LogoLogo
OS Docs HomeOS NGDOS APIsOS Download ProductsMore than MapsContact Us
  • More than Maps
  • Geographic Data Visualisation
    • Guide to cartography
      • Introduction to cartography
      • Types of maps
      • Symbology
      • Colour
      • Text on maps
      • Generalisation
      • Coordinate reference systems
      • Projections
      • Scale
      • Map legends
      • Map layout
      • Relief representation
      • North arrows
    • Guide to data visualisation
      • Introduction to data visualisation
      • GeoDataViz design principles
      • Types of visualisation
      • Thematic mapping techniques
      • Data visualisation critique
      • Accessible data visualisation
      • Ethical data visualisation
      • Software
      • Data
    • GeoDataViz assets
      • GeoDataViz basemaps
      • Stylesheets
      • GeoDataViz virtual gallery
      • Equal area cartograms
      • How did I make that?
        • Apollo 11 Landing
        • North York Moors National Park, 70 years
        • Snowdonia National Park, 70 years
        • Great Britain's National Parks
        • Great Britain's Islands
        • Great Britain's AONB's and National Scenic Areas
        • Famous shipwrecks of Pembrokeshire
        • Trig pillars today
        • Britain's most complex motorway junctions
      • #30DayMapChallenge
  • Data in Action
    • Examples
  • Demonstrators
    • 🆕Product Viewer
    • Addressing & location demonstrators
      • Address Portfolio overview
      • Which address product should you use?
      • AddressBase
      • AddressBase Core
      • AddressBase Plus
      • AddressBase Premium
      • Address Classifications
      • Addressing Lifecycle
      • OS Emergency Services Gazetteer
      • What are Vertical Streets?
      • Why are there differences in boundaries?
    • Contextual demonstrators
    • Customer best practice
      • Channel Shift
      • Data Management and OS Data Hub
      • End User Licence vs Contractor Licence
      • 🆕 IDs vs Spatial Relationships
      • Why we should capture good quality addresses at source
      • Why we Snap and Trace
    • Network Demonstrators
      • OS Detailed Path Network
      • OS Multi Modal Routing Network
        • OS Multi Modal Routing Network
      • Water Networks overview
      • OS MasterMap Highways Network and OS NGD Speeds
      • OS MasterMap® Highways Network and OS Open Roadsâ„¢
    • OS MasterMap Generation APIs
      • Using the OS Features API
      • Using the OS Features API Archive
      • Using the OS Downloads API
      • Using OS APIs in ESRI Software
    • 🆕OS NGD (National Geographic Database)
      • OS NGD Address
      • OS NGD Boundaries
      • 🆕OS NGD Buildings
        • 🆕Building and Building Access Feature Types
        • Building Part and Building Line Feature Types
      • 🆕OS NGD Geographical Names
      • OS NGD Land
      • OS NGD Land Cover enhancements
      • 🆕OS NGD Land Use
      • OS NGD Land Use enhancements
      • 🆕OS NGD Structures
        • 🆕OS NGD Structures
        • Field Boundaries
      • 🆕OS NGD Transport Features
      • 🆕OS NGD Transport Network
      • OS NGD Transport RAMI
      • OS NGD Water Features
      • OS NGD Water Network
      • OS NGD API - Features
      • Ordering OS NGD data
      • Change only updates
      • OS NGD Versioning
      • Creating a topographic map from OS NGD Data
      • Analytical styling for OS NGD data
    • OS MasterMap® demonstrators
    • 🆕Product & API Comparisons
      • 🆕Comparison of Water Network Products
  • Tutorials
    • GeoDataViz
      • Thematic Mapping Techniques
      • Downloading and using data from the OS Data Hub
      • How to download and use OS stylesheets
      • How to use the OS Maps API
      • Creating a bespoke style in Maputnik
    • GIS
      • Analysing pavement widths
      • Basic routing with OS Open Data and QGIS
      • Walktime analysis using OS Multi-modal Routing Network and QGIS
      • Creating 3D Symbols for GIS Applications
      • Constructing a Single Line Address using a Geographic Address
      • Creating a Digital Terrain Model (DTM)
      • Visualising a road gradient using a Digital Terrain Model
      • Visualising a road gradient using OSMM Highways
    • 🆕APIs
      • 🆕Using OS APIs with EPC API
      • 🆕OS APIs and ArcGIS
  • Deep Dive
    • Introduction to address matching
    • Guide to routing for the Public Sector
      • Part 1: Guide to routing
      • Part 2: Routing software and data options
      • Part 3: Building a routable network
    • Unlocking the Power of Geospatial Data
    • Using Blender for Geospatial Projects
    • A Guide to Coordinate Systems in Great Britain
      • Myths about coordinate systems
      • The shape of the Earth
      • What is position?
        • Types of coordinates
        • We need a datum
        • Position summary
      • Modern GNSS coordinate systems
        • Realising WGS84 with a TRF
        • The WGS84 broadcast TRF
        • The International Terrestrial Reference Frame (ITRF)
        • The International GNSS Service (IGS)
        • European Terrestrial Reference System 1989 (ETRS89)
      • Ordnance Survey coordinate systems
        • ETRS89 realised through OS Net
        • National Grid and the OSGB36 TRF
        • Ordnance Datum Newlyn
        • The future of British mapping coordinate systems
        • The future of British mapping coordinate systems
      • From one coordinate system to another: geodetic transformations
        • What is a geodetic transformation?
        • Helmert datum transformations
        • National Grid Transformation OSTN15 (ETRS89–OSGB36)
        • National Geoid Model OSGM15 (ETRS89-Orthometric height)
        • ETRS89 to and from ITRS
        • Approximate WGS84 to OSGB36/ODN transformation
        • Transformation between OS Net v2001 and v2009 realisations
      • Transverse Mercator map projections
        • The National Grid reference convention
      • Datum, ellipsoid and projection information
      • Converting between 3D Cartesian and ellipsoidal latitude, longitude and height coordinates
      • Converting between grid eastings and northings and ellipsoidal latitude and longitude
      • Helmert transformation worked example
      • Further information
  • Code
    • Ordnance Survey APIs
    • Mapping
    • Routing with pgRouting
      • Getting started with OS MasterMap Highways and pgRouting
      • Getting started with OS MasterMap Highways Network - Paths and pgRouting
      • Getting started with OS NGD Transport Theme and pgRouting
      • Getting started with OS NGD Transport Path features and pgRouting
  • RESOURCES
    • 🆕Data Visualisation External Resources
Powered by GitBook

Website

  • Ordnance Survey

Data

  • OS Data Hub
On this page

Was this helpful?

  1. Deep Dive
  2. A Guide to Coordinate Systems in Great Britain
  3. Modern GNSS coordinate systems

The WGS84 broadcast TRF

The primary means of navigating in the WGS84 coordinate system is via the WGS84 positions of the GPS satellites, which are continuously broadcast by the satellites themselves. This satellite constellation is a TRF – that is, it is a general-purpose access tool making the WGS84 coordinate system available to users.

The WGS84 satellite positions are determined by the US Department of Defense using a network of tracking stations, the positions of which have been precisely computed. The tracking stations observe the satellites and hence determine the WGS84 coordinates of the satellites. The quality of the resulting satellite coordinates depends on the quality of the known tracking station coordinates. These were initially not very good (probably 10 metre accuracy) but have been refined several times. The tracking station coordinates are now very close agreement with the International Reference Meridian and International Reference Pole.

The network of GPS tracking stations can be considered the original WGS84 TRF. The satellite constellation, which is a derived TRF, can be seen as a tool to transfer this realisation ‘over the horizon’ to wherever positioning is needed in the world. The current coordinates of the tracking station antennae implicitly state the physical origin, orientation and scale of the system – they have been computed such that these elements are as close as possible to the theoretical requirements listed above. Of course, no TRF is perfect – this one is probably good to five centimetres or so.

Prior to May 2000, the full accuracy of the US tracking station TRF was not made available to non-military users. In the transfer of this TRF to satellite positions, positional accuracies were deliberately worsened by a feature known as selective availability (SA). This meant that a civilian user, with a single GPS receiver, could not determine WGS84 position to an accuracy better than about 100 metres. In May 2000, this intentional degradation of the GPS signals was officially switched off.

With a pair of GNSS receivers we can accurately measure their relative positions (that is, the three‑dimensional vector between the two receivers can be accurately determined). We must put one of these receivers on a known point and leave it there. This is known as relative GNSS positioning or differential GNSS. Fortunately, there are methods of accurately determining the real WGS84 position of the known point and hence recovering correct WGS84 positions, using the civil GNSS TRFs which are the subject of the following pages.

PreviousRealising WGS84 with a TRFNextThe International Terrestrial Reference Frame (ITRF)

Last updated 4 months ago

Was this helpful?