- SPECIAL REPORTS
- THE MAGAZINE
Like electrical systems were a century ago, or cellular networks are today, Global Navigation Satellite System (GNSS) networks have become essential components of national and regional infrastructures. While the initial advantage of instant, precise measurement capabilities was seen as a benefit in the land development sector, new applications in the agriculture, utilities management and other sectors quickly emerged to provide additional productivity and economic benefits. Because the utilization of GNSS networks is an essential first step to facilitating subsequent infrastructure work, and because these networks are relatively inexpensive to set up and operate compared to large-scale projects like roads and dams, GNSS networks are springing up everywhere-even in developing countries. From this widespread usage, some trends are beginning to emerge. Representative networks in Europe, Africa and Asia are providing positioning precision for worldwide construction and renovation endeavors.
Europe Moves AheadMany European countries today have implemented GNSS networks; a few notable network installations are those in Switzerland, Poland, Slovakia, Denmark and Greece.
Extremely variable, Switzerland’s topography includes 74 mountains over 4,000 m high and about 1,500 lakes, with the lowest-Lago Maggiore-at 193 m. To tackle the particular challenges of this landscape, the Swiss Federal Office of Topography (swisstopo) utilizes a permanent GNSS network for many measurement services.
Since 1999, swisstopo has operated the Automated GPS Network Switzerland (AGNES) as part of its official survey responsibilities. With 31 reference stations covering the entire 41,000 km2 country, swisstopo is able to offer the Swiss Positioning Service (swipos), which provides RTK GPS and DGPS corrections. Surveyors have also benefited from a kinematic model of Switzerland that has increased the accuracy of its geodetic corrections.
Swipos customers particularly benefit from the use of Trimble (Sunnyvale, Calif.) VRS (Virtual Reference Station) technology, a configuration provided with Trimble RTKNet software that computes a virtual reference station for the user in the field, increasing system reliability and allowing significantly greater distances between reference stations.
In August 2006, swisstopo decided to update the network to AGNES II by upgrading to combined GPS/GLONASS receivers. This was prompted by the recent availability of a new GPS frequency (L2C), the defined codes for the planned NAVSTAR L5 frequency and the expansion of the Russian GLONASS satellite system. As a result of greater satellite availability, AGNES II is able to optimize positioning performance, particularly in reception-critical areas such as areas with interrupted reception and zones with extreme topographic features. Currently, 31 Trimble NetR5 receivers and Zephyr Geodetic 2 antennae have been added to the network; the new antennae will allow AGNES II to take advantage of future planned constellations and frequencies. By summer 2007, all 31 reference stations should be installed and AGNES II will be fully operable. Ten existing AGNES I stations will continue to operate in parallel for several years to maintain continuity of time series data for particular projects.
To the east, another nationwide GNSS network installation is underway in Poland. Altitude differences are less marked in this country than in Switzerland, but with its approximately 313,000 km2, Poland covers a significantly larger area. The ASG (Aktywna Siec Geodezyjna or Network for Online Positioning User Service) network will be the second RTK network in Poland. Building on the success of the first network and operated using Trimble VRS technology in southern Poland, the ASG is expected to be one of the largest reference station networks worldwide and the largest in Eastern Europe. The operator, the Polish National Office for Cartography and Geodesy (GUGiK), is constructing the network in accordance with the standards of the European geospatial infrastructure initiative EUPOS (European Position Determination System).
Currently, GUGiK is implementing 78 Trimble Continuously Operating Reference Stations (CORS) receivers with Trimble Zephyr Geodetic antennae; construction of reference stations covering the required area will be completed this year. In addition to the 78 ASG reference stations, GUGiK is planning to expand the network to up to 130 reference stations by including stations from neighboring countries. The ASG network will meet the requirements for high-accuracy RTK GPS data within the centimeter range by scientific institutes and surveying professionals. As a special user service, GUGiK will not charge any user fees during the first three years the national GNSS network is in operation.
Slovakia is investing heavily in network RTK infrastructure. This EUPOS member was the first country with its own nationwide modernized network, called SKPOS. Completed in January 2007, the SKPOS network currently includes 21 reference stations. Because Trimble NetR5 receivers and Trimble Zephyr Geodetic 2 antennae were used, the stations are able to use GPS L1, L2, L2C and L1 and L2 GLONASS signals, and are ready to track GPS L5 signals when available. An expansion of the SKPOS network using reference stations in neighboring countries is currently being pursued. Similar to operators in other countries, the network operator, publicly owned GKU Bratislava (Geodetický a kartografický ústav Bratislava), is taking advantage of the benefits provided by the Trimble RTKNet software, which enables state-of-the-art and highly accurate real-time corrections. The approximately 50,000 km2 surface area of Slovakia is completely covered by the SKPOS network, and GKU Bratislava states that measurement accuracies in the centimeter range is possible nationwide. GKU Bratislava intends to offer real-time and post-processing services. The network has already achieved a high acceptance level among users. With a commercial service now available, GKU Bratislava expects a rapid increase in users.
Public providers are not the only ones who have recognized the advantages of GNSS networks. In Denmark, for example, GEOTEAM-Trimble’s authorized survey dealer-operates the private GNSS network GPSnet.dk. In addition to covering the approximately 43,000 km2 of Danish heartland, GPSnet.dk also covers a large portion of the inshore waters. The network, which began operation in 2000 with Trimble 4700 CORS receivers, has also successfully been used in several offshore projects for positioning foundations for wind turbine parks. GPSnet.dk was the world’s first nationwide GPS network deployed with Trimble VRS technology. In December 2006, GEOTEAM implemented a crucial upgrade of the network with GNSS technology. The current system is a combined GPS and GLONASS network with 27 Trimble NetR5 receivers and Zephyr Geodetic 2 antennae. “Both municipal authorities and private survey companies use the GPSnet.dk network, and updating the network has provided even better availability of RTK GNSS corrections and an increased network performance to all our customers,” said Søren Ellegaard, GEOTEAM general manager. “In addition, customers using R8 GNSS rovers already report up to a 50 percent increase in productivity working in cities compared to using a standard GPS rover solution.”
In the south of Europe, Greece is also entering the GNSS infrastructure world. Including about 100 reference stations, the Hellenic Positioning System (HEPOS) will cover approximately 132,000 km2 of mainland and islands when completed by the end of this year. Trimble NetRS receivers and Trimble Zephyr Geodetic antennae constitute the technical backbone of HEPOS. Webserver connections and post-processing applications are both provided by HEPOS. Operated by the National Land Registry KTIMATOLOGIO SA, the HEPOS network is part of the modernization of the total land registry system in Greece and is supported by European Union funding1. KTIMATOLOGIO SA will benefit from the performance of HEPOS in coming years, and users will benefit from centimeter-level accuracy throughout the country.
GNSS Modernization in AfricaMany African countries are looking to modernize their national infrastructures. The Republic of South Africa (RSA) is among the first to gain GNSS network capabilities.
The Chief Directorate: Surveys and Mapping, of the Department of Land Affairs of South Africa, is responsible for the establishment and maintenance of a geodetic reference frame for South Africa. Known as TrigNet, the present system consists of a network of both passive trigonometrical beacons and reference marks, as well as a network of active GPS base stations.
The active network was established in 1999 to reduce the cost of GPS surveys for land reform projects in South Africa. The National Land Survey of Sweden provided technical assistance. The initial installation of four base stations has been expanded to a network of 44 stations spaced 200-300 km apart throughout the country. Once a day, GPS data is downloaded from 14 stations, while data from the remaining 30 stations is fed continuously to the central control center near Cape Town. Post-processing data is provided to users in RINEX format via a web-based service; a real-time service has recently been introduced using NTRIP protocols and data from the continuous feed stations.
In 2006, the Department of Land Affairs initiated a network-wide upgrade with Trimble RTKNet software and 80 Trimble NetRS receivers to make management and control more efficient and less complex, benefitting TrigNet data users. Using Trimble GPSNet software, a country-wide DGPS service is being implemented while two Trimble VRS RTK networks are being developed for Gauteng and the Western Cape.
Rapid Growth in AsiaGNSS infrastructure growth in parts of Asia has been phenomenal, and is expected to continue at high rates for decades to come. In addition to networks in Malaysia and China, advanced GNSS networks are being implemented in Korea, Singapore and other Asian countries.
Within 12 hours of the 2004 tsunamis, Malaysia’s new Trimble VRS network was able to provide the Southeast Asian country with extensive data on how much deformation the country had experienced-and the network had not yet officially launched.
The network’s 27 Trimble 5700 CORS supplied the Department of Surveying and Mapping, Malaysia (JUPEM) with data showing that the country had moved approximately 15 cm in some areas over a period of 10 days-and that it was still in motion. Neighboring countries with no network required many weeks to determine deformation rates using conventional surveying methods.
Owned and hosted by JUPEM, the network, dubbed MyRTKNet, includes 25 reference stations in West Malaysia and two stations in East Malaysia. Powered by Trimble RTKNet software, the network’s Coordinate Monitor feature provides an independent quality check of the stations’ positions. This feature is particularly useful for networks like Malaysia’s where crustal deformation may-and did-take place. In addition, alarms can be added if stations move more than a user-defined amount.
But the network is used for more than deformation measurements. The first system of its kind in southeast Asia, the Malaysian infrastructure also provides high-accuracy GPS measurements for a variety of surveying applications. With Malaysia undergoing massive development projects, costs for conventional surveying methods due to the challenging terrain can be prohibitive. The Trimble VRS network enables more efficient surveying and increased cost savings.
“With state-of-the-art Trimble VRS technology, the MyRTKNet network ensures that high-performance real-time centimeter solutions will become customary among Malaysia’s surveying and positioning service industry,” says David Chang, JUPEM principal assistant director of survey, at the official launch of MyRTKNet in May 2005. “MyRTKNet will also accelerate the implementation of a coordinated cadastral system (CCS), a computer-assisted topographic mapping system (CATMAPS) and utility mapping in the country.”
Along with developing the Trimble VRS infrastructure, the Malaysian government initiated both a height modernization project and the development of a new national geoid, a very precise mathematical model of specific portions of the Earth that enables more precise elevation measurements using RTK GPS. Currently, JUPEM is densifying and expanding the network on the Peninsula and in East Malaysia in order to cover the entire country with Trimble VRS network capabilities. In addition, JUPEM is modernizing the entire network with Trimble NetR5 reference stations, which provide comprehensive GNSS support to network users. This expanded network will be the largest of its kind in Asia. The VRS network is hosted by JUPEM and is funded through a grant provided by the Malaysian government.
Since 1999, 10 RTK networks have been established throughout China using Trimble VRS technology. Located in Shanghai, Wuhan, DongGuan, Tianjin, Shenzhen, Sichuan (Chengdu), Suzhou, Qingdao, Beijing and Guangdong, the multi-purpose networks will provide a geospatial infrastructure in each area.
Of these networks, the Guangdong CORS project is China’s first province-wide GNSS network. Owned by the Land and Resources Department of the Guangdong province-China’s largest and fastest-growing province-the network includes 34 GNSS reference stations. The Guangdong network overlaps with smaller municipal networks in the cities of Shenzhen and DongGuan. The network’s focus is on surveying and mapping, offering centimeter-level real-time positioning and submeter GIS data collection capabilities.
Also of note is the VRS network in Beijing, which is expected to play an important role in construction and transportation for the 2008 Olympic Games. Operated by the Beijing Information Resource Management Center, the Beijing network includes 14 Trimble GPS reference stations and Trimble GPSNet and RTKNet software.
In addition, Trimble plans to install three new networks in the first part of 2007 for the city of Harbin in China’s Heilongjiang Province, the Chengdu Survey and Mapping Institute’s separate network in Sichuan, and the city of Nanning in the Guangxi Province.
As global GNSS networks proliferate, and as new signals come online, precise positioning is becoming a global utility as widespread as cell phone service. Advanced infrastructure solutions will continue to be refined in order to satisfy worldwide demand for GNSS networks. Allowing users to connect to the network and immediately begin gaining positioning precision, GNSS networks enable national infrastructure construction and refurbishment efforts to move ahead rapidly. Offering inherent operating efficiencies for a wide range of tasks, GNSS infrastructure is today changing the workflow of the high-accuracy positioning professional worldwide.