Comparing DGPS corrections prediction using neural network, fuzzy neural network, and Kalman filter

Position information obtained from standard global positioning system (GPS) receivers has time variant errors. For effective use of GPS information in a navigation system, it is essential to model these errors. A new approach is presented for improving positioning accuracy using neural network (NN), fuzzy neural network (FNN), and Kalman filter (KF). These methods predict the position components’ errors that are used as differential GPS (DGPS) corrections in real-time positioning. Method validity is verified with experimental data from an actual data collection, before and after selective availability (SA) error. The result is a highly effective estimation technique for accurate positioning, so that positioning accuracy is drastically improved to less than 0.40 m, independent of SA error. The experimental test results with real data emphasize that the total performance of NN is better than FNN and KF considering the trade-off between accuracy and speed for DGPS corrections prediction.     

An experimental performance analysis of real-time kinematic positioning with NASA’s Internet-Based Global Differential GPS

The Global Differential GPS (GDGPS) system developed by JPL aims at seamless global real-time positioning at the dm accuracy level for dual-frequency receivers either fixed or mobile, anywhere and at any time. The GDGPS system relies on GPS data transmitted in real-time to a central processing center at JPL from a global network of permanently operating GPS dual-frequency receivers. At the processing center, the Internet-based Global Differential GPS (IGDG) system, the heart of JPLs GDGPS, generates and disseminates over the open Internet special 1-s global differential corrections (IGDG corrections) to the GPS broadcast ephemerides. The IGDG corrections enhance the accuracy of GPS broadcast orbits and clocks down to the dm level and serve as the key-factor for high-precise real-time positioning of a stand-alone receiver. An experimental verification of the dm positional accuracy of the IGDG system was carried out in the Netherlands, by means of both a static and a kinematic test. During the static test GPS data were collected for 5 consecutive days using a fixed immobile receiver and processed as if in real-time. Within the framework of the kinematic test, an experiment was carried out using a kinematic platform. Our tests confirmed the dm accuracy of stand-alone receiver positioning with IGDG. The standard deviation for positioning both in static and kinematic mode appears to be 10 cm in each horizontal component and 20 cm in the vertical component. More than 99% of the IGDG corrections were received with the expected 1-s interval in the field via mobile communication, the latency of the corrections was generally from 7 to 8 s.     

GPS手机的差分定位系统研究

提高GPS手机的定位精度具有很广阔的应用前景,本文研究GPS手机的实时定位差分系统的设计和实现方法。该系统采用基准站广播位置差分值、移动站实时处理的差分GPS方案,其主要优点是定位精度高、改正速度快、设备简单、操作方便;分析了位置差分GPS的定位误差,给出了测量结果。理论分析和测试结果都表明,该系统的差分定位精度比手机GPS直接定位相对提高31m左右,定位误差达4m左右,能够满足相关领域测量精度要求。     

Generation of differential GPS corrections along with performance of quality control

The DGPS technique can provide considerably better relative positioning accuracy than the stand-alone GPS positioning, but the improvement depends on the distance between the user and the reference station (spatial correlation), the latency of differential corrections (temporal correlation), and the quality of differential corrections. Therefore, how to correctly generate differential corrections as well as their pricision is very important to the DGPS positioning technique. This paper presents a new algorithm for generating differential GPS corrections. This algorithm directly uses code and carrier observations in the measurement model of a Kalman filter, so that it is possible to use a simple stochastic model and to use the standard algorithm of the Kalman filter. The algorithm accounts for biases like multipath errors and instrumental delays in code observations and it shows how differential corrections are differently affected by code biases when dual or single frequency data is used. In addition, the algorithm can be integrated with a real time quality control procedure. As a result, the quality of differential corrections can be guaranteed with a certain probability.     

Generation of differential GPS corrections along with performance of quality control

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Precise Real-Time Positioning in WADGPS Networks

In recent years the importance of real-time positioning and navigation with the Global Positioning System (GPS) has grown rapidly. Starting from the establishment of differential GPS (DGPS) reference stations for marine and land navigation, new users and applications have emerged that resulted in a high demand for the establishment of a high-density network of reference stations around the world. Many countries have established their own DGPS service, which is either governmentally or commercially owned. These services are referred to as Local Area DGPS Systems (LADGPS). However, the costs for the establishment and maintenance of a dense network of reference stations are very high. Therefore Wide Area DGPS Systems (WADGPS) are being developed to overcome the main drawbacks of LADGPS. In this case, only a few reference stations are used to cover a large area, such s a continent like Europe. To achieve high positioning accuracies, real-time modeling of the main error sources for long-range baselines is required as errors in the satellite orbit and ionospheric refraction do not cancel entirely in double differencing. In this article, a real-time correction model based on the Kalman filter for WADGPS and networked LADGPS services is discussed and results of field tests in a WADGPS network in Europe are presented. ? 2000 John Wiley & Sons, Inc.     

Performance assessment of single- and dual-frequency BeiDou/GPS single-epoch kinematic positioning

The first results of the short baseline single-epoch kinematic positioning based on dual-frequency real BeiDou/GPS data are presented. The performance of the BeiDou/GPS single-epoch positioning is demonstrated in both static and kinematic modes and compared with corresponding GPS-only performance. It is shown that the availability and reliability of the single-frequency BeiDou/GPS and dual-frequency BeiDou single-epoch kinematic positioning are comparable to those of the dual-frequency GPS. The fixed rate and reliability of ambiguity resolution for the single- and dual-frequency BeiDou/GPS are remarkably improved as compared to that of GPS-only, especially in case of high cutoff elevations. For positioning accuracy with fixed ambiguities, the BeiDou/GPS single-epoch solutions are improved by 23 and 4 % relative to the GPS-only case for two short baseline tests of 8 km, respectively. These results reveal that dual-frequency BeiDou real-time kinematic (RTK) is already applicable in Asia–Pacific areas and that single-frequency BeiDou/GPS RTK is also achievable but only with initialization of several seconds. More promisingly, the dual-frequency BeiDou/GPS RTK can overcome the difficulties with GPS-only RTK under the challenging conditions assuming, of course, that the additional BeiDou satellites are visible.     

广域差分GPS中用户定位算法的研究

广域差分GPS中用户接收机多为较廉价的单频接收机，可以接收差分播发站发播的差分改正信号进行DGPS计算，由于WADGPS的算法和常规算法不一致，差分改正信号难以是标准的RTCM格式，因此必须研制专用的差分信号接收机和DSP运算平台，并在平台上集成用户定位的算法，用户定位软件可以实时对观测数据、差分改正信号进行解码，计算用户的三维位置和速度。     

A combined algorithm of improving INS error modeling and sensor measurements for accurate INS/GPS navigation

Although the integrated system of a differential global positioning system (DGPS) and an inertial navigation system (INS) had been widely used in many geodetic navigation applications, it has sometimes a major limitation. This limitation is associated with the frequent occurrence of DGPS outages caused by GPS signal blockages in certain situations (urban areas, high trees, tunnels, etc.). In the standard mechanization of INS/DGPS navigation, the DGPS is used for positioning while the INS is used for attitude determination. In case of GPS signal blockages, positioning is provided using the INS instead of the GPS until satellite signals are obtained again with sufficient accuracy. Since the INS has a very short-time accuracy, the accuracy of the provided INS navigation parameters during these periods decreases with time. However, the obtained accuracy in these cases is totally dependent on the INS error model and on the quality of the INS sensor data. Therefore, enhanced navigation parameters could be obtained during DGPS outages if better inertial error models are implemented and better quality inertial measurements are used. In this paper, it will be shown that better INS error models are obtained using autoregressive processes for modeling inertial sensor errors instead of Gauss–Markov processes that are implemented in most of the current inertial systems and, on the other hand, that the quality of inertial data is improved using wavelet multi-resolution techniques. The above two methods are discussed and then a combined algorithm of both techniques is applied. The performance of each method as well as of the combined algorithm is analyzed using land-vehicle INS/DGPS data with induced DGPS outage periods. In addition to the considerable navigation accuracy improvement obtained from each single method, the results showed that the combined algorithm is better than both methods by more than 30%.     

Accurate absolute GPS positioning through satellite clock error estimation

 An algorithm for very accurate absolute positioning through Global Positioning System (GPS) satellite clock estimation has been developed. Using International GPS Service (IGS) precise orbits and measurements, GPS clock errors were estimated at 30-s intervals. Compared to values determined by the Jet Propulsion Laboratory, the agreement was at the level of about 0.1 ns (3 cm). The clock error estimates were then applied to an absolute positioning algorithm in both static and kinematic modes. For the static case, an IGS station was selected and the coordinates were estimated every 30 s. The estimated absolute position coordinates and the known values had a mean difference of up to 18 cm with standard deviation less than 2 cm. For the kinematic case, data obtained every second from a GPS buoy were tested and the result from the absolute positioning was compared to a differential GPS (DGPS) solution. The mean differences between the coordinates estimated by the two methods are less than 40 cm and the standard deviations are less than 25 cm. It was verified that this poorer standard deviation on 1-s position results is due to the clock error interpolation from 30-s estimates with Selective Availability (SA). After SA was turned off, higher-rate clock error estimates (such as 1 s) could be obtained by a simple interpolation with negligible corruption. Therefore, the proposed absolute positioning technique can be used to within a few centimeters' precision at any rate by estimating 30-s satellite clock errors and interpolating them. Received: 16 May 2000 / Accepted: 23 October 2000     

An Overview of Multi-Reference Station Methods for cm-Level Positioning

Over the past few years, a significant amount of research has been conducted on the formulation of carrier phase corrections in order to enhance ambiguity resolution and to increase the distances over which precise positioning can be achieved. Recently the use of a network of multiple GPS reference stations for generating carrier phase-based corrections has emerged with great promise for use in real-time environments. However, little research has been conducted on the distribution of these corrections to potential GPS users located within, and surrounding, the network coverage area. This is an integral part of real-time kinematic DGPS, and it must be adequately addressed before a practical realization of the multireference station concept is implemented. The focus of this paper is to present a comprehensive summary of some of the multiple reference station methods, with specific attention directed toward the correction generation and dissemination processes. More specifically, the various multi-reference station methodologies have been categorized according to their underlying correction generation framework, but will be discussed in terms of the correction dissemination options presented by the various authors. The for main categories of methods investigated in this paper are: (a) partial derivative algorithms, (b) linear interpolation algorithms, (c) condition adjustment algorithms, and (d) virtual reference station methodologies. ? 2001 John Wiley & Sons, Inc.     

顾及卫星姿态的多系统精密钟差产品综合

国际GNSS服务(IGS)提供的GPS综合产品被广泛应用于各种高精度科学研究中. 随着各国卫星导航系统的发展,亟需研究针对多系统全球卫星导航系统(GNSS)产品的综合策略. 由于卫星姿态与钟差相互耦合,综合钟差时额外考虑姿态改正将进一步提高综合产品精度,因此研究了一种顾及卫星姿态的GNSS钟差综合策略,改正姿态后GPS综合残差最大可减小80%. 对142个IGS测站进行精密单点定位(PPP)解算发现,综合产品比单个分析中心产品更加稳定,东(E)、北(N)、高(U)方向的动态定位精度最大可提升22.7%、16.7%和18.3%. 相对于未顾及姿态改正的综合产品,顾及姿态改正的综合产品的动态定位精度最大可提升65.3%.      

Empirical assessment of obstruction adaptive elevation masks to mitigate site-dependent effects

GPS Solutions - Site-dependent effects are accuracy-limiting factors in static and kinematic GNSS-based positioning applications since they cannot be minimized using differential techniques. In...     

Analysis of long-range network RTK during a severe ionospheric storm

The network-based GPS technique provides a broad spectrum of corrections to support RTK (real-time kinematic) surveying and geodetic applications. The most important among them are the ionospheric corrections generated in the reference network. The accuracy of these corrections depends upon the ionospheric conditions and may not always be sufficient to support ambiguity resolution (AR), and hence accurate GPS positioning. This paper presents the analyses of the network-derived ionospheric correction accuracy under extremely varying – quiet and stormy – geomagnetic and ionospheric conditions. In addition, the influence of the correction accuracy on the instantaneous (single-epoch) and on-the-fly (OTF) AR in long-range RTK GPS positioning is investigated, and the results, based on post-processed GPS data, are provided. The network used here to generate the ionospheric corrections consists of three permanent stations selected from the Ohio Continuously Operating Reference Stations (CORS) network. The average separation between the reference stations was ∼200 km and the test baseline was 121 km long. The results show that, during the severe ionospheric storm, the correction accuracy deteriorates to the point when the instantaneous AR is no longer possible, and the OTF AR requires much more time to fix the integers. The analyses presented here also outline the importance of the correct selection of the stochastic constraints in the rover solution applied to the network-derived ionospheric corrections.     

北斗区域导航系统的PPP精度分析

北斗卫星导航系统的开放运行为其在高精度领域的应用提供了可能,系统精密单点定位性能受到了极大关注。本文首先介绍了北斗区域导航系统的星座和BDS/GPS跟踪网,分析了基于国内布站定轨的北斗卫星精密轨道和钟差精度。在此基础上研究了北斗区域导航系统静态、动态精密单点定位精度,并与GPS定位结果进行比较。实测算例表明:北斗精密单点定位可以实现静态厘米级、动态分米级的定位精度,达到目前GPS精密单点定位水平。     

嵌入式GPS/BDS实时精密单点定位方法

实时性和动态性是精密单点定位面临的关键问题。本文基于实时轨道钟差改正产品,讨论了GPS/BDS实时精密单点定位数学模型及误差处理方法。搭建嵌入式软硬件平台,研制出一套具有实时GPS/BDS双系统高精度定位功能的嵌入式设备。测试结果表明,对于嵌入式GPS/BDS实时精密单点定位,静态条件下平面方向优于3 cm,高程方向优于8 cm;动态条件下平面方向优于12 cm,高程方向优于15 cm。     

Real-time registration of airborne laser data with sub-decimeter accuracy

This paper presents a methodology for the precise registering of airborne laser data directly in flight with an accuracy that is sufficient for the majority of derived products, such as digital terrain models. We first present the strategy that integrates GPS/INS/LiDAR data for generating laser point clouds directly in flight and analyzes their accuracy. The latter requires the implementation of a functional covariance propagation on-line for all the system components (i.e. trajectory, laser, system calibration) to which influences of scanning geometry are added at the end of a flight line. The study of scanning geometry necessitates the classification of vegetation and coarse estimation of the terrain normal. This is achieved by a method that we formerly proposed for off-line quality analysis. The second part of the paper focuses on the positioning component. In high resolution scanning performed close to the terrain, the absolute accuracy of the resulting point cloud depends mainly on the quality of the trajectory which is related to the type of GPS solution (e.g. absolute positioning, DGPS, RTK). To reach sub-decimeter accuracy for the point cloud in the real-time, an RTK-GPS solution is needed. This requires the establishment of a communication link for the transmission of GPS corrections (or measurements). We analyze the usability of RTK-GPS/ALS acquired during several flights using different communication methods in the particular context of helicopter based missions. We focus mainly on the exploitation of nation-wide reference GNSS networks and confirm experimentally that the real-time registration of airborne laser data is feasible with sub-decimeter accuracy. Such quality is sufficient not only for a wide range of applications, but it also opens new opportunities for monitoring missions that require a short reaction time. Finally, we concentrate on situations when the phase and code corrections cannot be transmitted, and the quality of the differential carrier-phase positioning needs to be predicted. We validate the previously introduced indicators of positioning quality by simulated degradation of the input data.     

SBAS orbit and satellite clock corrections for precise point positioning

The quality of real-time GPS positions based on the method of precise point positioning (PPP) heavily depends on the availability and accuracy of GPS satellite orbits and satellite clock corrections. Satellite-based augmentation systems (SBAS) provide such corrections but they are actually intended to be used for wide area differential GPS with positioning results on the 1-m accuracy level. Nevertheless, carrier phase-based PPP is able to achieve much more accurate results with the same correction values. We applied SBAS corrections for dual-frequency PPP and compared the results with PPP obtained using other real-time correction data streams, for example, the GPS broadcast message and precise corrections from the French Centre National d’Etudes Spatiales and the German Deutsches Zentrum für Luft- und Raumfahrt. Among the three existing SBAS, the best results were achieved for the North American wide area augmentation system (WAAS): horizontal and vertical position accuracies were considerably smaller than 10 cm for static 24-h observation data sets and smaller than 30 cm for epoch-by-epoch solutions with 2 h of continuous observations. The European geostationary navigation overlay service and the Japanese multi-functional satellite augmentation system yield positioning results with biases of several tens of centimeters and variations larger by factors of 2–4 as compared to WAAS.     

一种联合载波相位观测值的GPS/BDS滤波导航算法

提出了一种基于历元间相位差分的GPS/BDS单机实时动态定位算法。该方法采用历元间载波相位差分数据准确计算出载体的位置变化量;并以此描述载体的运动状态变化,建立动态定位滤波模型的状态方程。同时以历元间载波相位差分数据与伪距数据作为主要观测值,采用扩展Kalman滤波实时估计载体的位置和钟差。采用自主编制的软件对静态与车载GPS/BDS实测数据进行处理,结果表明:采用该方法,定位结果精度优于传统的标准单点定位算法与载波相位平滑伪距算法;而且算法具有较好的稳定性,与载体的运动状态无关。