AN INSTANTANEOUS AMBIGUITY RESOLUTION PROCEDURE SUITABLE FOR MEDIUM-SCALE GPS REFERENCE STATION NETWORKS

This contribution describes a recent Australian Research Council (ARC) project funded under the ARCLinkage Scheme. The research team comprises researchers from RMIT University, UNSW, University of Melbourne, Spatial Information Infrastructure and the Department of Lands, NSW. The aim of the project is to enhance the utility of continuously operating reference station (CORS) networks in the states of Victoria and New South Wales by developing improved atmospheric correction models to support high accuracy, real-time positioning even when the density of reference stations is insufficient for standard operational GPS techniques such as RTK (‘real-time kinematic’). Many applications of Global Navigation Satellite System (GNSS) technology, such as surveying, mapping and precise navigation, require real-time positioning accuracies to centimetre levels. To support these applications, many countries are establishing dense CORS networks with stations, positioned typically a few tens of kilometres apart. However, for Australia with its large and sparsely populated landmass, such dense networks cannot be justified economically. This ARC project will investigate enhancements of sparse networks to maintain similar levels of accuracy as dense CORS networks. It will seek a better understanding and modelling of atmospheric conditions, currently a major limitation in the use of sparse networks for high accuracy techniques. This paper will describe the status of current developments in CORS network infrastructure in Australia, namely GPSnet in Victoria and SydNet in New South Wales. The major research components of the project will be outlined and the technical and practical challenges will be discussed, including some methodologies that will be investigated.

One of the main tasks of geodesy is to determine coordinates with high accuracy using GNSS-observations. To perform such tasks in geodesy usually use the relative method of determining the coordinates in static mode. The technique of the relative method is based on determining the coordinates of an unknown point relative to a known one. The coordinates of the base point must be known, often precisely specified on the basis of the State Geodetic Networks. Static mode of observation is most often used to build geodetic networks, because it has the highest accuracy. In this study, the base points were active reference stations, the coordinates of which are set from the weekly combined solution of the GNSS-network. The purpose of the work is to investigate the accuracy of the application of the method of determining the coordinates according to GNSS-observations with reference to the network of active reference stations. Methodology. The data of GNSS-observations performed at the triangulation points of the State Geodetic Network and data of reference stations are used in the work. Based on these data, conditional GNSS-networks were formed for their processing in the software package, which consisted of three reference stations and one triangulation point. The data processing process assumed that the nearest reference stations were taken as a control point, the specified coordinates of which were recorded as control and set in the ETRS2000 system. The obtained sets of coordinates of the same points and stations were transformed into the USK2000 system. The accuracy of the coordinates determined in this way was analyzed on the basis of coordinate differences and their standard deviations. Differences for reference stations were determined relative to the refined weekly coordinates, and for triangulation points relative to the mean. Results. The obtained differences at the points of the state network are within 1–2 cm, acquiring both positive and negative values. The only exception is the fifth observation session, where the differences are 2–4 cm with a positive sign. At the KOVL reference station, the coordinate differences vary from –1.2 cm to +1.8 cm, and at the MEL2 station from 0 cm to 5.4 cm. Coordinate changes at triangulation points were estimated by standard deviations of 2.1 cm, 1.1 cm, and 1.9 cm for XYZ, respectively. The accuracy of all other coordinates of the reference stations is 0.3–1.6 cm, with an average range from –2.7 cm to +1 cm. Scientific novelty and practical significance. The paper shows the method of determining the coordinates in the system USK2000 according to GNSS-observations with reference to the network of active reference stations. The proposed technique allows the use of satellite methods to determine the coordinates in the state system USK2000 with an accuracy of 1–2 cm, as well as significantly speed up and speed up the process of field work.

High accuracy acceleration determination by using multiple GNSS reference stations

The Japanese Quasi-Zenith Satellite System (QZSS) has recently (October 2017) reached its first 4-satellite constellation. In this contribution, the standalone performance of this 4-satellite QZSS constellation is assessed by means of its triple-frequency (L1 + L2 + L5) real-time kinematic (RTK) integer ambiguity resolution and precise positioning capabilities. Our analyses are carried out for data collected in Perth, Australia, and include a study of the noise characteristics of the QZSS code and phase data, particularly concerning their precision, time correlation and multipath. Our results show that while the phase observations on different frequencies are of similar precision, the code observations on different frequencies show considerably different precisions and can be ordered, from high to low, as L5, L2 and L1. As to positioning and ambiguity resolution, we demonstrate that the Position Dilution Of Precision (PDOP) and the Ambiguity Dilution Of Precision (ADOP) exhibit complementary characteristics, both of which are important for predicting precise positioning capabilities. We show that despite the large PDOPs, the ADOPs are sufficiently small to indicate (almost) instantaneous successful ambiguity resolution. This is confirmed by our empirical data analyses, demonstrating that instantaneous ambiguity resolution is feasible, despite the relatively poor 4-satellite receiver-to-satellite positioning geometry over Australia, thus showing that already now centimeter-level stand-alone QZSS positioning is possible with the current 4-satellite constellation (February–March 2018).

High-precision vehicle positioning is key to the implementation of modern driving systems in urban environments. Global Navigation Satellite System (GNSS) carrier phase measurements can provide millimeter- to centimeter-level positioning, provided that the integer ambiguities are correctly resolved. Abundant code measurements are often used to facilitate integer ambiguity resolution (IAR); however, they suffer from signal blockage and multipath effects in urban canyons. In this contribution, a light detection and ranging (lidar)-aided instantaneous ambiguity resolution method is proposed. Lidar measurements, in the form of 3D keypoints, are generated by a learning-based point cloud registration method using a pre-built high-definition map and integrated with GNSS observations in a mixed measurement model to produce precise float solutions, which in turn increase the ambiguity success rate. Closed-form expressions of the ambiguity variance matrix and the associated Ambiguity Dilution of Precision (ADOP) are developed to provide a priori evaluation of such lidar-aided ambiguity resolution performance. Both analytical and experimental results show that the proposed method enables successful instantaneous IAR with limited GNSS satellites and frequencies, leading to centimeter-level vehicle positioning.

The correct ambiguity resolution between reference stations is the key to calculate the high precision Network Real-Time Kinematic (NRTK) differential information. For long range reference stations (≧50 km), the double difference troposphere model residuals should be considered as the parameters being solved, but this will aggravate the ill conditioning of ambiguity resolution (AR) model between reference stations; as a result, the ambiguity fixing becomes more difficult for the case of long range station ambiguity resolution. In the paper, a new method with double troposphere parameters restriction is put forward for ambiguity resolution of long range reference stations. The proposed method applies GPT2 model, which is called the state of the art empirical troposphere model, to form a high precision troposphere a priori estimation to provide high accuracy double difference troposphere delay estimation. Based on the principles of TIKHONOV regularisation, a regularisation criterion for the double difference restriction model is then built. The difference between the troposphere estimation and the truth value is used as a restriction parameter to improve the estimation of the unknown parameters and optimisation of the ambiguity search range. Trials verify the significant reduction in the ill conditioning of the parametric resolution functions when the double troposphere restriction model is applied. The success rate of ambiguity resolution within 60 s is above 98% for baselines over 80 km, which is an immense improvement from conventional methods.

In order to solve the problem of low precision of tropospheric delay interpolation under the long-distance sparse reference station, a method of tropospheric delay interpolation estimation based on support vector machine (SVM) theory was proposed. Firstly, the troposphere was assumed to be an infinitely thin single-layer membrane, and with the known information of reference stations, the puncture point coordinates and zenith tropospheric delay of each reference station on each visible satellite in the single-layer membrane were obtained. Then, the puncture point coordinates and zenith tropospheric delay were taken as training samples to optimize the appropriate core parameters of SVM and establish the SVM model of tropospheric delay. Finally, two sets of network RTK data with different lengths were selected to compare and analyze the effect of the interpolated tropospheric delay. The results showed that under the long-distance reference station, with the SVM tropospheric delay model established in this paper, the tropospheric delay accuracy of the interpolation estimation is better than 2cm, and the estimation error is generally stable, and the interpolation effect of satellite in different systems is basically the same.

Influence of the input layer signals of ANNs on wind power estimation for a target site: A case study

When processing code- and carrier-phase ranges sampled at individual epochs, the control baseline of a pair of reference receivers can bring forth relative integer-valued ambiguities and tropospheric delays. The estimates are usually so accurate that they may act as constraints, when a roving user who operates in the network of references requires instantaneous position solutions. This paper has the objective of taking advantage of a tropospheric zenith delay surface for the user’s ranging corrections. A local network of six reference stations was used to study the single-epoch position estimation, which revealed that with the increased reliability of ambiguity resolution on both L1 and L2 carriers, global positioning system-derived height accuracy was refined by more than 60%.

For precise positioning in arctic regions with Global Navigation Satellite Systems (GNSS), such as GPS and Glonass, there are a number of issues to be resolved or taken into account. The most important ones are satellite geometry, ionospheric effects and distribution of correction data. Precise Point Positioning (PPP) is currently the standard for precise offshore positioning. PPP requires a sparse global network of GNSS reference stations to estimate precise satellite orbit and clock parameters in real-time. These parameters are transmitted to users, who can compute their position with an accuracy of 0.1 m using code and carrier observations. The main disadvantage of PPP is that in general it takes a long time to converge to this accuracy, about 30–45 minutes. GNSS carrier observations are similar to pseudo ranges, but have an additional bias which needs to be estimated. Under certain conditions, these biases are integer. This property is used in what is generally referred to as Real-Time Kinematic (RTK) positioning, a technique which uses corrections generated at reference stations to make sure these ambiguities are integer. Integer Ambiguity Resolution (IAR) results in RTK accuracies of 0.01–0.03 m or better and short convergence times, ranging from instantaneous to several minutes for long inter-station distances. The disadvantage is the limited area of applicability, because RTK in general does not use precise orbits and clocks. Current research focuses on the combination of PPP and RTK techniques. The main goal is to achieve cm accuracy with a sparse network of reference stations. Another goal is to reduce convergence time. Applications are in the field of precise real-time offshore positioning in general, and subsidence monitoring and tidal estimation in particular. Results will be presented from standard PPP and PPP combined with IAR for arctic regions and from a test bed in the Gulf of Mexico. These results, although preliminary, indicate centimeter level accuracy is indeed possible, also in the arctic. We will also look at further improvements, mainly in convergence time, using all operational satellites and signals from current and future navigation systems.

<p>The snow water equivalent (SWE) is a key parameter in hydrology. In the past years, the signals of Global Navigation Satellite System (GNSS) receivers were discovered to be very attractive for SWE monitoring. The set-up of GNSS-based SWE monitoring typically consists of two GNSS receivers, whereas one is placed on the ground to sense the signal attenuation and time delay being caused by the snow pack. A second receiver is placed above the snow and serves as reference receiver. The measurements of both receivers are differenced to eliminate the common effect of errors in the satellite orbits and clocks, satellite phase and code biases and atmospheric errors, while the information on the snow is kept.</p><p>In this talk, we discuss the replacement of the reference receiver by a virtual reference station (VRS). The VRS is a virtual GNSS reference station, whose corrections are obtained by interpolation of the corrections from multiple surrounding reference stations to achieve a higher accuracy at the user location. The concept of VRS was first developed by Trimble and is widely used in today's real-time kinematic (RTK) positioning receivers. The concept of VRS is also attractive for snow monitoring, since the GNSS reference receiver could be avoided resulting in a lower power consumption and less costs. Moreover, this could be a big advantage for applications in slopes, which are, e.g., potentially avalanche prone. Within the hardware setup of our GNSS SWE sensors, an internet communication link for the reception of the corrections from the VRS corrections at the SWE monitoring site is already available.</p><p>However, there are also two challenges: First, the SWE monitoring stations in Alpine areas are typically at a significantly different altitude than the geodetic reference receivers. The differential tropospheric zenith delay is not negligible for altitudinal differences of more than 100 m. Therefore, the differential tropospheric delay needs to be considered either in the determination of VRS corrections or alternatively in the SWE determination. For altitudinal differences of less than 1000 m, the differential tropospheric zenith delay could be approximated by a model with sufficient accuracy. The residual modelling error is projected to the SWE estimate. Second, the use of a VRS instead of a conventional GNSS reference station requires a stronger data link, since the GNSS raw data (pseudoranges, carrier phases and carrier-to-noise power ratio measurements from all tracked satellites) need to be transmitted besides the final SWE results. However, an LTE link is totally sufficient.</p><p>Besides the methodology, we will also focus on specific hardware implementations.</p>

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.

Precise kinematic differential positioning using the global positioning system (GPS) at a marine platform usually requires a relatively short distance (e.g. <500 km) to a land-based reference station. As an alternative, precise point positioning (PPP) is normally considered free from this limiting requirement. However, due to the prerequisite of network-based satellite products, PPP at a remote marine platform may still be affected by its distance to the reference network. Hence, this paper investigates this scenario by configuring rings of reference stations with different radii centered on a to-be-positioned marine platform. Particularly, we applied ambiguity resolution at a single station to PPP by estimating uncalibrated phase delays (UPDs). We used three rings of reference stations centered on a vessel, with radii of roughly 900, 2,000 and 3,600 km, to determine satellite clocks and UPDs independently. For comparison, we also performed differential positioning based on a single reference station with baseline lengths of about 400, 1,700 and 2,800 km. We demonstrate that, despite the increasing ring-network radius to a few 1,000 km, the overall change in accuracy of the satellite clocks that are used at the vessel is smaller than 0.02 ns, and the RMS values of differences between the three sets of narrow-lane UPD estimates are around 0.05 cycles only. Moreover, the kinematic positioning accuracy of PPP is affected by the increasing ring-network radius, but can still achieve several centimeters after ambiguity resolution when the vessel is over a few 1,000 km away from the ring network, showing better performance than that of differential positioning. Therefore, we propose that ambiguity-fixed PPP can be used at remote marine platforms that support precise oceanographic and geophysical applications in open oceans.

By using multiple reference stations, we have developed a method to get reliable ULF global geomagnetic variations. This background is extremely useful for detecting local anomalous behaviors. This paper demonstrates developed variable tools to identify the anomalies in two frequency ranges; daily variation and 10-1000 seconds. For estimating background daily variations, the periodical model has been applied for data observed at 3 reference stations and a study station. Comparison between the first principal component of the periodical data from the reference stations and the periodical data derived from the target station provides high correlation in general. For data with 100 sec periods after wavelet filtering, the nighttime energy variations have been investigated among 3 reference stations and a study station. The similar principal component analysis as the diurnal variation has been performed and results also show high correlation between the variation at the target and the global background. These tendency suggest that the developed two methods are effective to identify the anomalies in automatic. Examining the original data, we can know details of waveform and distinguish whether the anomalies is related to underground activities or just some artificial noises.

The fast ambiguity resolution method, especially that using single epoch single frequency data, has been a challenging issue in the kinematic GPS positioning. In this paper, artificial immune algorithm (AIA) was introduced to search GPS carrier phase integer ambiguities. It is a new optimization method through simulating biological immune systems to search the best solution. AIA can succeed in improving the poor efficiency and stability of the genetic algorithm (GA). If the normal equation is consisted only by single epoch carrier phase, it is always rank deficient. The general method is to combine the C/A code and the L1 carrier phase together, so as to eliminate rank deficiency. In this situation, the normal equation is ill-conditioned. In addition, the pseudo-range is often influenced by high noise, which would result in lower reliability of the float ambiguity solutions. So, the efficiency of kinematic positioning is influenced because the differences are large between the float ambiguity solution and the correct integer ambiguity. To improve the float solution, an unbiased estimation method named Iteration by Modifying Normal Equation (IMNE) is applied. IMNE can efficiently improve the ill-condition of normal equation. By using it, more precise ambiguity float solution can be acquired and the search space of ambiguity can be greatly reduced. The improved float solution increases the success rate of fixing the carrier phase integer ambiguities. After obtaining more accurate ambiguity float solution, AIA was used to search GPS carrier phase integer ambiguity. The results of some examples show that the new approach (INME-AIA) based on the improving float solution is efficient.