An open source GPS multipath simulator in Matlab/Octave

Multipath is detrimental for both GPS positioning and timing applications. However, the benefits of GPS multipath for reflectometry have become increasingly clear for monitoring soil moisture, snow depth, and vegetation growth. In positioning applications, a simulator can support multipath mitigation efforts in terms of, e.g., site selection, antenna design, receiver performance assessment, and in relating different observations to a common parameterization. For reflectometry, in order to convert observed multipath parameters into useable environmental products, it is important to be able to explicitly link the GPS observables to known characteristics of the GPS receiver/antenna and the reflecting environment. Existing GPS multipath software simulators are generally not readily available for the general scientific community to use and/or modify. Here, a simulator has been implemented in Matlab/Octave and is made available as open source code. It can produce signal-to-noise ratio, carrier phase, and code pseudorange observables, based on L1 and L2 carrier frequencies and C/A, P(Y), and L2C modulations. It couples different surface and antenna types with due consideration for polarization and coherence. In addition to offering predefined material types (water, concrete, soil, etc.), it allows certain dimensional properties to be varied, such as soil moisture and snow density.     

A multi-technique approach for characterizing the SVN49 signal anomaly, part 1: receiver tracking and IQ constellation

A characterization of the signal anomaly of SVN49 is presented. A mathematical model is developed to relate the observed multipath to the internal signal reflection. The analyses provided are based on measurements, which have been collected during a dedicated tracking campaign with a 30-m dish antenna. Data on the L1 and L2 frequency have been collected with four different receivers. In addition, IQ samples have been recorded directly with a spectrum analyzer. The multipath combination of the receiver measurements on L1 and L2 is analyzed to demonstrate the effect of the signal reflections on different correlator spacing. The capability to suppress the signal reflection with receiver multipath mitigation methods is demonstrated. Finally, preliminary estimates of the attenuation, delay, and phase shift over elevation are obtained from an IQ sample analysis.     

Multipath detection based on single orthogonal dual linear polarized GNSS antenna

GPS Solutions - Based on the polarization difference between the multipath and the line-of-sight (LOS) signal, a method for multipath detection using a single antenna is proposed. The antenna has...     

有限带宽信道条件下的Double-delta技术多径抑制性能分析及优化设计

以一种基于码跟踪多径误差包络的多径抑制性能评估准则为基础,首先根据Double-delta技术的基本原理,首次推导出了其在无限带宽时的码跟踪多径误差公式,并由此得出了Double-delta技术的理论性能计算公式;然后以数值计算的方法分析了不同信道带宽条件下Double-delta技术的多径抑制性能。仿真结果表明,当信...     

Quantitative evaluation of multipath rejection capabilities of GNSS antennas

Multipath error remains the largest error source in many high precision GPS applications. To counteract this problem, solutions at both software and hardware level have been studied. Software processing by means of measurement redundancy or error predictability can be used in order to mitigate the multipath effects. In general, these techniques work properly only when the length of a reflection path exceeds that of the direct path by more than 10–20 m. Unfortunately, in most cases, reflections are generated in the area near the receiving antenna. For this reason, multipath rejection actuated at the antenna level is one of the most valid means to improve the accuracy of GPS systems. The scope of this work is twofold. First, a review of low-multipath reception requirements will be proposed for comparing different classes of high precision GNSS antennas. Based on this discussion, we introduce a quantitative evaluation of multipath rejection capabilities of a GNSS antenna. The proposed assessment technique is focused on the antenna pattern, but, in contrast to other parameters evaluating the antenna radiation characteristics, it is specifically conceived to capture the effects of multipath signals.     

双极性天线在多路径效应探测中的应用

多路径效应是影响卫星定位精度和稳定性的重要因素。针对多路径环境复杂多变难以进行分析的问题,该文提出了一种利用双极性天线探测多路径的方法。详细介绍了反射信号的极性特征;通过判断信号的极性可以有效地区分直射信号和反射信号。分析了通过信号极性实现多路径探测的理论:组成双极性天线的右螺旋圆极化天线和左螺旋圆极化天线能够分别输出卫星信号中的右螺旋圆极化分量和左螺旋圆极化分量,如果两个分量的载噪比差值大于一定的门槛值,就可以推断卫星信号在传输过程中经历了多路径效应。最后通过仿真实验验证了该方法的可行性和有效性。     

GPS多路径效应实例计算与分析

本文讨论了一个GPS伪距多路径值计算及其频谱分析的方法。文中首先介绍了GPS多路径试验测量和观测数据采集的程序。然后探测和修复周跳、再利用伪距多路径计算公式,获得GPS测量中的伪距多路径值。实例证明该方法有效地计算出了伪距多路径值。     

Carrier phase multipath error characterization and reduction in single aircraft relative positioning

This paper describes GPS carrier phase (CP) multipath characterization and error reduction techniques and their application in single aircraft relative positioning. In particular, the single aircraft relative positioning scenario has applications for high-accuracy multi-sensor stabilization where CP multipath is a major error source that limits system performance. We will briefly review the requisite multipath theory and discuss models for quantifying the error characteristics. Field-test data will be used to validate the multipath models considering the underlying assumptions. A basic geometric reflection point theory will be presented to demonstrate the physical environmental sensitivity of CP multipath to parameters such as surface flatness and antenna height. Several different quantities will be described as multipath indicators for time-domain detection and will be compared with a frequency-domain technique. A new multipath detection approach will be introduced that is suitable to track multipath from time-varying reflection/diffraction points. Finally, the narrow-lane antenna baseline processing technique will be presented as a real-time approach to mitigate CP noise and multipath errors that is well suited for a very short baseline single aircraft high accuracy relative positioning system. Field-test results and analysis will be used to illustrate the key concepts in this paper and to help characterize the total navigation system performance.     

GPS码跟踪环自适应多径消除算法

根据多径信号的产生机理,在对GPS接收机中的码跟踪环多径信号模型研究的基础上,提出了采用自适应滤波的来消除GPS多径效应的算法。自适应滤波的方法不需要估计模型的系统参数,而直接通过自适应滤波将多径信号滤除。在有噪声的情况下,自适应滤波的RLS算法是最小二乘意义下的最优估计,仿真的结果表明采用自适应滤波算法可以快速的消除多径的影响,修正鉴相函数的过零点偏差,提高码跟踪环的跟踪精度。由于自适应滤波算法是递推算法,易于软、硬件实现。     

利用信噪比削弱GPS多路径效应的研究

由于多路径误差的非时空相关性,使其成为双差模型中较难解除的误差源。本文利用观测值的信噪比对观测值质量进行评价,通过降低受多路径效应影响的观测值的权重,从而达到削减多路径误差的目的。最后通过实验数据解算结果的重复性验证了此方法的有效性和可靠性。     

一种顾及先验约束的北斗观测数据多路径一步修正模型

卫星观测数据质量是衡量系统服务性能的关键指标之一。多路径延迟作为导航定位服务中的主要误差源,是全球导航卫星系统数据处理领域的研究热点。针对多路径分步处理策略无法获得全局最优解的问题,提出一种基于先验约束的北斗卫星导航系统（BeiDou navigation satellite system, BDS）观测数据多路径实时修正模型。首先,基于提取的多路径延迟序列,利用全变分正则化算法实现多路径序列的分离与降噪;然后,利用最小二乘和自回归模型对多路径序列中趋势项与随机项进行一步建模处理;最后,以求解的多路径模型系数为先验信息,精化实时服务条件下BDS多路径处理模型。BDS实时精密单点定位实验表明,相较于传统多路径处理策略,经顾及先验约束的BDS观测数据一步修正后,BDS-2定位精度在E、N、U方向分别提升了10.6%～64.9%、0.0%～59.1%和12.6%～67.2%(B1I+B3I),BDS-3定位精度在E、N、U方向分别提升了13.9%、60.0%、45.9%(B1I+B3I)与19.1%、46.5%、23.9%(B1C+B2a)。所提的多路径处理模型可有效改善BDS实时定位应用中...     

GPS snow depth meter with geometry-free linear combinations of carrier phases

Multipath in global positioning system (GPS) is the interference of the microwave signals directly from satellites and those reflected before reaching the antenna, typically by the ground. Because reflected signals cause positioning errors, GPS antennas are designed to reduce such interference. Recent studies show that multipath could be utilized to infer the properties of the ground around the antenna. Here, we report one such application, i.e. a fixed GPS station used as a snow depth meter. Because the satellite moves in the sky, the excess path length of reflected waves changes at rates dependent on the antenna height. This causes quasi-periodic variations of the amplitude and phase of the received signals. Accumulation of snow reduces effective antenna heights, and we can see it by analyzing multipath signatures. Signal-to-noise ratios (SNR) are often used to analyze multipath, but they are not always available in raw GPS data files. Here, we demonstrate that the geometry-free linear combination (L4), normally used to study the ionosphere, can also be used to analyze multipath signatures. We obtained snow depth time series at a GPS station in Hokkaido, Japan, from January to April in 2009 using L4 and SNR. Then, we compared their precisions. We also discuss mechanisms responsible for the possible underestimation of the snow depth by GPS. Finally, we investigate the possibility of inferring physical conditions of the snow surface using amplitudes of multipath signatures.     

GNSS接收机观测数据多径效应分析方法研究

多径效应是影响GNSS接收机观测数据质量的主要误差因素之一。首先,介绍了多径效应及其信号特性,分析了多径效应对GNSS接收机伪距测量和载波相位测量的影响,阐述了三种GNSS接收机观测数据多径效应分析方法,并结合实际观测数据给出了多径效应分析结果,最后介绍了几种常用的多径效应抑制方法。     

GPS参考站网络的多路径误差建模技术研究

通过研究多路径误差的周日重复性特征,将参考站网络多路径历史数据用于减弱参考站网络基线解算中的多路径效应影响,以提高网络解算(尤其指低高度角卫星和新升卫星)的初始化进程及可靠性。通过分析多路径误差的产生机理,研究多路径误差与信噪比信息的强相关关系,建立信噪比相关网络内插模型(SNRIM),用于流动用户位置高精度多路径改正数的实时计算。最后通过四川GPS观测网络实验,验证上述方法在网络高精度实时定位领域应用的有效性及可靠性。     

GPS multipath mitigation: a nonlinear regression approach

Under the assumption that the surrounding environment remains unchanged, multipath contamination of GPS measurements can be formulated as a function of the sidereal repeatable geometry of the satellite with respect to the fixed receiver. Hence, multipath error estimation amounts to a regression problem. We present a method for estimating code multipath error of GPS ground fixed stations. By formulating the multipath estimation as a regression problem, we construct a nonlinear continuous model for estimating multipath error based on well-known sparse kernel regression, for example, support vector regression. We will empirically show that the proposed method achieves state-of-the-art performance on code multipath mitigation with 79 % reduction on average in terms of standard deviation of multipath error. Furthermore, by simulation, we will also show that the method is robust to other coexisting signals of phenomena, such as seismic signals.     

Effects of azimuthal multipath asymmetry on long GPS coordinate time series

Carrier phase multipath is currently one source of unmodeled signals that may bias GPS coordinate time series significantly. We investigate the effect of simulated carrier phase multipath on time series of several sites covering the period 2002.0–2008.0 and spanning a range of observation geometries. High-, mid-, and low-latitude IGS sites are investigated as well as sites with significant signal obstructions. We examine the effect of multipath in different sectors of the sky, considering time-constant, horizontal reflectors at each of 0.1, 0.2, and 1.5 m below the antenna. The differences between a horizontally uniform multipath source are analyzed, and it is shown that positioning errors are generally larger when unmodeled carrier phase multipath is azimuthally heterogeneous. Using the adopted multipath model, height biases reach ±1 mm in case of the symmetric multipath and ±5 mm for the asymmetric multipath but this increases to being ±10 mm in the worst case. In addition to mean bias, low-frequency variations in the bias also exist, including periodic signals and leading to velocity biases of up to ±0.1 mm/year in the symmetric case and ±1 mm/year in the asymmetric case over the considered period. In contrast to the generally slowly varying observation geometry that is typically experienced, we show the effects of an abrupt change in geometry due to receiver/antenna hardware changes; in the case considered, we see changed pattern of temporal variation in the bias in addition to an instantaneous offset.     

减小伪卫星多路径效应误差的试验研究

伪卫星定位技术可以补偿GPS系统本身一些先天不足，然而GPS和伪卫星组合进行定位又带来了新的误差源，伪卫星多路径效应就是误差源之一。针对多路径的产生原理及特点，在经典最小二乘理论的基础上，推求稳健最小二乘估计的权函数，并结合实际观测数据验证了稳健最小二乘法消除多路径误差的有效性。     

Evaluating pseudorange multipath effects at stations in the National CORS Network

A preliminary study was conducted to evaluate the amount of pseudorange multipath at 390+ sites in the National Continuously Operating Reference Station (CORS) Network. The National CORS Network is a cooperative effort involving over 110 different agencies, universities, and private companies who seek to make GPS data from dual-frequency receivers located throughout the United States and its territories available to the general public. For CORS users, pseudorange multipath can seriously degrade the accuracy of any application that relies on precise measurements of the pseudorange observable over a short period of time, including differential pseudorange navigation, kinematic and rapid-static surveying, and ionospheric monitoring. The main objectives of this study were to identify the most affected and least affected sites in the network, to closely investigate problematic sites, and to compare various receiver/antenna combinations. Dual-frequency carrier phase and pseudorange measurements were used to estimate the amount of L1 and L2 pseudorange multipath at each site over a one-year period. Some of the most severely affected sites were maritime Differential GPS and Nationwide Differential GPS (DGPS/NDGPS) sites. Photographs obtained for these sites verified the presence of transmission towers and other reflectors in close proximity to the GPS antennas. Plotting the variations of the L1 and L2 pseudorange multipath with respect to azimuth and elevation further verified that even above a 60° elevation angle there was still as much as five meters of pseudorange multipath at some sites. The least affected sites were the state networks installed in Ohio and Michigan; these sites used excellent antenna mounts, choke ring antennas, and new receiver technology. A comparison of the 12 most commonly used receiver/antenna combinations in the CORS Network indicated that newer receivers such as the Ashtech UZ-12, Leica RS-500, and Trimble 5700 help to significantly mitigate pseudorange multipath, while the receivers/antennas at some DGPS/NDGPS sites, and the antennas formerly used at the Wide Area Augmentation System (WAAS) sites, are among those most affected by pseudorange multipath. The receiver/antenna comparison did not take into account the potential presence of reflectors at the sites (i.e., it is possible that a well-performing receiver/antenna combination could have been consistently placed at very poor site locations, and vice-versa).Product Disclaimer: Mention of a commercial company or product does not constitute an endorsement by the National Oceanic and Atmospheric Administration. Use for publicity or advertisement purposes of information from this paper concerning proprietary products or the comparison of such products is not authorized.     

GNSS导航信号的开环补偿多径抑制方法

多径的存在会给全球导航卫星系统的接收机带来较大的定位误差。因此,高精度的接收机须对多径信号进行抑制。针对目前常用的多径抑制方法的优缺点,提出了一种基于多门延迟和曲线拟合的多径抑制方法。该方法通过多门延迟来重塑伪码的自相关函数,用于找到直射信号的伪码真实位置和接收机码跟踪环路鉴别结果之间的偏差,进一步通过曲线拟合方法更加精确地计算出该偏差,最后将该偏差通过开环方法补偿给伪距计算,使得接收机在不改变环路跟踪性能和抗动态干扰性能的前提下实现定位性能的提升。仿真结果表明新算法在前端带宽的影响下对短、中长多径均能进行有效地抑制。      

Development and testing of a new ray-tracing approach to GNSS carrier-phase multipath modelling

Multipath is one of the most important error sources in Global Navigation Satellite System (GNSS) carrier-phase-based precise relative positioning. Its theoretical maximum is a quarter of the carrier wavelength (about 4.8 cm for the Global Positioning System (GPS) L1 carrier) and, although it rarely reaches this size, it must clearly be mitigated if millimetre-accuracy positioning is to be achieved. In most static applications, this may be accomplished by averaging over a sufficiently long period of observation, but in kinematic applications, a modelling approach must be used. This paper is concerned with one such approach: the use of ray-tracing to reconstruct the error and therefore remove it. In order to apply such an approach, it is necessary to have a detailed understanding of the signal transmitted from the satellite, the reflection process, the antenna characteristics and the way that the reflected and direct signal are processed within the receiver. This paper reviews all of these and introduces a formal ray-tracing method for multipath estimation based on precise knowledge of the satellite–reflector–antenna geometry and of the reflector material and antenna characteristics. It is validated experimentally using GPS signals reflected from metal, water and a brick building, and is shown to be able to model most of the main multipath characteristics. The method will have important practical applications for correcting for multipath in well-constrained environments (such as at base stations for local area GPS networks, at International GNSS Service (IGS) reference stations, and on spacecraft), and it can be used to simulate realistic multipath errors for various performance analyses in high-precision positioning.