLOS identification, composed of a decision tree and feedforward neural network, is proposed to realize indoor locations. NLOS ranging errors are classified into three types, and corresponding mitigation strategies and recall mechanisms are developed, which are also extended to partial line-of-sight (LOS) errors. Extensive experiments involving three obstacles (humans, walls, and glass) and two sites show an average NLOS identification accuracy of 95.05%, with LOS/NLOS recall rates of 95.72%/94.15%. The mitigated LOS errors are reduced by 50.4%, while the average improvement in the accuracy of the three types of NLOS ranging errors is 61.8%, reaching up to 76.84%. Overall, this method achieves a reduction in LOS and NLOS ranging errors of 25.19% and 69.85%, respectively, resulting in a 54.46% enhancement in positioning accuracy. This performance surpasses that of state-of-the-art techniques, such as the convolutional neural network (CNN), long short-term memory-extended Kalman filter (LSTM-EKF), least-squares-support vector machine (LS-SVM), and k-nearest neighbor (K-NN) algorithms.

A field test of compact active transponders for InSAR geodesy

Abstract Compact active transponders (CATs) – also termed electronic corner reflectors – are compact electronic devices designed to receive, actively amplify and re-transmit a radar signal, e.g. a C-band radar signal received from a Sentinel-1 satellite. CATs can potentially be useful for a number of purposes, e.g. if co-located with geodetic infrastructure. However, CATs have only recently become commercially available, and therefore, the usability and long-term performance of CATs are not well known. In this study, two CATs are tested under realistic operating conditions for a period of 14 months, from July 2020 to September 2021. The displacement time series of the CATs are determined from a persistent scatterers interferometric synthetic aperture radar processing of four tracks of Sentinel-1A/-1B data with a passive corner reflector (CR) as the spatial reference. The displacement time series of the CATs are evaluated against a ground truth established from repeated levellings between the CR and the CATs. Based on the results of this study, it is found that a sudden vertical displacement of a CAT can be determined with an accuracy better than 1 cm, possibly a few millimetres. Furthermore, it is found that the mean vertical velocity of a CAT, calculated from 14 months of interferometric synthetic aperture radar displacement time series, can be determined with an accuracy of a few mm/year. Finally, the line of sight (LoS) phase error is generally found to be moderately correlated with temperature, with an instrument-specific linear relationship between LoS error and temperature ranging between approx. 0.1 and 0.2 mm/°C. This correlation between LoS phase error and temperature can in principle be used for instrument-specific calibrations, which is a topic that should be addressed in future studies.

Appointed-Time Cooperative Guidance Law With Line-of-Sight Angle Constraint and Time-to-go Control

This study proposes a three-dimensional appointed-time cooperative guidance law for multiple missiles. The maneuvering target can be hit simultaneously by multiple missiles with different expected line-of-sight (LOS) angles. It is worth noting that the actual convergence time of the LOS error or time-to-go can be treated as a tuning parameter along the LOS direction and the normal direction of LOS. Firstly, the simultaneous attack problem of multiple missiles can be regarded as a consensus problem. In the LOS direction, a distributed cooperative guidance law is presented to guarantee the time-to-go of all missiles to be consensus at the appointed time. Subsequently, a guidance law is also presented in the normal direction of LOS to achieve the expected LOS angles at the appointed time. Finally, to improve the robustness of the cooperative guidance system, an appointed-time extended state observer (ATESO) is developed for compensating the guidance commands. The stability of the proposed cooperative guidance law is fully proved by the rigorous theoretical derivation. Numerical simulation results verify the efficacy of this scheme.

Enhanced Observer Control of a Cubic Stewart Platform With Image Sensor Measurement

A cubic Stewart platform with an image sensor meausring is proposed for a line-of-sight (LOS) pointing and stabilization. The charge couple device (CCD) as a image sensor mounted on the Stewart platform can provide the LOS error only in the azimuth and elevation directions, so a decoupled control strategy through the transformation of the Jacobian matrix is introduced to transform a Multiple-Input Multiple-Output (MIMO) control into Single-Input Single-Output (SISO) control. Due to the low bandwidth of the Stewart platform and the finite sampling rate of the CCD, the closed-loop performance is greatly restricted. An optimal observer control based on Youla parameterization is proposed to improve the tracking performance in the CCD-based Stewart platform control system. This improved controller relies on the LOS error combined with the controller output to produce high gain, plugging into the tradditional closed-loop structure, which results in minimizing the error attenuation function by designing an appropriate Q-filter. Both the simulations and experiments are carried out to prove the effectiveness of the proposed observer compared with the classical feedback controller.

Improving the Positioning Accuracy of UWB System for Complicated Underground NLOS Environments

The achievement of accurate and reliable localization is invariably a predominant bottleneck to the realization of shearer automation. Unfortunately, the positioning accuracy may significantly degrade and the yield may be unsatisfactory due to the existence of the line-of-sight (LOS), non-line-of-sight (NLOS) along with mixed LOS-NLOS situations in complicated under-ground environments. To address this issue, this article proposes a novel localization algorithm called GMM-NNIMM-CLMFO, which is a combination of the Gaussian mixture model (GMM), neural network (NN)-based interacting multiple model (NNIMM), Caffery localization (CL), and moth-flame optimization (MFO). The GMM is first used to re-estimate the range, and the variational Bayesian cubature Kalman filter with the assistance of NN under the IMM framework is then respectively employed to diminish the LOS and NLOS errors. Then, based on the result of GMM-NNIMM, the CL method is used to compute the position coordinates of the target node. Finally, the MFO algorithm is implemented to optimize the positioning outcome sourced from the GMM-NNIMM-CL method. The proposed method is evaluated by using P440 sensors, and the experimental results demonstrate that the proposed technique is able to remarkably enhance the overall positioning accuracy, and outperforms other approaches. It can therefore efficiently alleviate the influence of NLOS errors and achieve more accurate position estimation, thereby manifesting better positioning performance and higher robustness in mixed LOS/NLOS situations.

Prescribed-time cooperative guidance law against maneuvering target based on leader-following strategy

This paper proposes a prescribed-time cooperative guidance law (PTCGL) against maneuvering target with variable line-of-sight (LOS) angle constraint for leader-following missiles, where the convergence times of the state errors can be arbitrarily set. The leader missile against the maneuvering target is provided as the modified proportional navigation (MPN) guidance law. The proposed PTCGL for follower missiles consist of two parts, in LOS direction, the range-to-go (Rgo) is selected as a co-variable, avoiding the estimation of time-to-go (Tgo), and a novel second-order nonlinear consensus protocol is developed to design the PTCGL; in normal LOS direction, considering the variable LOS angle constraint, the cooperative guidance law is designed with the proposed prescribed-time sliding model control (PTSMC) method. Besides, the prescribed-time convergence of Rgo and LOS errors are proved. Finally, the effectiveness and superiority of the proposed PTCGL with leader-following strategy is illustrated by numerical simulation results.

LEO Onboard Real-Time Orbit Determination Using GPS/BDS Data with an Optimal Stochastic Model

The advancements of Earth observations, remote sensing, communications and navigation augmentation based on low Earth orbit (LEO) platforms present strong requirements for accurate, real-time and autonomous navigation of LEO satellites. Precise onboard real-time orbit determination (RTOD) using the space-borne data of multiple global navigation satellite systems (multi-GNSS) becomes practicable along with the availability of multi-GNSS. We study the onboard RTOD algorithm and experiments by using America’s Global Positioning System (GPS) and China’s regional BeiDou Navigation Satellite System (BDS-2) space-borne data of the FengYun-3C satellite. A new pseudo-ambiguity parameter, which combines the constant phase ambiguity, the orbit and clock offset error of the GPS/BDS broadcast ephemeris in the line-of-sight (LOS), is defined and estimated in order to reduce the negative effect of the LOS error on onboard RTOD. The analyses on the variation of the LOS error in the GPS/BDS broadcast ephemeris indicate that the pseudo-ambiguity parameter could be modeled as a random walk, and the setting of the power spectral density in the random walk model decides whether the pseudo-ambiguity can be estimated reasonably and the LOS error could be reduced or not. For different types of GPS/BDS satellites, the LOS errors show different variation characteristics, so the power spectral density should be set separately and differently. A numerical search approach is presented in this paper to find the optimal setting of the power spectral density for each type of GPS/BDS satellite by a series of tests. Based on the optimal stochastic model, a 3-dimensional (3D) real-time orbit accuracy of 0.7–2.0 m for position and 0.7–1.7 mm/s for velocity could be achieved only with dual-frequency BDS measurements and the broadcast ephemeris, while a notably superior orbit accuracy of 0.3–0.5 m for position and 0.3–0.5 mm/s for velocity is achievable using dual-frequency GPS/BDS measurements, due to the absorption effect of the estimated pseudo-ambiguity on the LOS error of the GPS/BDS broadcast ephemeris. Compared to using GPS-alone data, the GPS/BDS fusion only marginally improves the onboard RTOD orbit accuracy by about 1–3 cm, but the inclusion of BDS satellites increases the number of the tracked GNSS satellites and thus speeds up the convergence of the filter. Furthermore, the GPS/BDS fusion could help suppress the local orbit errors, ensure the orbit accuracy and improve the reliability and availability of the onboard RTOD when fewer GPS satellites are tracked in some anomalous arcs.

Application and analysis of interacting multimode Kalman filter in location algorithm

Aiming at the non-line-of-sight (NLOS) problem that affects the accuracy of wireless location in cellular networks, the IMM method is introduced. The method based on interactive multimode Kalman filter (IMMKF) is studied and analyzed in detail. A more complex second-order and fourth-order Markov transfer model is established to describe the wireless communication channel. The positioning performance and the factors that affect the performance of IMMKF filter are analyzed by simulation. The simulation results show that IMMKF can adaptively switch between models with the change of channel propagation environment, suppress non line of sight error, and show good positioning performance and filtering performance.

Rotating GNSS Antennas: Simultaneous LOS and NLOS Multipath Mitigation

Urban environments present significant challenges to global navigation satellite system (GNSS) positioning. GNSS signals are often obstructed by buildings and other obstacles, leading to reflection and diffraction, which cause major line-of-sight (LOS) and non-line-of-sight (NLOS) multipath positioning errors. We propose a new, precise GNSS positioning technique that can simultaneously mitigate LOS and NLOS multipath errors by rotating the GNSS antenna arm horizontally at a certain angular velocity. The key idea behind this technique is to use the antenna motion to mitigate the LOS multipath effect and detect the NLOS multipath signals. The relative carrier phase between the direct signal and the multipath signal frequently changes when a GNSS antenna is moving. As a result, the effect that the strong reflected signal affects the direct signal can be reduced. We use this characteristic to mitigate LOS multipath errors by generating antenna motion. By rotating a GNSS antenna, the Doppler frequency is produced by the antenna motion. If the received GNSS signal is the NLOS signal, the direction of incoming signals is different from that of direct signals. In this situation, the phase of the generated Doppler frequency will be different, and NLOS signals can be detected by comparison with the ideal Doppler frequency. We conducted an experiment to evaluate the proposed method. The positioning error of the proposed method using LOS multipath mitigation and NLOS exclusion by the rotating antenna was decreased from 18.96 to 2.83 m. In addition, the availability of GNSS positioning increased from 44.29 to 100%.

Nonlinear Suboptimal Guidance Law With Impact Angle Constraint: An SDRE-Based Approach

In this article, a guidance law with impact angle constraint is developed using the state-dependent Riccati equation (SDRE) technique. First, the nonlinear guidance dynamics consisting of line-of-sight (LOS) angle rate and error is formulated under a planar engagement scenario. After the state-dependent coefficient parameterization, a nonlinear regulator problem is formed to concurrently zero LOS angle rate and error. Second, a preliminary guidance law is directly developed under the framework of the SDRE technique. By applying the Lyapunov stability analysis, a nonlinear suboptimal guidance law is obtained through further parameter modifications. Then, the proposed guidance law is extended to attack maneuvering targets. Finally, the effectiveness of the proposed guidance law is demonstrated through numerical simulations.

A Novel Mobile Target Localization Approach for Complicate Underground Environment in Mixed LOS/NLOS Scenarios

Accurate positioning of the shearer remains a challenge for automation of the longwall coal mining process. In this paper, the popular Ultra-wideband (UWB) positioning system that has attracted considerable attention is adopted to obtain the target node location. Unfortunately, localization accuracy is still unsatisfactory and unreliable in mixed line of sight (LOS) and non-line of sight (NLOS) scenarios. To ameliorate localization accuracy of UWB for complicate underground environment where the positioning scenarios suffered from frequently switching among LOS, NLOS, and mixed LOS-NLOS condition, the novel positioning algorithm GMM-IMM-EKF was proposed. Gaussian mixed model (GMM) was employed to re-estimate the measurement distance, and two parallel variational Bayesian adaptive Kalman filters (VBAKFs) under the structure of interacting multiple model (IMM) was utilized to smoothen the result of GMM to eliminate the LOS and NLOS errors, respectively. Then, the position of the target node was determined by exploiting extended Kalman filter (EKF) based on the outcome of IMM-VBAKF. The proposed approach was assessed by exploiting UWB P440 modules. Comparative experimental verification demonstrated that GMM-IMM-EKF strategy outperformed other positioning approaches, which can effectively reduce the adverse effect of NLOS errors and achieve higher positioning accuracy in underground environment with LOS/NLOS/LOS-NLOS transition conditions.

Error-Based Feedforward Control for a Charge-Coupled Device Tracking System

High control bandwidth is usually restricted in a finite sampling rate of a charge-coupled device (CCD)-based tracking loop, which hinders a good closed-loop performance. Conventionally, a rate feedforward controller is usually used to improve the control performance; however, it is restricted by the line-of-sight (LOS) rate, which is required to be estimated due to only the LOS error available in the CCD-based tracking control system. Besides that, it is also affected by the inverse of the control model. A new error-based feedforward (EBF) control mode is proposed to overcome these difficulties. Due to being plugged into an existing feedback control loop, the closed-loop performance of the CCD-based control system can be minimized by optimizing a Q-filter. The closed-loop stability and robustness with this EBF control system are investigated on the condition of the gain margin and the phase margin of the open-loop transfer function. Furthermore, this paper proposes a new criterion to enhance the robustness of the EBF control system and reveals the sensitivity function property. Because of not being restricted by an accurate model, this new feedforward controller cannot lead to a compromise between error attenuation and noise propagation in the control loop. Simulations and experiments are both used to testify the effectiveness of the improved control mode.

Design and Modeling of the Off-Axis Parabolic Deformable (OPD) Mirror Laboratory

Coronagraph-equipped direct imaging missions need an active wavefront control system to cancel out the optical aberrations that degrade the performance of the coronagraphs. A fast steering mirror is used to control Line-of-Sight (LoS) pointing error caused by the telescope jitter. In addition to controlling other low-order aberrations such as astigmatism and coma, high stroke, high actuator density deformable mirrors (DMs) are also used to control the electric field at the required high spatial frequencies. We are designing a testbed to verify a different deformable architecture, where the powered optic in the optical train are controllable and have lower actuator count compared to the existing DMs with flat nominal surfaces. This simplifies the packaging issue for space missions and reduces both cost and risk of having the entire coronagraph instrument's performance depending on one or two high-actuator count DMs. The testbed would also be capable of testing different low-order wavefront sensing algorithms, which focuses in the near-term on a new adaptive Kalman filtering and gradient decent method to estimate the harmonic LoS errors that affect space telescopes. In long run, we would test different machine learning techniques to estimate low-order aberrations and non-linear algorithms for digging the region of high contrast called the dark holes (DH).

An acceleration feed-forward control method based on fusion of model output and sensor data

In this paper, to enhance the tracking ability of a charge-coupled device (CCD) and micro-electro-mechanical-system (MEMS) accelerometer based low-cost tracking control system, an acceleration feed-forward control method based on fusion of model output and sensor data is proposed. To achieve the acceleration feed-forward control, the target acceleration is fed into the traditional acceleration-position dual closed-loop. Considering the CCD can only detect the line-of-sight (LOS) error of the target, the double integration of accelerometer measurement is regarded as the platform's position and is combined with the LOS error to build the target trajectory. The target acceleration is predicted by a Kalman filter, in which the combined target trajectory is used as the observed value. However, the basic acceleration feed-forward control method fails to improve the tracking ability at low frequencies due to inaccuracy of the accelerometer measurement caused by drift and noise. To solve this problem, we further propose to calculate an acceleration using the system acceleration model, and then use this calculated acceleration to fuse with the measured acceleration to obtain a new acceleration. The new acceleration, which is more accurate than the original measured acceleration in low frequency, is utilized to rebuild the target trajectory and predict the target acceleration again. Experimental results confirm the effectiveness of the proposed method.

Error-Based Observer of a Charge Couple Device Tracking Loop for Fast Steering Mirror.

The charge couple device (CCD) tracking loop of a fast steering mirror (FSM) is usually used to stabilize line of sight (LOS). High closed-loop bandwidth facilitates good performance. However, low-rate sample and time delay of the CCD greatly limit the high control bandwidth. This paper proposes an error-based observer (EBO) to improve the low-frequency performance of the CCD tracking system. The basic idea is by combining LOS error from the CCD and the controller output to produce the high-gain observer, forwarding into the originally closed-loop control system. This proposed EBO can improve the system both in target tracking and disturbance suppression due to LOS error from the CCD’s sensing of the two signals. From a practical engineering view, the closed-loop stability and robustness of the EBO system are investigated on the condition of gain margin and phase margin of the open-loop transfer function. Two simulations of CCD experiments are provided to verify the benefits of the proposed algorithm.

UWB Positioning Performance Tests Based on Chan Algorithm in IEEE 802.15.4a Channel Model

Chan algorithm is a closed form solution to the non-recursive equation set. This algorithm needs only a small amount of calculations but has a high degree of precision on positioning. It is valuable for academic reference. Firstly, it obtains the preliminary solution by using WLS (Weighted Least Squares) twice. Then, it uses the preliminary solution to linearise the nonlinear equation and finally makes the estimation of the position. The channel model can provide the model of indoor office environment ranging from 2 GHz to 10 GHz. Through the UWB (Ultra WideBand) positioning system of the channel model, the LOS(line-of-sight) environment can be simulated and TOA(Time-Of-Arrival) data measured by distance can also be obtained by sampling. However, small LOS errors included in the TOA data may lead to big ones in the positioning of 3D(three-dimensional) space and the precision of positioning may be undermined, when the data are directly applied to the Chan algorithm which is based on the TOA. In order to solve this issue, the TOA data obtained can be processed with MA(Moving Average) algorithm and the precision can be improved.