The South African Network of Active GPS Base Stations

National Control center at the office of Chief Directorate: Surveys and Mapping. Mowbray, Cape Town Background The Chief Directorate : Surveys and Mapping is responsible for the establishment, extension and maintenance of the national control survey network for the entire country. The network is represented by highly visible trigonometrical beacons on mountains and high hills and buildings and easily accessible town survey and levelling benchmarks in town and city streets. Intervisibility between marks and beacons and the users survey area is essential which reduces the effectiveness of the network at night, in poor weather conditions or in dense bush. The use of GPS as a positioning tool has greatly reduced the intervisibility requirement of traditional survey methods. In order to achieve the high accuracy requirements of surveying, GPS users must, however, purchase at least two GPS receivers which increases the capital investment required to carry out any survey operations. The establishment of a network of active GPS base stations will effectively provide the user with the second receiver thus reducing the cost of the surveys. Motivation GPS as a positioning tool is used by a wide variety of disciplines such as surveyors, engineers, GIS practitioners, navigators and so on. In order to achieve acceptable levels of accuracy, users must purchase at least two suitable GPS receivers one of which must be located on a point of known position. This increases both the capital investments in such equipment and the operational cost of GPS surveys. The need to purchase at least two receivers and to locate one on a known point can be reduced by establishing a network of active GPS base stations which effectively provides the second receiver on a known position. Up to forty active GPS base stations are to be established throughout South Africa which will be operated, controlled and monitored from a control centre to be established in the offices of the Chief Directorate : Surveys and Mapping in Mowbray. GPS corrections will be computed and the data monitored and quality controlled before being made available to users either in real time or as a post processing service. 1. Passive Networks 2. Active Networks 2. High Accuracy Positioning 4. Active GPS Base Station Network 5. Trignet 6. Trignet Service and Products
1. Passive Networks ØThe Chief Directorate: Surveys and Mapping (CDSM) is responsible for the establishment and maintenance of the National Control Survey Network for South Africa. ØThe current network consists of approximately 29 000 highly visible trigonometrical beacons built on the tops of mountains, hills and high structures, approximately 20 000 easily accessible town survey marks and many thousands of precisely levelled benchmarks.                                                  ØSuch a network could be classified as a passive network since, apart from representing the co-ordinate or height, the beacon or mark does not play an active role in determining its position and/or height . ØIn such a network, intervisibility between beacons, marks and the survey area is essential when using conventional instruments e.g. theodolites and levels.  The effectiveness of passive networks is thus reduced by poor visibility, bad weather conditions, hilly and mountainous terrain and vegetation.
2. Active Network ØIn contrast to traditional networks, an active control survey network could be considered as one in which the reference points (beacons or marks), actively and continuously determine their positions and/or heights. ØSatellite based positioning systems are ideally suited for providing the key element in an active control survey network. GPS and, to a lesser extent, GLONASS are the most commonly used technologies to determine the position of reference points. 2.1 Global Navigation Satellite Systems (GNSS) Ø“Global Navigation Satellite Systems” is a term used to describe all forms of satellite based navigation systems, which includes : ØGPS (Global Positioning Systems) is designed and controlled by the United States Department of Defense. ØGLONASS (Global Orbiting Navigation Satellite System) is designed and controlled by the Ministry of Defense of  the Russian Federation. ØGNSS is capable of providing : Rapid 3D positioning. Velocity information Time information 24 hours per day operation Weather independent operation. Real time\instantaneous position determination Navigation satellite systems work on the principle of triangulation : Ø by determining the pseudorange (distance) from three or more satellites, the position (horizontal and vertical) of a point can be calculated
The positions of these satellites are known at every instant in time and can be understood as “orbiting” trigonometrical beacons.
2.2 Applications of GNSS ØGIS data capture ØVelocity information ØAerial photography control ØSurveying ØNavigation ØWeather forecasting and modeling ØTime transfer and control ØMonitoring of movements of the Earth’s crust. ØAsset management Ø...and numerous other applications
2.3 GNSS  Error Sources Positions derived from satellite navigation systems are subject to numerous sources of errors such as : ØSatellite and receiver clock instability. ØThe broadcast satellite ephemeris is a predicted orbit and is subject to error. ØSatellite signals are transmitted through an unstable atmosphere (ionosphere and troposphere). ØDeliberate degradation of signals by the U.S. Department of Defense (Selective Availability).         This has been de-activated since 1 May 2000.
ØThe effect of errors on points in close proximity to one another  is almost identical since the above errors are highly spatially correlated. ØThe error in position  at the Point1 is almost identical to the error at Point2 at a certain instant in time (a~b). ØThe relative distance and direction between Point1 and Point2 remains virtually the same at all times .
3. High      Accuracy Positioning  (Differential Positioning) Any single receiver, from the basic type to the most precise geodetic grade, is only capable of yielding positional accuracies of ± 15m in autonomous mode (since Selective Availability is deactivated).         ØIn order to achieve the high accuracy requirements of surveying, at least two GPS receivers must be used simultaneously. ØOne receiver is placed at a known position while a second receiver is at the unknown point or is travelling (roving). ØBoth receivers simultaneously log GPS data to resolve most errors. Accuracies of a few centimeters are achievable depending on the type of receiver and technique used.
This does, however, almost double the  capital cost of using GPS  to achieve highly accurate results since two receivers are needed. It further requires that the second receiver acting as “base” is operational (additional labour and logistics) for the duration of the project.
4. Active GPS Base Station Networks ØAn active GPS network provides GPS data which, when added to the users own GPS data, reduces most of the errors that affect GPS derived positions. ØThese networks provide continuously operating GPS receivers set on precisely known positions. ØUsers therefore only need to purchase one receiver to achieve high accuracy position results.
4.1 Fundamentals of Active GPS Networks ØThe GPS receivers operate 24 hours a day. ØUsers either access the GPS observations after the survey has been completed (post processing) or receive corrections in real time (DGPS, RTK). ØPost processing data is effectively the “base” receiver data in a differential pair. ØReal time data provides corrections to be applied to stand alone roving or mobile  receivers.
Interesting GPS related Websites
General                                              HartRAO Space Geodesy Home Page http://www.hartrao.ac.za/geodesy/geodesy_index.ht IGS -- International GPS Service http://igscb.jpl.nasa.gov/                        Swepos (Swedish Active GPS Network) http://swepos.lmv.lm.se/ U.S. Geodetic Survey http://www.ngs.noaa.gov/GPS/GPS.html	U.S. Coast Guard                       http://www.uscg.mil/                                University NAVSTAR Consortium (UNAVCO)   http://www.unavco.ucar.edu/      Publications GPS World :            http://www.gpsworld.com
TrigNet
ØThe CDSM has recognised the need to provide a modern National Control Survey Network, to ensure its compatibility with current positioning technologies, and to facilitate easy access to it. ØTo realise this, CDSM is establishing a Network of Active GPS Base Stations which has been named TrigNet. ØThe data from TrigNet will be used by navigators, GIS data collectors, surveyors, engineers, scientists etc. in a wide variety of applications.
5.1 General Considerations The design of the network is based on the following general principles : ØThe system must be receiver independent, hence data will be provided in receiver independent formats for post processing and real time applications. ØThe system must be reliable through integrity monitoring and redundancy. ØIn case of the WWW server for the distribution of post processing data, the data must be as current and up to date as possible. ØThe cost of the services must not be exorbitant. ØThe system must be expandable and flexible.
5.2  Basic Design
TrigNet has been modeled on the Swedish GPS network called SwePos. ØOne master control station based at Mowbray, Cape Town. ØDual frequency geodetic GPS receivers and high precision choke ring antennas are used throughout the network. ØThe stations are about 200 km apart. ØInitially only  a post-processing service will be provided.. ØLater phases will see the introduction of real time services such as DGPS and RTK.
5.3 Distribution of TrigNet Station
For more info on each site please go to www.trignet.co.za
5.4 Redundancy and  Integrity  Monitoring In designing TrigNet a great deal of attention has been placed on ensuring that the service has as little down time as possible. ØThis has been achieved by building as much redundancy into the network as was practical. ØEach of the base stations is equipped with two GPS receivers, two PC’s for data collection and storage.
For real time stations, each station has both frame relay and ISDN telecommunication links. ØThe distribution of stations is such that should one fail, data from the next closest station will still meet the user’s needs. ØRedundant servers have also been installed at the master control station to process incoming data. ØBoth post-processing and real time data is continuously monitored to ensure a high level of integrity and quality, while the positions of base stations are monitored monthly to ensure the stability of the stations.
6. TrigNet Services & Products TrigNet, in future, will provide  two basic services, namely post processing and real time services.
6.1 Post Processing  GPS Data  ØThe post processing service essentially provides the user with GPS data from a receiver (at base station) established at a well defined and monitored position. ØData is supplied in the internationally accepted Receiver INdependent EXchange format (RINEX) .
ØFor people doing precision work aiming at sub 0.05m accuracy, eg. Engineers, Land Surveyors, Geodesists, the standard products are :   1 hour datasets  (1 sec epoch L1, L2 data )   24 hour datasets (15 sec epoch L1, L2 data ) ØFor work aimed at 0.5m to 1m accuracy, eg. GIS data collection , utility surveys, asset management, the standard product  is : 1 hour datasets (5 sec epoch L1 only data.)
ØStandard prices for the above data sets will be set annually. ØData files can be obtained directly from the CDSM website (URL). ØAfter three months the data will be archived.  A surcharge will be payable to cover retrieval costs of archived data. ØSpecial requests for alternative epochs can be accommodated but will also be subjected to special preparation costs.
6.2 Real Time      Correction Services ØThe real time services that are to be provided by TrigNet will fall into two categories depending on the level of accuracy which each service will be able to deliver. ØIn both cases, corrections will be provided in the Radio Technical Commission: Maritime (RTCM) formats which are equivalent to RINEX in the post-processing situation. ØConsideration is being given to using The Radio Data Service (RDS) or Data Radio Channel (DARC) of the FM radio transmitters, Geostationary communication satellites or  Cellular telephone networks (GSM) as possible means of distributing the RTCM corrections to users.
6.2.1  Differential GPS (DGPS)
ØUsers of the DGPS service will be able to determine the position of their GPS receivers in real time to within 0.5m to 1m depending on receiver type and the level of service to which the user subscribes. ØTypical uses of this service will be vehicle navigation and tracking, crop spraying, relocation of infrastructure such as manhole covers, electricity sub-stations etc.
Ø 6.2.2  Real Time Kinematic (RTK)
RTK services will be able to provide users with a position in real time to within a few centimetres. Typical users of this service will be surveyors, engineers and GIS data collectors for whom a position in real time accurate to within about 5cm is essential.
Further information, please contact :
A. Parker, GPS Telephone: +27 (0)21 658 4300 Fax: +27 (0)21 689 1351 E-mail: trignet@sli.wcape.gov.za Address: Private Bag X10, Mowbray, 7705 South Africa Web Site: www.trignet.co.za
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Legal Disclaimer | Revised: 26/03/04 12:50:292 Copyright © 1997 Chief Directorate: Surveys & Mapping Private Bag X10, Mowbray 7705. Tel. +27-(0)21-658 4300 (Dept of Land Affairs, Republic of South Africa)