TrigNet, South Africa’s GNSS base station network

November 26th, 2014, Published in Articles: PositionIT

 

The TrigNet network currently consists of 67 base stations, which have a maximum inter-station spacing distance of 300 km. The data from these base stations is streamed, via dedicated leased lines, to the NGI office in Cape Town where it is processed and made available, free of charge, to national and international users. Future plans for TrigNet are to increase the number of base stations, reduce their maximum inter-station spacing distance, provide an online post-processing service, and incorporate future GNSS systems as and when they become operational. The co-operation that exists between the NGI and national and international organisations/agencies, regarding TrigNet, is to be maintained and enhanced.

The Chief Directorate: National Geospatial Information (NGI) is, in terms of the Land Survey Act (Act 8 of 1997), responsible for the establishment and maintenance of the South African national horizontal and vertical control survey networks. The control survey network comprises the reference system and the reference frame. The South African horizontal reference system is based on one of the realisations of the International Terrestrial Reference System, being the International Terrestrial Reference Frame 1991 (ITRF91). The ITRF91 reference point used as origin for the South African reference frame is the Hartebeesthoek radio astronomy telescope as at 1 January 1994 (epoch 1994.0). The South African datum is, therefore, referred to as the Hartebeesthoek94 datum.

The South African national horizontal and vertical control survey networks comprise of approximately 29 000 trigonometrical beacons, 20 000 town survey marks and approximately 30 000 bench marks. The positions and heights of these monuments do not change over time and are, therefore, termed a passive network.

The advent of Global Navigation Satellite Systems (GNSS) has realised the establishment of networks of permanently installed GNSS receivers that continuously record data from navigation satellites, termed continuously operating reference stations (CORS). The South African CORS network is known as TrigNet and is an extension of the national control survey network.

Fig. 1: TrigNet and IGS stations as at April 2014.

Fig. 1: TrigNet and IGS stations as at April 2014.

The past

The establishment of a network of permanent GNSS base stations in South Africa was first proposed in 1997. The motivation for its establishment was to provide a modern positioning service to aid land reform surveys and other land related projects. The proposal was approved by the then Department of Land Affairs, now the Department of Rural Development and Land Reform, in 1998.

The first four stations were installed by staff of the Chief Directorate: Surveys and Mapping, now the Chief Directorate: National Geospatial Information, in 1999. These four stations were Langebaan, being a dial-up connection, and Cape Town, Hermanus and Pretoria, being leased line connections. The National Land Survey of Sweden, sourced through a Swedish international aid agency named Swede Survey, provided assistance and guidance with the design and first stages of installation of the South African CORS network, named TrigNet. TrigNet’s design was, therefore, initially aligned to the Swedish network of continuously operating GNSS reference stations, named SWEPOS.

The TrigNet service commenced with the provision of GPS data for post-processing purposes to users, via email, as per user requests. An internet-based data provision service was introduced in 2003. Users could then access the file transfer protocol (FTP) site and download data as and when required, thus greatly improving the efficiency of the service.

Present TrigNet networkIn 2005 the NGI received funding in order to replace and upgrade the outdated receivers and network management software. The original choke ring antennas were retained. The upgraded network management software enabled the introduction of various real-time services in 2006.

The TrigNet network currently comprises 64 operational stations. The NGI has installed stations at the offices of the South African weather service, Eskom (South Africa’s electricity utility) and local authorities. Co-operative agreements with public and private entities, that have agreed to supply and maintain the required equipment, have resulted in the establishment of TrigNet stations at those offices.

The continuity of TrigNet’s services, particularly real-time services, relies on the continual operation of equipment, power and data communication. To mitigate these risks redundancies were incorporated into TrigNet’s infrastructure.

Remote stations redundancy

Redundancy was built into initial TrigNet stations through the installation of two receivers at each station, each receiver communicating through its own controlling computer. The densification of the TrigNet network, particularly in the Gauteng, Kwazulu-Natal and Western Cape provinces, has provided redundancy in the network itself. The redundant two-receiver configuration was discontinued in high station density areas and is currently only applied at 14 TrigNet stations that have low network redundancy.

Control centre redundancy

TrigNet’s Control Centre consists of four processing servers that house TrigNet’s functions, processes, data and services. This control centre has been duplicated, resulting in two separate control centres, the one primary and the other secondary. The primary and secondary control centres have a fibre optic connection between them that allows them to run in parallel, serving as backup to each other should either control centre suffer a failure. The secondary control centre has been housed in a second server room in a separate building to the primary control centre, a necessary contingency should a fire or other major disaster occur in one of the server rooms. Both server rooms have environment monitoring and control, fire suppression and access control devices.

Table 1. TrigNet post-processing products.
Post processing products Comments
1 second LIL2 hourly files Available ±40 minutes after the hour – Unchecked for completeness
1 second L1L2 24 hour files Available the following day – Checked for completeness
5 second L1 24 hour files Available the following day – used by GIS & mapping community
30 second L1L2 24 hour files Available the following day – used mainly by scientific community

Backup power supply

Each TrigNet station is connected to an uninterrupted power supply (UPS) or to a local emergency power supply. The primary control centre is connected to both a UPS and an emergency diesel generator.

Data services

Data from TrigNet stations are streamed back to the control centre, located in Mowbray, at a 1 second epoch rate, and this is processed into the various post-processing products and real-time services. Post-processing data products, being daily files of L1L2 1 second epoch and L1L2 30 second data, are backed up and archived.

Communication between the TrigNet stations and the Mowbray Control Centre occurs via leased lines, some via VSAT (Very Small Aperture Terminal), in a frame relay cloud. The frame relay cloud provides alternative routing of the leased lines should failure occur in a particular exchange. The TrigNet stations’ leased lines converge at the Rondebosch exchange and two leased lines are used between the Rondebosch exchange and Mowbray: one leased line to the primary and another to the secondary control centre.Communication

TrigNet’s products and services are provided to users via the internet, which is administered by the Internet Service Provider (ISP). The server that provides the products and services to the ISP is housed within the primary control centre. The demand for TrigNet products and services, especially Real-Time Kinematic (RTK) services, has necessitated that this connection’s bandwidth be increased incrementally to ensure delivery to the user.

Network management

TrigNet’s network management software controls all aspects relating to TrigNet’s operations, which includes monitoring network stability, data download, data processing and the generation and distribution of products and services. The software processes the data received into TrigNet’s post-processing products and generates Radio Technical Commission Maritime (RTCM) and Compact Measurement Record (CMR+) to be streamed to TrigNet’s real-time services. The Virtual Reference Station (VRS) technology, that forms part of the RTK service, has two-way communication where the software receives the user’s three-dimensional position and generates virtual corrections for that position and supplies these to the user. These virtual corrections are derived from data sourced from a group of TrigNet stations with inter-station distances of not more than 80 km. The effects of the ionosphere are modelled within these virtual corrections. The real-time services also provide network status information such as tracking behaviour and atmospheric activity.

TrigNet users

Users are required to register on the TrigNet website (www.trignet.co.za) in order to access TrigNet’s real-time services. Post-processing data is downloaded via the anonymous FTP site (ftp.trignet.co.za). Registered users receive e-mails, letters and notifications relating to TrigNet.

The TrigNet website contains Status Messages notifying users of station outages, maintenance shut downs, system changes, system upgrades and related messages. Users generally do not make use of this service and, therefore, either an SMS notification service or a social media platform is being considered as an additional means of communicating TrigNet notifications.

National users

Current feedback suggests that the TrigNet RTK data service is the most frequently used service for cadastral and engineering survey projects. Since 2012 the CMR+ service, primarily being used by the farming sector in the northern and western Free State, has seen a steady increase in demand. TrigNet data for post-processing is primarily being used by aerial imagery contractors and the GIS community.

Table 2: User-defined dataset via the Reference Data Shop.
Variables Selection example
TrigNet Station Cape Town (CTWN)
Data epoch rate 4 seconds
Data period Start: 11/04/2014 11h36 SAST, Duration: 6h 13m
Data Content L1 & L2 (GPS and GLONASS)

International users

TrigNet data is used extensively by the international scientific community. The International GNSS Service (IGS) has adopted seven TrigNet stations as IGS stations for its post-processing data services. The Crustal Dynamics Data Information System (CDDIS), an IGS Global Data Centre, uses high rate post-processing data from these stations to compute near real-time products that include satellite ephemerides, clocks, and earth rotation parameters. TrigNet also streams real-time data from three stations to the IGS for its real-time services that commenced in April 2013. The National Ocean and Atmospheric Administration (NOAA) draws TrigNet data from seven stations in order to compute variables used in global weather forecasting and global weather models. The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) project uses radio signals from GPS satellites, which includes data from one TrigNet station, in conjunction with its six low earth orbit satellites, to compute radio occultation soundings for atmospheric research.

Services and products

Post-processing products

Post-processing products, containing GPS observables, are made available in Receiver Independent Exchange Format (RINEX). RINEX is an open format and can be used in any GNSS data processing software suite. The table below provides a summary of TrigNet post-processing products that can be downloaded via the FTP TrigNet site ftp.trignet.co.za.

The Reference Data Shop facility, which is available on the TrigNet website, enables users to customise their post-processing data product by specifying the data epoch rate, data period and data content from a specific TrigNet station.

The maximum data epoch rate is 1 second (1 Hz), which is the highest rate at which TrigNet stations record data. The receivers within the network are, however, capable of recording data at an epoch rate of 0,05 seconds (20 Hz). Users requiring data at epoch rates exceeding 1 second for a defined period may lodge a special request. Each request will be considered on a case by case basis.

Real-time services

Real-time services are provided in RTCM format which is receiver independent and is an open format. Differential GPS (DGPS) corrections are provided in RTCM version 2.3. RTK corrections are transmitted in RTCM version 3.1. CMR+ formatted data is specifically provided for and used in precision agricultural applications and machine control.

Third party co-operation

A third party co-operation model has evolved as a result of third parties requesting assistance from the NGI in installing CORS stations for incorporation into the TrigNet network.

In this model the third party is responsible for:

  • The purchase of the antenna and receiver that is compliant with NGI’s TrigNet specifications.
  • The provision of a suitable facility to house the operational CORS (TrigNet) station.
  • The provision of a suitable and reliable power source and backup power supply.
  • The maintenance, repair and upgrade of the receiver and antenna.

The NGI is responsible for:

  • Selecting the antenna position and installing the equipment at the third party’s site.
  • Providing the operational communication facilities to stream the data to the Control Centre in Mowbray.
  • Incorporating the CORS (TrigNet) station into the TrigNet network.

Seven third party co-operative installations have occurred to date. Three of these stations have been installed in conjunction with two local authorities; three stations have been installed in conjunction with three mining companies and one with Eskom.

Future TrigNet GNSS products and services

The established TrigNet stations include 41 stations that are GLONASS enabled, providing GLONASS observables within the real-time data stream. The remaining 26 TrigNet stations are operating GPS receivers, purchased in 2005, that are to be upgraded to GLONASS enabled receivers. GLONASS data will then be included within TrigNet’s post-processed products. Other available GNSS, such as Japan’s Quasi Zenith Satellite System (QZSS) and China’s BeiDou Navigation Satellite System (BDS), cannot be incorporated into TrigNet at this time as they do not have satellite coverage over South Africa. Should observables from other GNSS be tracked by TrigNet stations in the future, these will be incorporated into the post-processing and real-time services.

TrigNet currently provides services and products that are well utilised, however, additional and related services that are driven by user needs and changes in technology are to be investigated and incorporated, should these be feasible.

Fig. 2. Monthly post-processing data volumes.

Fig. 2. Monthly post-processing data volumes.

Space weather monitoring

The South African National Space Agency’s (SANSA) office in Hermanus, formerly known as the Hermanus Magnetic Observatory, has utilised TrigNet data for a number of years for space weather research.

TrigNet provides SANSA with 30 second epoch data in 15 minute data files from 22 TrigNet stations for the estimation of Total Electron Content (TEC) in the ionosphere. The modelling of the ionosphere using the TEC is of particular benefit to single frequency GNSS users, amongst others.

Space weather researchers gather data for ionospheric scintillation monitoring by using GPS receivers that are designed to have improved lock retention on GPS satellites during periods of increased solar activity. These receivers record data at an epoch rate of 0,02 seconds (50 Hz). The trend is to supplement this data with data from geodetic receivers, such as those within the TrigNet network.

Co-operation between the NGI and SANSA in the application of TrigNet data in space weather research is ongoing.

Online GNSS post processing services

A number of national and international organisations host free online GPS data processing services that provide post-processing services to users anywhere in the world. GPS data that has been collected by the user is uploaded to the service in RINEX format. The service then computes an optimum solution by sourcing data from GNSS base stations in the IGS network and applying differential processing techniques. Users receive e-mailed reports containing these optimum co- ordinates in an International Terrestrial Reference Frame realisation or some other national reference frame.

An online GNSS Post-Processing Service is to be investigated for incorporation into TrigNet’s products and services. The user will provide GNSS data to which differential processing techniques will be applied using base station data from the TrigNet network and, where applicable, the IGS network. The results and quality indicators will be provided to the user.

Support for EGNOS

The European Geostationary Navigation Overlay Service (EGNOS) provides an augmentation signal to the GPS service. The objective is to provide correction and integrity information to improve positioning navigation services over Europe.

A network of ground based GNSS receivers, known as Receiver Integrity Monitoring Stations (RIMS), provide the augmentation signal content that is broadcast to users through geostationary telecommunications satellites that primarily cover Europe. Each RIMS comprises of three receivers from different manufacturers with each receiver connected to a different type of antenna. The three receiver/antenna configurations must be located within a radius of about 100 m.

The EGNOS space segment consists of an EGNOS transponder located on each of the three geostationary satellites that broadcast the corrections and integrity information for GNSS satellites in the L1 frequency band (1575,42 MHz). When Galileo, Europe’s satellite navigation system, is fully operational it will become a key component of the EGNOS space segment.

The Department of Science and Technology (DST) and the South African National Space Agency (SANSA) are collaborating with the European Union (EU) to extend EGNOS to Southern Africa to aid aeronautical navigation between Europe and Southern Africa. Preliminary testing of the South African extension of EGNOS has commenced where approximately 18 TrigNet stations are being used as RIMS. The current TrigNet station configuration, however, does not comply with the RIMS specification.

TrigNet network expansion and densification

TrigNet stations are not evenly distributed across South Africa and current inter-station spacing ranges from approximately 40 km to 300 km. The areas of highest density are the developing areas which are Gauteng, Kwa-Zulu Natal and the southern part of the Western Cape. Areas of lowest density are the Northern Cape and the northern part of the Western Cape. Future densification of the TrigNet network will focus on the Western and Eastern Cape, Free State, Kwa-Zulu Natal, Mpumulanga and Limpopo Provinces, with a view to realising an inter-station spacing of between 40 km and 150 km in these areas. The reduction in inter-station spacing in the Northern Cape and North West Provinces must also be realised.

Equipment installation and upgrade

GNSS receivers

The upgrade of TrigNet receivers to be GPS and GLONASS capable has been discussed.

The purchase and replacement of receivers and antennas is planned and budgeted for at a rate of five receivers and five antennas per year. An once-off capital funding request is to be made in order to upgrade the 26 TrigNet stations to GLONASS enabled TrigNet stations.

Antennas

Many of the antennas in the TrigNet network are the original choke ring antennas purchased when TrigNet commenced in 1999. The three-dimensional position of a TrigNet station is referred to the antenna. The antenna must, therefore, remain in an undisturbed operational state for as long as possible in order to provide continuity between computed co-ordinate positions at various points in time. The TrigNet network currently has a variety of antennas from three manufacturers. Antennas installed in the TrigNet network must have an IGS approved absolute calibration rating.

Fig. 3: Monthly real-time data volumes.

Fig. 3: Monthly real-time data volumes.

Communication

Fixed land-line communication

TrigNet has used dedicated fixed land-lines since its inception and these have proven to have excellent reliability with an average latency of less than 1 second. All but four TrigNet stations are directly connected via such dedicated fixed landlines. Cable theft has, in recent times, seriously affected communication to a number of these TrigNet stations.

VSAT

VSAT satellite telecommunication has been installed at four TrigNet stations that are in remote rural areas with no communication infrastructure or where cable theft has occurred. Initial tests have shown an average latency of just over 1 second. VSAT should become the preferred method of communication for new TrigNet stations, subject to the site owner approving the installation of the VSAT antenna, which is rather bulky.

WiFi

The Richards Bay and Thyspunt TrigNet stations do not have access to fixed land-line connections. The Richards Bay station has an omni-directional WiFi radio connection to a VSAT connection, which is approximately 200 m away. The Thuyspunt station is to have a 15 km line-of-sight WiFi radio connection installed.

Internet

The use of the internet to deliver TrigNet’s services and products to users has been discussed. A single internet connection exists between the Primary Control Centre and the ISP. A second internet connection between the Secondary Control Centre and the ISP is required.

Network management

TrigNet’s network management software contains functionality that has not been activated as yet. These functions are to be investigated and, where feasible, will be incorporated within TrigNet’s services and products.

Conclusions

TrigNet must continue to provide quality GNSS products and services to geospatial users in the business and scientific sectors, nationally and internationally, through the upgrade and improvement of its infrastructure and the implementation of technological advances. The network must be expanded and undergo further densification in order to realise a maximum inter-station spacing of 150 km. Ongoing user needs assessments and investigations must be undertaken to identify products
and services to be added to the TrigNet bouquet, where these are deemed feasible.

Acknowledgement

Richard Wonnacott retired as Director: Survey Services in July 2013. His direction and guidance in the establishment and development of TrigNet has formed the basis for this paper.

This paper was presented at AfricaGEO 2014 and is republished here with permission.

References

[1] The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) 2014, University Corporation for Atmospheric Research, viewed 9 April 2014, www.cosmic.ucar.edu/ro.html
[2] The Crustal Dynamics Data Information System (CDDIS) 2013, National Aeronautics and Space Administration (NASA), USA, viewed 9 April 2014. http://cddis.gsfc.nasa.gov/Data_and_Derived_Products/GNSS/high-rate_data.html
[3] G Hedling, A Parker, R Wonnacott: “TrigNet The Network of Active GPS Base Stations for South Africa”, Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, 19 -22 September 2000, pp. 1865-1870, 2000.
[4] PM Kintner, T Humphreys, J Hinks: “GNSS and Ionospheric Scintillation”, Inside GNSS, July/August 2009, pp. 22-30, 2009.
[5] E Rubinov, R Wonnacott, S Fuller, P Collier: “Integrity Monitoring of CORS Networks – TrigNet Case Study”, South African Journal of Geomatics, Vol. 1, No. 1, January 2012.
[6] PJ Cilliers, D Gouws, B Opperman, R Wonnacott, L Combrinck: “The South African network of dual frequency global positioning system satellite receiver base stations – a national asset with many applications and research opportunities”, South African Journal of Science, Vol. 99, Issue 1&2, January/February 2003, pp.51-55, 2003.

Contact Patrick Vorster, Department of Rural Development and Land Reform, pvorster@ruraldevelopment.gov.za and Stephan Koch, skoch@ruraldevelopment.gov.za

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