Neist Point

Technics of the Light's

In Salutem Omnium
For the Safety of All
Neist Point - Isle of Skye
Flag of Scotland
© Composted by:
Bob Schrage
updated: 03-11-2017

Differential Global Positioning System (DGPS)


The DGPS system is a radio-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 Radio 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.


Table of DGPS stations in Scotland
Station name Reference Station Transmitting Station Lat, Lon Nom. Range
at 50 ∝V/m (km)
Since Integrity Monitoring Transmitted Message
type
Freq. (kHz) Bit Rate (bps)
Butt of Lewis 684 444 58°30.940' N, 06°15.717' W 370 1998 Yes 3 7 9 16 295.5 100
Sumburgh Head 685 445 59°51.243' N, 01°16.476' W 370 2002 Yes 3 7 9 16 291.5 100
Gridle Ness 686 446 57°08.339' N, 02°02.916' W 370 1998 Yes 3 7 9 16 297.0 100
Stirling 693 443 56°04.000' N, 04°03.000' W 370 2002 Yes 3 7 9 16 285.5 100

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.

Modulator

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.

Transmitter

The transmitter is basically a power amplifier which is connected to an antenna system. The modulated carrier is amplified and driven to the antenna. In the United States Coast Guard system, the transmitter is 250-1000 Watts and operates in the 300Khz frequency range. The amplified signal is radiated via the antenna to remote DGPS receivers for real-time position correction. DESCRIPTION (Differential GPS Stations) Information is tabulated in eight columns as follows: Column 1: The number assigned to each DGPS Station by this Agency. Column 2: Name of the DGPS Station. Column 3: Approximate latitude and longitude of the DGPS Transmitting Station to the nearest tenth of a minute. Column 4: Station ID which can be found in the IALA Master list. No two stations have the same ID. T denotes the Transmitting Station, R denotes the Reference Station. Column 5: Range (approximate) in nautical miles. Column 6: Frequency in kHz. Column 7: Transfer Rate which equates to the baud rate and will be published as a whole number without any additional abbreviations such as “bps” (bits per second). Column 8: Remarks. This column contains information about the reference stations and messages types transmitted.

GPS Message Type Numbers are 1, 3, 4, 5, 6, 7, 9, 15 and 16. (Refer to message type descriptions below) GPS MESSAGE TYPE NUMBER INDICATORS 1 Differential GNSS corrections (full set of satellites) 3 Reference stations parameters 4 Datum used 5 Constellation health 6 Null frame (no information) 7 Radiobeacons Almanacs 9 Sub-set differential GNSS corrections 15 Ionospheric corrections 16 Special messages

The Differential Global Positioning System (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. Girdle Ness - East Coast Sumburgh Head - Orkney & Shetland Butt of Lewis - North West Stirling - Central Scotland 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 radiobeacons 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 radiobeacon 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 radiobeacons 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 Radio Signals. 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 radiobeacon DGPS services in over 45 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 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. Who can be contacted regarding any problems with the DGPS service? The GLAs DGPS system is provided for maritime use. Implementation was completed in August 1998 and the system achieved Operational Capability on 1 July 2002. Problems with DGPS receiver equipment should be taken up with the supplier, whereas users who experience problems with the system are invited to complete and return an Incident Report Form, which can be obtained as follows: Differential Global Positioning System (DGPS) Girdle Ness is one of a network of twelve (three in Scotland) ground-based reference stations providing DGPS transmissions around the coasts of the United Kingdom and the Republic of Ireland. The other Scottish Stations are Butt of Lewis in the Outer Hebrides and Sumburgh Head in Shetland. The DGPS transmissions were introduced, on a trial basis, on 31 July 1998. DGPS is transmitted from the MF Radio Beacon and a Standby Generator has been installed to ensure the intergity of this service. The generator is also used to power the other Aids to Navigation in the event of a mains failure. The public marine DGPS is a satellite based navigation system. It is the newest element of the mix of visual, audible and electronic aids to navigation provided by the three General Lighthouse Authorities 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. The light and DGPS is monitored using the NLB telemetry system over the PSTN telephone network to the Northern Lighthouse Boards headquarters in Edinburgh. This is manned 24 hours and checks the operation of over 70 lighthouses around Scotland and the Isle of Man. It should be noted that at some sites the Northern Lighthouse Board have sold some redundant buildings within the lighthouse complex and are not responsible for the maintenance of these building.

DGPS Marine Differential Global Positioning System How does the Global Positioning System work? DGPS makes GPS more accurate What if the DGPS datum is different from that used for maritime charts? How can DGPS help maritime transport? Is DGPS provided elsewhere? Who can be contacted regarding any problems with the DGPS service? Station Frequencies DGPS Station Locations Marine Differential Global Positioning System The General Lighthouse Authorities' (GLAs) public marine Differential Global Positioning System (DGPS) is part of the mix of visual, audible and electronic aids to navigation provided by the three General Lighthouse Authorities 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 shipping and other income paid into the General Lighthouse Fund. DGPS can assist the safe passage of all classes of vessels from cargo ships, 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. Girdle Ness - East Coast Sumburgh Head - Orkney & Shetland Butt of Lewis - North West Stirling - Central Scotland 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 radiobeacons 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 radiobeacon 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 radiobeacons 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 Radio Signals. 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 radiobeacon DGPS services in over 45 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 operators' 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. Who can be contacted regarding any problems with the DGPS service? The GLAs DGPS system is provided for maritime use. Implementation was completed in August 1998 and the system achieved Operational Capability on 1 July 2002. Problems with DGPS receiver equipment should be taken up with the supplier, whereas users who experience problems with the system are invited to complete and return an Incident Report Form, which can be obtained as follows:

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