Australian Maritime Safety Authority

Differential Global Positioning System

This page has been designed to provide up-to-date information on the status of the DGPS Service, along with other important and general information.

What is DGPS?

Differential Global Positioning System (DGPS) is a method of providing differential corrections to a Global Positioning System (GPS) receiver in order to improve the accuracy of the navigation solution. DGPS corrections originate from a reference station at a known location. The receivers in these reference stations can estimate errors in the GPS because, unlike the general population of GPS receivers, they have an accurate knowledge of their position. As a result of applying DGPS corrections, the horizontal accuracy of the system can be improved from 100m (95% of the time) to better than 10m (95% of the time).

More importantly, the reference stations provide integrity monitoring, warning users to disregard a satellite which is operating outside of specification. With DGPS, this warning happens within a few seconds of the satellite becoming 'unhealthy', compared to GPS warnings where some hours can elapse.

The most common form of DGPS used internationally for maritime navigation operates in the MF Radiobeacon band (285-325 kHz) and conforms to the RTCM Recommended Standards for Differential Navstar GPS Service for the transmission of data. This form of DGPS uses pseudorange corrections and range-rate corrections from a single reference station which has sufficient channels (typically 12) to track all satellites in view. Pseudoranges (distance measurements) are simultaneously measured to all satellites in view, and using the known (surveyed) position of the receiver's antenna and the positional (ephemeris) data from each satellite, the errors in the pseudoranges are calculated. These errors are converted to corrections and are broadcast to user receivers.

DGPS overview

The users GPS receiver applies the corrections to the pseudoranges measured to each satellite used in its position calculation. The GPS receiver always applies the latest (ie. newest) corrections received.

Using this method, and depending on the user-to-reference station separation and the age of the corrections being applied, accuracies better than 10 metres (95%) are achievable.

Effect of User-to-Reference Station Separation on the Navigational Accuracy

As the user-to-reference station separation increases, the signal from the satellite to the user takes a different path through the atmosphere compared to the signal from the satellite to the reference station. Due to variations in the atmosphere, there is a different signal delay at the user receiver compared with the delay at the reference station.

The greater the user-to-reference station separation, the less the satellite ephemeris errors are corrected. This is because different line-of-sight vectors to the satellite from the reference station and from the user produce different perceived ephemeris errors.

As a result of the range decorrelation of the atmospheric and ephemeris errors, the further the user is from the reference station, the less accurate the navigational accuracy can be.

Accuracy of AMSA's DGPS Reference Station Antenna Positions

The AMSA DGPS reference station antenna positions have been surveyed in terms of the US Department of Defense World Geodetic System 1984 (WGS84). WGS84 was chosen because this coordinate system is used by the Global Positioning System (GPS), and the Australian Hydrographic Office produce charts, navigational publications and digital products in WGS84. AMSA initially determined these antenna positions using the now superseded AGD datum, and transformed them to WGS84. This means that the transformed WGS84 position includes the uncertainty of the original AGD position and an additional uncertainty of up to several metres due to the transformation process.

In order to get decimetre accuracy and to meet legal traceability requirements, several days of raw phased GPS observations need to be processed with data from Geoscience Australia's Australian Regional GPS Network (ARGN) which provides the positions in terms of the International Terrestrial Reference Frame (ITRF). AMSA will continue the process of determining the GPS reference station antenna positions for all DGPS facilities by this method. As WGS84 is aligned with ITRF at the level of a few centimetres, these positions can be used as WGS84 at this level of uncertainty. It should be noted that since AMSA’s DGPS network meets the needs of mariners this is a relatively low priority activity that will be completed in due course.

The Geocentric Datum of Australia (GDA) is the current Australian coordinate system adopted by the Intergovernmental Committee on Surveying and Mapping (ICSM). Due to the Australian tectonic plate moving in a north-easterly direction at a rate of about 7 centimetres per year, ITRF & WGS84 are now some 60 centimetres different from GDA94, and this will continue to grow at a rate of about 7 centimetres per year. Accurate transformation from ITRF to GDA94 is possible, but an AMSA DGPS user ITRF/WGS84 position will still have an inherent uncertainty of the order of a metre.

At present the Sydney (NSW) and Cape Flattery (Qld) DGPS facility’s reference station antenna positions (WGS84) were derived from GDA adjusted trigonometric marks and therefore have about 60 cm uncertainty. Albany, Corny Point, Darwin, Exmouth, Karratha, Mallacoota, Perth and Weipa have been determined in terms of ITRF and so have an uncertainty of no more than a decimetre. The remaining DGPS facility’s reference station antenna positions (WGS84) were surveyed using the previous AGD84 datum, and transformed to WGS84 using now superseded parameters. Hence these antenna positions have an accuracy of one to several meters (depending on the order of the survey mark used).

Users should notify AMSA of any degradation or loss of DGPS service.

Users may experience interruptions to the DGPS Service without advanced warning. The service should not be used under any circumstances where a service failure or error could create a safety hazard.

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Shown in the table below are the methods used to derive the WGS84 positions for the DGPS reference station antenna, along with the estimated WGS84 uncertainty and AMSA DGPS reference station antenna positions (WGS84)

Stations WGS-84 Accuracy Ref Stn 1 antenna location Ref Stn 2 antenna location IALA Ref Stn ID # IALA Tx Stn ID # Baud rate (bps) RTCM Messages Used Radiated power Survey
marks/
transformation
used

Albany,
WA

315kHz

(on-air)

0.1

35 05' 15.168" S 117 54' 01.752" E 35 05' 15.093" S 117 54' 01.682" E 011 711 200 Types 3, 5, 7, 9, 16 300

ITRF97

Brisbane,
Qld

294kHz

(on-air)

0.1

27 04' 08.797" S 153 03' 18.941" E 27 04' 08.855" S 153 03' 18.887" E 007 707 200 Types 3, 5, 7, 9, 16 180

Several 1st order AGD84 (Higgins parameters) AGD positions GDA-adjusted

Cape Flattery, Qld

304kHz

(on-air)

0.6

14 57' 56.730" S 145 18' 02.814" E 14 57' 56.728" S 145 18' 03.000" E 005 705 200 Types 3, 5, 7, 9, 16 150

Zero order GDA94 - B065 Piebald & high precision GPS solution

Corny Point, SA

316kHz

(on-air)

0.1

34 53' 58.305" S 137 00' 52.539" E 34 53' 58.392" S 137 00' 52.593" E 010 710 200 Types 3, 5, 7, 9, 16 220

ITRF97

Crib Point,
Vic

314kHz

(on-air)

0.1 38° 21' 36.706" S 145° 10' 11.554" E 38° 21' 36.667" S 145° 10' 11.401" E 000 - 200 Types 3, 5, 7, 9, 16 195 ITRF2000

Darwin,
NT

294kHz

(on-air)

0.1

12 26' 42.989" S 130 57' 30.543" E 12 26' 42.934" S 130 57' 30.460" E 014 714 200 Types 3, 5, 7, 9, 16 165 ITRF2000

Exmouth,
WA

297kHz

(on-air)

0.1

21 53' 03.3041" S 114 08' 02.3719" E 21 53' 03.3091" S 114 08' 02.2800" E 015 715 200 Types 3, 5, 7, 9, 16 240 ITRF2000

Gladstone, Qld

313kHz

(on-air)

<0.1

24 02' 12.544" S 151 21' 31.570" E 24 02' 12.543" S 151 21' 31.460" E 006 706 200 Types 3, 5, 7, 9, 16 320

ITRF2005

Horn Island, Qld

320kHz

(on-air)

<0.1

10 35' 31.021" S 142 16' 28.810" E 10 35' 30.856" S 142 16' 29.505" E 002 702 200 Types 3, 5, 7, 9, 16 150

ITRF2005

Ingham,
Qld

306kHz

(on-air)

<0.1

18 33' 20.248" S 14618' 21.087" E 18 33' 20.177" S 14618' 21.025" E 008 708 200 Types 3, 5, 7, 9, 16 350

ITRF2005

Karratha,
WA

304kHz

(on-air)

0.1

20 42' 24.840" S 116 46' 26.152" E 20 42' 24.874" S 116 46' 26.385" E 001 701 200 Types 3, 5, 7, 9, 16 350

ITRF97

Mackay,
Qld

315kHz

(on-air)

<0.1

21 06' 11.642" S 149 12' 40.872" E 21 06' 11.703" S 149 12' 40.822" E 004 704 200 Types 3, 5, 7, 9, 16 289

ITRF2005

Mallacoota, Vic

318kHz

(on-air)

0.1

37 34' 05.523" S 149 44' 09.804" E 37 34' 05.437" S 149 44' 09.746" E 013 713 200 Types 3, 5, 7, 9, 16 350

ITRF97

Perth,
WA

306kHz

(on-air)

0.1

31 47' 52.854" S 115 56' 00.582" E 31 47' 52.935" S 115 56' 00.523" E 012 712 200 Types 3, 5, 7, 9, 16 230

ITRF97

Sydney,
NSW

308kHz

(on-air)

0.1

33 59' 10.763" S 150 58' 40.282" E 33 59' 10.847" S 150 58' 40.272" E 003 703 200 Types 3, 5, 7, 9, 16 140

Zero order GDA94

Weipa,
Qld

316kHz

(on-air)

0.1

12 39' 12.658" S 141 51' 36.631" E 12 39' 12.669" S 141 51' 36.725" E 009 709 200 Types 3, 5, 7, 9, 16 120

ITRF97

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