Technology profile: Bringing GNSS to South Africa.
Located at the southernmost tip of Africa, the Republic of South Africa is enclosed by the Atlantic and Indian oceans to its south, west and east sides, and capped by the remainder of the African continent to its north. It’s only about twice the size of Texas, yet it is the world’s largest producer of platinum, gold and chromium, and a leader in producing gem-quality diamonds. The republic’s economy is Africa’s largest and the 28th largest in the world. In many ways, South Africa is the most developed country on the African continent--and this includes its infrastructure.
A vital part of that infrastructure is an extensive geodetic reference network consisting of nearly 50,000 passive trigonometrical beacons and reference marks and an active network known as TrigNet. Begun in 1999 by the Chief Directorate: Surveys and Mapping (CDSM) of the Department of Land Affairs, TrigNet has evolved into a countrywide network of more than 40 active Global Navigation Satellite System (GNSS) reference stations. The CDSM set up the network, pays for its maintenance and telecommunications costs and offers the service free of charge to users--most of whom are in the surveying, mapping and GIS industries. The network has also been used in scientific studies and experiments involving atmospheric science, space weather and other investigations. TrigNet provides Differential Global Positioning System (DGPS) coverage over the entire country; convenient, accurate real-time kinematic (RTK) positioning in much of the country; and an RTK network in two principal urban areas based on NTRIP for the delivery of data to clients.
Building a Reference NetworkFrom four reference stations in 1999, the TrigNet network has grown to 44 stations spaced roughly 200–300 kilometers (124–186 miles) apart over most of the country.
Data from all 44 stations can be used for post-processing applications, and 30 of the stations (shown in red on the map) have real-time telecom infrastructure enabling users to perform RTK positioning. The remaining 14 TrigNet stations (shown in yellow) currently do not have suitable telecom infrastructure for RTK work; their data is downloaded at night for post-processing applications. Of the 30 stations with telecom infrastructure, 21 are used for single base station RTK fixes. The remaining nine are configured into two RTK network “islands.”
TrigNet's IslandsTwo urban areas, which are more developed and have a greater demand for accurate positioning, are set up as RTK network “islands.” Within these islands, users have the ability to use Trimble VRS technology to improve their real-time survey accuracy (see sidebar on page 29).
One island with four reference stations services the Gauteng province, including the cities of Johannesburg and Pretoria (the first and fifth most populous cities in the country). The other RTK network island consists of five reference stations and services Cape Town, the country’s third most populous city.
In 2006, CDSM equipped the entire TrigNet network with Trimble NetRS GPS reference stations (two at each location for redundancy). Trimble GPSNet and Trimble RTKNet infrastructure software tie the stations together from a single control center to provide countrywide DGPS service as well as RTK service within 30 kilometers (18.6 miles) of the stations with adequate real-time telecom service. The DGPS service is similar in its effect to the Wide Area Augmentation System (WAAS) in North America. It provides code-phase correction information over a wide area rather than the more accurate carrier-phase corrections used for RTK work in a more localized area.
In November 2007, all nine of the RTK network reference stations were upgraded to Trimble NetR5 GNSS reference stations that support the modernized GPS L2C and L5 signals as well as Glonass L1/L2 signals. The previously installed RTKNet software provides network RTK corrections enabling rover users to achieve fast high-precision performance over much larger areas than standard RTK.
Advantages of GNSS Infrastructure
- The convenience and efficiency of working with just a rover because there is no need to set up a base station when working within the network area.
- Accurate positioning over a large area, and a common coordinate reference frame for consistent accuracy and fewer errors.
- Precision GNSS surveying with a shorter learning curve.
- Cost savings for the customer or greater profit margins for the contractor--or both.
TrigNet was originally set up to provide these advantages to reduce the cost of surveys for land-reform projects. Free of charge to users, the network certainly provides the efficiency improvements on which cost savings are based. But how accurate is it?
Verifying Network Accuracy
- Network RTK solution repeatability;
- Initialization times;
- and Comparison of the network RTK solution to that of a single base station.
The repeatability of the network RTK solution and the single-base RTK solution was excellent. All points fell within less than 1 centimeter of each other. The initialization times averaged eight seconds except when the baselines were longer than 30 kilometers (18.6 miles). See Table 1.
Additionally, a Trimble GeoXT receiver was used to test the accuracy of the countrywide DGPS coverage. Five measurements were made on a point with a wait time of one minute between measurements. After 30 minutes (allowing the satellite geometry to change), another series of five measurements was made. All positions fell well within submeter accuracy.
The tests verified that the TrigNet network provides extremely accurate results for surveyors in South Africa.
A future step in the evolution of TrigNet is the integration of all TrigNet stations into a virtual private network (VPN). The VPN will also enable the 14 stations, which are currently restricted to post-processing work, to provide real-time data. South Africa’s GNSS infrastructure is not only keeping pace with the times, it continues to be one of the most advanced infrastructures on the African continent.
VRS TechnologyTrimble VRS technology uses a network of GNSS reference stations connected to a control center via data links. The control center continuously collects the data from all the receivers and creates a dynamic database of regional area corrections. These corrections are used to develop a mathematical model for the ionosphere and troposphere for the overall area.
When a surveyor uses a rover receiver in any part of the covered area, the system software creates a virtual station that provides corrections based on the model as if the reference station were located almost next to the rover’s position. The rover uses these highly localized corrections to determine an extremely accurate position--more accurate than standard RTK results.
Standard RTK operation requires the rover to be within roughly 30 kilometers (18.6 miles) of a reference station under calm ionospheric conditions--shorter baselines as the solar max increases toward 2012.
Accuracy--typically specified as +/-(10 mm + 1 ppm)(x baseline length) RMS--can fall off rapidly beyond that distance primarily due to increasing differences in the atmospheric conditions between the rover and the reference station. Trimble VRS technology essentially “moves” the reference station to appear to be next to the rover’s position to eliminate the atmospheric differences between the two. Using Trimble VRS technology, highly improved RTK positioning can be performed anywhere within the covered network.