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Update: 23-01-2024

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Differential Global Positioning System (DGPS)

The General Lighthouse Authorities of the UK and Ireland – Trinity House, the Northern Lighthouse Board and Irish Lights,
has discontinued their DGPS service in March 2022.
The DGPS system is a Radar-based navigation system that eliminates errors in a GPS receiver that will allow the accuracy level to be significantly enhanced. DGPS accuracy can be 10 meters or better, compared with 100 meters or better with GPS. This is possible by placing a high-performance GPS receiver (reference station) at a known location. Because the receiver knows its exact location, it can determine the errors in the satellite signals.

The satellite measures the ranges to each satellite using the signals received and comparing these measured ranges to the actual ranges calculated from its known position. The total error is the difference between the measured and calculated range. The error data for each tracked satellite is formatted into a correction message and transmitted to GPS users. The correction message format follows the standard established by the Radar Technical Commission for Maritime Services, Special Committee 104 (RTCM-SC104). These differential corrections are then applied to the GPS calculations, thus removing most of the satellite signal error and improving accuracy.


Former DGPS stations in Scotland
Girdle Ness
Butt of Lewis
Sumburgh Head

Station name Ref. Station Transm. Station Lat, Lon Nom. Range
at 50 ∝V/m (km)
Since Integrity Monitoring Transmitted Message
Freq. (kHz) Bit Rate (bps)
Butt of Lewis 684 444 58°30.923'N, 006°15.717'W 370 1998 Yes 3 7 9 16 295.5 100
Sumburgh Head 685 445 59°51.2231'N, 001°16.515'W 370 1998 Yes 3 7 9 16 291.5 100
Girdle Ness 686 446 57°08.339'N, 002°02.916'W 277 1998 Yes 3 7 9 16 297.0 100
Stirling 693 443 56°04.000' N, 004°03.000' W 370 2002 Yes 3 6 7 9 16 285.5 100

DGPS is part of the mix of visual, audible and electronic aids to navigation, provided by the three General Lighthouse Authorities (GLA's) of the UK and the Republic of Ireland under their Marine Navigation Plan. It is an open system - available to all mariners - and is financed from light dues charged on commercial vesselping and other income paid into the General Lighthouse Fund. DGPS can assist the safe passage of all classes of vessels from cargo vessels, cruise liners and fishing vessels to small yachts, by:-

- Monitoring the integrity of the US NAVSTAR Global Positioning System (GPS);
- Improving accuracy for safe navigation in waters where the freedom to manoeuvre is restricted.

The DGPS is a network of 14 ground-based reference stations providing transmissions with coverage of at least 50 nautical miles around the coasts of the United Kingdom and the Republic of Ireland, of which 4 are in the Northern Lighthouse Board area of Scotland and the Isle of Man. Details of the locations, ranges and transmission characteristics of all of the GLAs DGPS reference stations at the lighthouse sites are given in the accompanying tables.

How does the Global Positioning System work?

GPS was developed by the US Department of Defense as a worldwide all weather navigation and positioning and timing resource, mainly for military use. It is based on a constellation of 24 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 these measurements to calculate its latitude, longitude, altitude, course and speed. GPS positions are accurate 95% of the time to ± 9 metres and updates can be received every second. A higher accuracy level is required for some specialised navigation and positioning purposes and the integrity of the system needs to be monitored for safety-related applications.

DGPS makes GPS more accurate

DGPS can provide higher accuracies of 5 metres (95%) or better in moving applications and even better in stationary situations, by cancelling out most of the natural and man-made errors arising from normal GPS measurements. DGPS works by having a fixed receiver at a known ground-based reference station 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. DGPS is provided from Radarbeacons by modulating the carrier with the correction data and information regarding the system:

DGPS receivers should comply with the International Electrotechnical Commission (IEC) Standard 1108-4 (BS EN 61108-4) and may be combined with the GPS receiver; the messages provided include information about the health of the reference station and the transmitter.

The Radarbeacon system uses the LF/MF band 283.5 - 315 kHz in Europe. A full description of the technical characteristics of the system can be found in ITU-R Recommendation M.823-2. This incorporates the RTCM SC-104 protocol and the International Association of Lighthouse Authorities has adopted these standards for use with maritime Radarbeacons to transmit DGPS corrections.

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

DGPS is referenced to the World Geodetic System 1984 (WGS84) Datum. Charts covering the waters within the GLAs' areas of responsibility produced by the Hydrographic Office of the United Kingdom are referred to WGS84. Most DGPS receivers will have a facility to transform positions from WGS84 Datum to the regional datum of the chart, but the resulting accuracy will depend on the transformation parameters contained within the software of the user's receiver. The GLAs strongly advise mariners to keep the DGPS receiver referenced to WGS84 Datum and to apply the position shift values denoted on the relevant nautical chart before plotting a position. Reference should also be made to the current Volume 2 of the Admiralty List of Radar Signals.

How can DGPS help maritime transport?

GPS 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 Radarbeacon DGPS services in over 45 countries throughout the world including the majority of North West European States. With advances in technology, the signals from Radarnavigation 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 vessel operators' costs. A high level of availability of the right mix of aids to navigation systems also plays some role in determining which vessels will visit our ports and in demonstrating that effective measures have been taken to minimise the risk of pollution from vessels.

Terms for understanding DGPS:

DGPS Correction Receiver

A DGPS correction receiver decodes the signals received from a reference site. Data is formatted into a serial RTCM SC104 data stream and provided to the remote GPS receiver. There are many types of DGPS correction receivers.

GPS Receivers

The GPS receiver measures ranges to each satellite, but before the measurements are used to calculate position, corrections received from the DGPS receiver are applied to the measurements. The position is then calculated using the corrected range measurements providing vastly increased accuracy.


Depending on the transmission format, the modulator encodes the data as necessary for transmission.

Reference Station

The reference station GPS receiver knows exactly the position of its antenna, therefore it knows what each satellite range measurement should be. It measures the ranges to each satellite using the received signals just as if it was going to calculate position. The measured ranges are subtracted from the known ranges and the result is range error. The range error values for each satellite are formatted into messages in the RTCM SC104 format and transmitted continuously.


The transmitter is basically a power amplifier which is connected to an antenna system. The modulated carrier is amplified and driven to the antenna. The amplified signal is radiated via the antenna to remote DGPS receivers for real-time position correction.

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