CHIEF DIRECTORATE: SURVEYS AND MAPPING

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CHIEF DIRECTORATE SURVEYS AND
MAPPING TrigNet The South African Network
of Active GPS Base Stations
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• 1. Passive Networks
• The Chief Directorate Surveys and Mapping (CDSM)
  is responsible for the establishment and
  maintenance of the National Control Survey
  Network for South Africa.
• The 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.
• Such 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 .
• In 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.

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• 2. Active Network
• In 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.
• Satellite 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)
• Global Navigation Satellite Systems is a term
  used to describe all forms of satellite based
  navigation systems, which includes
• GPS (Global Positioning Systems) is designed and
  controlled by the United States Department of
  Defense.
• GLONASS (Global Orbiting Navigation Satellite
  System) is designed and controlled by the
  Ministry of Defense of the Russian Federartion.
• GNSS is capable of providing
• Rapid 3D positioning.
• Velocity information
• Time information
• 24 hours per day operation
• Weather independent operation.
• Real time\instantaneous position determination

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• Navigation satellite systems work on the
  principle of triangulation
• 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
• GIS data capture
• Velocity information
• Aerial photography control
• Surveying
• Navigation

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• 2.3 GNSS Error Sources
• Positions derived from satellite navigation
  systems are subject to numerous sources of errors
  such as
• Satellite and receiver clock instability.
• The broadcast satellite ephemeris is a predicted
  orbit and is subject to error.
• Satellite signals are transmitted through an
  unstable atmosphere (ionosphere and troposphere).
• Deliberate degradation of signals by the U.S.
  Department of Defense (Selective Availability).
  This has been de-activated since 1 May 2000.

Predicted Orbit
Actual Orbit
Atmospheric Refraction
b
a
Apparent Po sition
Point2
Point1
True Position
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• 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).
• In order to achieve the high accuracy
  requirements of surveying, at least two GPS
  receivers must be used simultaneously.
• One receiver is placed at a known position while
  a second receiver is at the unknown point or is
  travelling (roving).
• Both 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.

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• 4. Active GPS Base Station Networks
• An 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.
• These networks provide continuously operating GPS
  receivers set on precisely known positions.
• Users therefore only need to purchase one
  receiver to achieve high accuracy position
  results.
• 4.1 Fundamentals of Active GPS Networks
• The GPS receivers operate 24 hours a day.
• Users either access the GPS observations after
  the survey has been completed (post processing)
  or receive corrections in real time (DGPS, RTK).
• Post processing data is effectively the base
  receiver data in a differential pair.
• Real 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.h
  t
• IGS -- International GPS Service

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• TrigNet
• The 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.
• To realise this, CDSM is establishing a Network
  of Active GPS Base Stations which has been named
  TrigNet.
• The 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
• The system must be receiver independent, hence
  data will be provided in receiver independent
  formats for post processing and real time
  applications.
• The system must be reliable through integrity
  monitoring and redundancy.
• In case of the WWW server for the distribution of
  post processing data, the data must be as current
  and up to date as possible.
• The cost of the services must not be exorbitant.
• The system must be expandable and flexible.
• 5.2 Basic Design
• TrigNet has been modeled on the Swedish GPS
  network called SwePos.
• One master control station based at Mowbray, Cape
  Town.

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• Dual frequency geodetic GPS receivers and high
  precision choke ring antennas are used throughout
  the network.
• The stations are about 200 km apart.
• Initially only a post-processing service will be
  provided..
• Later phases will see the introduction of real
  time services such as DGPS and RTK.
• 5.3 Distribution of TrigNet Station

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• 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.
• This has been achieved by building as much
  redundancy into the network as was practical.
• Each of the base stations is equipped with two
  GPS receivers, two PCs for data collection and
  storage.
• For real time stations, each station has both
  frame relay and ISDN telecommunication links.
• The distribution of stations is such that should
  one fail, data from the next closest station will
  still meet the users needs.
• Redundant servers have also been installed at the
  master control station to process incoming data.
• Both 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.

2 GPS Receivers
2 PCs
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• 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
• The post processing service essentially
  provides the user with GPS data from a
  receiver (at base station) established at a
  well defined and monitored position.
• Data is supplied in the internationally
  accepted Receiver INdependent EXchange format
  (RINEX) .
• For 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 )
• For 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.)
• Standard prices for the above data sets will be
  set annually.
• Data files can be obtained directly from the
  CDSM website (URL).
• After three months the data will be archived. A
  surcharge will be payable to cover retrieval
  costs of archived data.

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• 6.2 Real Time Correction
  Services
• The 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.
• In both cases, corrections will be provided in
  the Radio Technical Commission Maritime (RTCM)
  formats which are equivalent to RINEX in the
  post-processing situation.
• Consideration 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.

RINEX data on CDSM web site
Raw Data
Post Processing Users
Network Control Centre
Active GPS Base station
User accesses CDSM website for RINEX Base Station
data for post processing
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• 6.2.1 Differential GPS (DGPS)
• Users 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.
• Typical uses of this service will be vehicle
  navigation and tracking, crop spraying,
  relocation of infrastructure such as manhole
  covers, electricity sub-stations etc.
• 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.

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• TrigNet
• Products
• GPS Datasets
• 1 hour datasets (1 sec epoch L1, L2 data from
  all stations)
• 1 hour datasets (5 sec epoch L1 only data from
  all stations)
• Real Time Corrections ( coming soon! )
• Countrywide real-time DGPS corrections service
  providing accuracies of up to 0.5m.
• RTK corrections for centimeter positioning in
  city areas and most large towns.
• CHIEF DIRECTORATE
• SURVEYS AND MAPPING
• Private Bag X10
• MOWBRAY, 7705
• SOUTH AFRICA
• For further information