Understanding DGPS

Redcar FishTechnical

Courtesy of Redcar Fish

Want to know about the Differential Global Positioning System? It’s all here.

Differential beacon transmissions for the correction of GPS satellite signals in the UK was originally only available in the south and southeast areas. Even in these areas, there had been a number of problems receiving the signal due to these areas being on the extreme limits of transmission of the European Differential Radiobeacons.

Since the 1st of August 1998, the General Lighthouse Authority (GLA) has operated a number of DGPS stations in the UK. There are three transmitters on the West coast of Ireland, two in Wales, two on the South coast of England, two on the East coast of England, one near Aberdeen, one in Shetland and one in the Western Isles of Scotland. These are intended to provide complete coastal coverage to a distance from the coast of about 50 nautical miles. 

Details of the locations, ranges and transmission characteristics are given in the accompanying table at the end of this feature DGPS makes GPS more accurate. DGPS, which became operational on 1 July 2002, is a network of 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. It is an open system—available to all mariners—and is financed from light dues charged on commercial shipping and other income paid into the General Lighthouse Fund.

DGPS can provide higher accuracies in the order of 5 metres 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 – such as Selective Availability (SA) which superimposes variability on the GPS satellite clocks to reduce their accuracy.

DGPS works by having a fixed receiver at a known well-surveyed ground-based reference station that continually monitors the GPS errors and transmits corrections to the measurements taken by mobile receivers operated by users of the system.

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, radio beacon DGPS can assist safe navigation in waters where freedom to manoeuvre is restricted. 

The output from the DGPS receiver could form the position sensor input to an integrated navigation system, an electronic chart system or an automatic identification system. In these applications the inherent integrity checking, high accuracy and stability are particularly important.

Is DGPS provided elsewhere?

There are marine radio beacon DGPS services in over 24 countries throughout the world including most member states of the European Union. As technology advances, the signals from radio navigation systems are progressively transcending national boundaries and provide long-range transmissions from a network of satellite and terrestrial stations. A significant benefit for the safety of navigation since it enables a consistent means of navigation throughout the phases of the voyage, during the day or night and in conditions of reduced visibility. 

What equipment do I need?

To obtain the precision of DGPS all you need is a “Differential ready GPS” either fixed or handheld and a Differential Beacon receiver. Most GPS sets currently available are Differential ready. It is then just a simple task of connecting the two units together and then selecting the frequency and bit rate of the beacon for your area and entering it into your GPS set.

Please note that since 1st May 2000 selective availability (S/A) has been turned off by the US Department of Defence. DGPS beacons not only correct the effects of S/A but also correct normal satellite clock errors, other system errors and errors caused by atmospheric conditions.

In other words, DGPS improves GPS tenfold.

UK DGPS STATIONS

Station Name Position Lat/Long Nominal Range (km) Frequency (kHz) Bit Rate (bps)
Lizard (CORNWALL) 49º 58′ N   05º 12′ W 185 284.00 100
St. Catherine’s (ISLE OF WIGHT) 50º 35′ N   01º 18′ W 185 293.50 100
North Foreland (KENT) 51º 23′ N   01º 27′ E 185 310.50 100
Flamborough Head (EAST COAST) 54º 07′ N   00º 05′ W 185 302.50 100
Girdle Ness (ABERDEEN) 57º 08′ N   02º 03′ W 277 311.00 100
Butt of Lewis (OUTER HEBRIDES) 58º 31′ N   06º 16′ W 277 294.00 100
Sumburgh Head (SOUTH SHETLAND) 59º 51′ N   01º 16′ W 277 304.00 100
Point Lynas (ANGLESEY) 53º 25′ N   04º 17′ W 185 305.00 100
Nash Point (BRISTOL CHANNEL) 51º 24′ N   03º 33′ W 185 299.00 100
Tory Island (NORTH IRELAND) 55º 16′ N   08º 15′ W 277 313.50 100
Loop Head (WEST IRELAND) 52º 34′ N   09º 56′ W 185 312.00 100
Mizen Head (SOUTH IRELAND) 51º 27′ N   09º 49′ W 185 300.50 100
Station Name Position Lat/Long Nominal Range (km) Frequency (kHz) Bit Rate (bps)