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KNOWING THE DATUM

Written on 16 September 2016

The development of the NAVSTAR Global Positioning System began under the cloak of military secrecy. The goal was to create a navigation and positioning system that would support military operations of the USA and its allies twenty-four hours a day, in all weather conditions, anywhere in the world – from beneath the surface of the oceans outward to the realm of near-earth satellites. Equally important, enemy forces were to be denied access to GPS.

By the late 1970s work on the secret multibillion-dollar GPS program was progressing well, with about a third of the planned twenty-four satellite constellation already in orbit, when an unexpected turn of events occurred. Academic researchers at the Massachusetts Institute of Technology (MIT) who had been working for a number of years on the development of Very Long Baseline Interferometry with no knowledge of GPS, proposed the launching of a constellation of artificial satellites that could be used to accurately determine the coordinates of points anywhere on earth.

Because the signals of received from nearby satellites would be many orders of magnitude more powerful than those received from natural radio sources located billions of light years from the earth, the huge radio telescopes, used for VLBI would not be required. First generation satellite receivers would be no larger than a small suitcase, and, if the demand proved sufficient to warrant the manufacture of special integrated chips, receivers might one day be small enough to fit into a wristwatch.

When the US Department of Defence (DOD) personnel learned of the MIT proposal, they realised that GPS already possessed many of the capabilities the scientists sought, but worried that providing access to civilian researchers might compromise national security. The DOD was particularly concerned that the signal-processing technologies, developed by the VLBI experts would effectively bypass the GPS Selective Availability (SA) system, making it impossible to deny access to unfriendly forces. The US Defence Mapping Agency (DMA) hosted a quickly organised meeting to explore the issue.

A period of rather awkward and disjointed discussions followed. Without revealing the details of how SA worked, the DOD personnel tried to ascertain if in fact the scientists could bypass SA; without knowing exactly how the system worked the scientists were unable to say how difficult would be to bypass the effects of SA. There was no doubt that the techniques that had been developed for VLBI could be used to record the satellite signals for later processing, but the delays involved would prevent their use in real time navigation, of highest concern to the military.

Eventually GPS emerged from the shadows of military secrecy and blossomed into perhaps the single most important achievement of the American space program.

Over the past thirty- plus years GPS had evolved into truly global “utility” that provides position and navigation services to users in all nations.

The day-to-day operation of GPS remains under the control of the DOD, but executive orders signed by the US President now provide strong assurance that, except during time of extreme national emergency, the civilian community will have access to the system.

Aided by the era of Satellite Navigation Systems, the Geodetic datum established as a standard and used world-wide is WGS84. For most of the common GPS receivers, the default datum is the WGS84 (World Geodetic System 1984).

However, OSGB36 is the common coordinate reference system (datum) used in the UK for the purposes of Agriculture, Construction, Surveying, Asset Management and is the National Grid of Great Britain. In fact, the three national coordinate systems are:

  • ETRS89 (European Terrestrial Reference System 1989) – the UK national coordinate system for 3D positioning. It has much higher precision that the better known WGS84 standard. The ETR89 coordinates are the WGS84 coordinates, but the general WGS84 coordinates do not necessary meet the standards of ETR89, as the latter is the better quality GPS coordinate system.
  • OSGB36 National Grid (Ordinance Survey Great Britain 1936) – the UK topographic system mapping
  • ODN (Ordinance Datum Newlyn) – The UK coordinate system for heights above mean sea level. Based on the tide gauge readings at Newlyn, Cornwall. This datum is defined in the UK Hydrographic Office publication NP5011 and pictorially displays (see below) the definition of height above Mean Sea Level in mainland Britain.
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  • NP5011 and pictorially displays (see above) the definition of height above Mean Sea Level in mainland Britain.

For the reliable coordinates transformation between the different datums, Ordinance Survey provides a (free of charge) dedicated web page, accessible to the general public

https://www.ordnancesurvey.co.uk/gps/transformation

This however operates within Great Britain and up to 10 km offshore.

It happened in 2007 that a naval vessel, using a WGS84 had entered another country’s waters, charted under the Russian PULKOVO datum: embarrassingly, the crew got arrested in front of the whole world eyes – so it is good to know the correct datum and be aware of the others.

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