See current research project 'SydNet GPS Network Research'

Singapore Integrated Multi-Reference Station Network (SIMRSN) and Applications

This project had as its aim to develop and test various GPS positioning techniques which take advantage of a network of continuously operating GPS base stations, linked by dedicated telephone lines to a central processing facility. A number of graduate students and staffers from SNAP and the Surveying and Mapping Laboratory, School of Civil & Environmental Engineering, Nanyang Technological University (NTU), Singapore, were involved. This close collaboration led to the initiative to jointly sponsor a "GPS/GIS Showcase" in Singapore. The first was held in November 1999. The second in November 2000.

NTU academic staffers Goh Pong Chai and Tor Yam Khoon, together with Chris Rizos and Shaowei Han, jointly submitted a National Science & Technology Board (NSTB) grant application, "Development of an integrated multiple base station infrastructure to support concurrent high precision differential GPS positioning applications", seeking funding to establish a GPS test laboratory in Singapore based on four high precision permanent GPS reference stations linked by high speed data lines to NTU. In September 1998 funding was approved by the NSTB. The Australian Research Council has supported the also project (1999-2001).

Shaowei Han was a visitor at NTU during July 1999 (his report). Ken Wong made a similar visit in September 1999 (his report). Chris Rizos spent 5 months of his sabbatical at NTU from September 1999 (his report).

During the period February-April 2000, NTU researcher Dr. Xiaoming Chen, worked with SNAP/UNSW researchers (primarily Shaowei Han and Horng-Yue Chen) to develop the basic processing 'engine' for the analysis of the reference receiver network data. This 'engine' is software that lies at the heart of network-based positioning, and must process data from multiple receivers in real-time. A first demonstration of the capability was made in late 2000.

Database management and QC lies at the heart of such a system. Preliminary work was carried out in 1999 by Ken Wong to develop the necessary data handling procedures to support both real-time applications, as well as web-based processing and data archiving requirements.

The SIMRSN has operated for several years. In 2003 it was decided to establish a 'mirror' site in Sydney. This real-time network is known as SydNet. Further algorithm development will be undertaken at UNSW from 2003 onwards.

Motivation: Use of multiple GPS reference stations allows for "network-based" positioning solutions, rather than the conventional single baseline (two receivers: one reference, the other the user receiver) approach. Network-based GPS is of course the mode used in ultra high accuracy GPS geodesy applications such as the determination ground fault motion through the integrated processing of data collected by a network of simultaneously operating GPS receivers. The global network of GPS tracking stations comprising the "core" network of the International GPS Service (IGS) is an example of such a multi-reference receiver network. However, the data is not streamed to a computation centre in real-time. Such a mode is also used in the pseudo-range-based Wide Area Differential GPS (WADGPS) real-time services such as those provided by organisations such as FUGRO (the "Omnistar" service). A commercial version has been launched by Trimble, known as the 'Virtual Reference System' (VRS), followed by Leica's version several years later.

Research Challenge: The multiple GPS reference station network such as the one established in Singapore is similar in concept to the WADGPS services. In both cases there is a separation between the reference station network infrastructure and the user(s). The reference station network is linked by dedicated communications links to a central facility, where all the reference receiver data are processed on a continuous basis, and "network correction messages" are generated. These correction messages (not unlike the RTCM messages in the case of single reference station DGPS implementations) are immediately transmitted to users via appropriate data channels. The crucial difference is that the network-based GPS technique uses carrier phase observations, and therefore is intended to address high accuracy surveying and navigation applications.

The advantages of network-based carrier phase-based positioning techniques over single baseline positioning are:

Various combinations of static and kinematic positioning are possible, in either real-time (if there is a data channel available to the user) or post-processed mode. Furthermore, users need not own and operate the reference station network infrastructure. The latter characteristic is significant, as it permits the development of network services to the user, to support high accuracy applications, without the need for a user to invest in base station receivers.

Building Monitoring Application: An example of how an 'off-the-shelf' system can be adapted to improve performance by using the reference station network is building monitoring. A joint NTU-UNSW project (commenced in early 2000) requires the continuous monitoring of Singapore's tallest building, the Republic Plaza Building, using dual-frequency RTK-capable GPS receivers. The resultant time series of positioning solutions will be analysed to detect vibrations of the building structure caused by wind effects. Filtering algorithms were developed as part of Clement Ogaja's PhD project. Download paper here describing monitoring system and time series analysis methodology.

L-Commerce Products & Services: The Singapore facility will permit research and development to be carried out at the infrastructure level (reference station data processing, network correction message generation, etc.), the data coms link level (via a variety of technologies including GSM, Digital Audio Broadcast, and packet radio services), and at the user/application level. A paper that discusses how the 'future of GPS surveying' could be influenced by the provision of 'services' such as those to be encouraged that take advantage of this network facility has been prepared (a PDF file can be downloaded here). Opportunities also exist to address so-called 'L-commerce' applications, loosely defined as be 'position-dependent services' delivered via consumer devices such as 'smartphones' or PDAs. The Singapore-based project therefore offers unique opportunities to address a wide range of positioning needs, from 'niche' applications such as surveying, to mass market 'location-based-services' or 'L-commerce' products and services. This research will be continued on the SydNet GPS network.

Some Relevant Publications: