Spider
From our Leica Partners...
Introduction
Leica Geosystems continues to be the class leader in GPS surveying with their fast and highly reliable 30km conventional RTK using SmartTrack and SmartCheck technology. In its role as industry leader in precision GPS, Leica Geosystems has been for many years actively researching, promoting and realising network RTK solutions and working towards an industry standard for network RTK corrections. It is in this role that Leica Geosystems has developed and driven the Master-Auxiliary Concept (MAC), the future of networked RTK and the basis for the forthcoming RTCM 3.0 network correction message standard. The Master-Auxiliary corrections (MAX) are compact and efficient to transmit to the rover, but also robust and flexible. The Master-Auxiliary Concept overcomes all of the weaknesses of the previous approaches that have been used until now by giving the rover the capability to derive the maximum benefit from the information provided by a reference station network even over a broadcast/one-way communication medium. In conjunction with this new standard, Leica Geosystems is proud to announce the Leica GPS Spider 2.0 software with its revolutionary new network RTK module SpiderNET, which draws on the latest zerodifference processing techniques to take the Master-Auxiliary Concept to the MAX: maximum reliability, maximum performance, maximum flexibility and maximum security.
The Basics of Network RTK
The motivation behind using multiple reference stations in a network for GPS corrections is to model and correct for distance-dependent errors that reduce the accuracy of conventional RTK or DGPS positions in proportion to the distance from a rover to its nearest reference station. It is well known that the most significant sources of error affecting precise GPS positioning are the ionosphere, troposphere and satellite orbits. These error sources may be categorized in two groups: dispersive and nondispersive. The ionosphere is a dispersive error because the magnitude of the resultant error is directly related to the frequency of the ranging signal (L1, L2, L5). The influence of the ionospheric error on different frequencies in the L-band used by satellite navigation systems is well understood. The ionosphere, which is subject to rapid and localised disturbances, is the main restriction on the station density in a reference network. The troposphere and orbit errors are classified as nondispersive because they are not frequency-dependent and have an equal effect on all ranging signals used by current (and proposed) satellite-based global navigation systems. The aim of a reference network is to model and estimate these error sources and provide this network correction information to rover users so that they may derive positions with a higher accuracy than with conventional RTK. Until the release of RTCM 3.0, there will have been no official internationally accepted standard for network RTK corrections. Prior to the release of RTCM 3.0, two approaches, namely those making use of area correction parameters and ‘virtual’ reference stations, were adopted by the user community as interim measures, both of which have serious flaws in their concept and methodology. Some of the problems with these approaches are listed below.
Problems Common to VRS and FKP:
1. The modelling performed by the network software, which is proprietary and unknown to the rover user, greatly influences the information that is provided to the rover. The outcome of this is that not all of the relevant error information is provided to the rover prohibiting it from using the optimal processing techniques (algorithms, models, interpolation) for the application. The fact that proprietary information is transmitted means that the corrections are not standard and therefore biased towards a particular brand of rovers.
2. The use of RTCM 2.3 as the data format for the existing approaches is inefficient, particularly for area correction parameters. The VRS and FKP approaches do not conform to the philosophy of RTCM's industry standard formats because the messages contain modelled data and not raw data as specified by RTCM. More importantly, proprietary, non-standardised messages are used to transmit part of the information. For area correction parameters most of the message is transmitted in proprietary messages.
Problems Unique to VRS:
1. Two-way data links are required between the network computation centre and the user, making access to the correction service costly for both the user and the service provider. Duplex communications also have the downside of limiting the number of simultaneous users who are able to receive the corrections from the network.
2. The rover is forced to reinitialise its position fix once it has travelled more than a certain distance from its initial position because the ‘virtual’ reference station must be moved to maintain the quality of the network corrections.
3. An arbitrary number of reference stations, typically three, which is determined by the reference station software, are used to calculate the corrections for the rover. This restriction limits the ability of the system to adapt to the prevailing atmospheric conditions by using an appropriate number of reference stations to, for example, model larger scale atmospheric activity. Such a constraint also influences the robustness through its impact on the network geometry and the sensitivity to data outages. If even one of the three stations is unable to provide data to the network, the network software must search for another suitable reference station and reset the calculation of correction for the user. During this search process, no network corrections are available to the rovers which impacts their productivity in the field.4. The virtual reference station approach represents the network to the rover as a single reference station. Thus in the end the rover still has a single baseline solution, albeit with a much shorter baseline length. However, misleading the rover in this way also denies it the opportunity to fully realise the increase in accuracy and reliability possible with a true network solution. It also limits the ability of the rover to do quality and integrity monitoring.
5. In some jurisdictions there are legal issues if the GPS correction data is not directly related to a real reference station. Georeferencing GPS corrections or observations to virtual reference stations is neither traceable nor repeatable. In light of these and other limitations of existing approaches to network corrections, Leica Geosystems has driven the development and adoption of the Master-Auxiliary Concept within the RTCM Special Committee 104.
