How does the Global Positioning System work?

GPS was developed by the US Department of Defence as a worldwide all weather navigation and positioning and timing resources, originally for military use.

It is based on a constellation of 24 or more satellites orbiting the earth which act as reference points. By measuring the travel time of signals transmitted from four satellites, a receiver can measure its distance from each satellite and combine those measurements to calculate its latitude, longitude, altitude, course and speed. GPS positions are accurate 95% of the time to +/- 13 metres and updates can be received every second. A higher accuracy level is required though for some specialised navigation and positioning purposes and the integrity of the system needs to be monitored for safety-related applications.

DGPS makes GPS safer and more accurate

DGPS is provided primarily for use in monitoring the integrity of GPS - giving a level of assurance that GPS cannot provide on its own. DGPS also provides accuracies of 5 metres (95% probability) or better in moving applications and even greater in stationary situations, by cancelling out most of the errors arising from normal GPS measurements. GPS can provide misleading information due to inherent and man-made errors in the system. DGPS works by having a fixed receiver at a known ground-based reference station (e.g. GLAs stations), which continuously monitors the GPS errors and transmits corrections to the measurements taken by mobile receivers operated by users of the system.

The GLAs' DGPS service is intended to offer accuracies for general navigation of 5 metres for vessel position fixing in areas where the freedom to manoeuvre is restricted. The intended coverage area is 50 nautical miles around the coasts of the United Kingdom and Republic of Ireland.

What are the Technical Characteristics of DGPS?

  • DGPS is provided using radio beacon technology by modulating a second carrier with the correction data and other information on the system.
  • The modulation is Minimum Shift Keying (MSK), a special form of Frequency Shift Keying (FSK).
  • The modulation rate is usually 100 or 200 bits per second.
  • In Europe, the channel spacing is 500 Hz and the identification code is included in this data.
  • DGPS receivers should comply with the International Electrotechnical Commission (IEC) Standard 1108-4 (BS EN 61108-04) and may be combined with the GPS receiver; the messages provided include information about the health of the reference station and the transmitter.
  • The International Telecommunications Union (ITU) Recommendation M.823 incorporates the RTCM SC-104 protocol and the International Association of Lighthouse Authorities has adopted these standards for use with maritime radio beacons to transmit DGPS corrections. The radio beacon system uses the LF/MF band 283.5 - 315 kHz in Europe (285 - 325 kHz in other parts of the world). The regulations governing the use of this band allow for DGPS signals to be transmitted.

What if the DGPS datum is different from that used for maritime charts?

DGPS is referenced to the World Geodetic System 1984 (WGS84) Datum. DGPS derived positions may be plotted directly onto charts referred to WGS84 Datum. Such charts covering the waters within the GLAs' areas of responsibility are now being produced by the Hydrographic Office of the United Kingdom.

Most DGPS receivers will have a facility to transform positions from WGS84 Datum to a regional datum of a chart, but the resulting accuracy will depend on the transformation parameters contained within the software of the users receiver. The GLAs strongly advise mariners to keep the DGPS receiver referenced to WGS84.

If a chart with a regional datum is used, the position shift values denoted on the chart must be applied before plotting a position. Position shifts for those charts which do not include a "Satellite Derived Positions" note are listed in the UK Hydrographic Office Annual Summary of Notice to Mariners. Reference should also be made to the relevant, most up-to-date volume of the Admiralty List of Radio Signals.

Who supplies DGPS receivers?

Most of the leading manufacturers of radio navigation equipment will supply a DGPS or GPS/DGPS integrated receiver. Users should contact the suppliers of their receivers for guidance on system installation, as incorrect siting and installation can cause interference leading to degradation of the coverage, accuracy and availability of the service.

How can DGPS help maritime transport?

DGPS is no longer simply a navigation technique - it is a means to measure positions and movement of any kind to a high degree of accuracy.

In the marine environment, the output from a suitable DGPS receiver could form the position sensor input to an Integrated Navigation System (INS), an Electronic Chart System (ECS) or an Automatic Identification System (AIS). In these applications the inherent integrity checking, high accuracy and stability of the differential system are particularly important.

In positioning mode, DGPS is used for buoy-laying and hydrographic surveying. DGPS may also be used, often in combination with other systems, for a variety of other marine applications such as dynamic positioning, rig moves, pipe-laying, cable laying and dredging.

Is DGPS provided elsewhere?

There are marine radio beacon DGPS services in over 40 countries throughout the world including the majority of North West European states.

With advances in technology, the signals from radionavigation systems have progressively transcended national boundaries to provide long-range transmissions from a network of satellite and terrestrial stations. This has significant benefits for the safety of navigation since it enables a consistent means of navigation throughout all phases of the voyage, during the day or night and in conditions of reduced visibility. Such systems will also help with improved voyage planning to reduce ship operator costs.

A high level of availability of the right mix of aids to navigation systems also plays some role in determining which ships will visit our ports and in demonstrating that effective measures have been taken to minimise the risk of pollution from ships.