Australian Maritime Safety Authority

Differential Global Positioning System

Service information and status

The Differential Global Positioning System accurately keeps ships on course.

What is DGPS?

Differential Global Positioning System (DGPS) provides differential corrections to a Global Positioning System (GPS) receiver in order to improve accuracy.
DGPS corrections originate from a reference station at a known location.

The receivers in these reference stations can estimate errors in the GPS because they have an accurate knowledge of their own position.

As a result, the horizontal accuracy of the system can be improved from 100m (95 per cent of the time) to better than 10m.

More importantly, the reference stations can warn 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 hours can elapse.

The most common form of DGPS (used internationally for maritime navigation) operates in the MF Radiobeacon band (285-325 kHz).

This form of DGPS uses pseudorange corrections and range-rate corrections from a single reference station which has enough channels (typically 12) to track all satellites in view.

Pseudoranges (distance measurements) are simultaneously measured to all satellites in view, and using the known position of the receiver's antenna, errors are calculated in the pseudoranges.

These errors are then converted to corrections and broadcast to users.
DGPS overview

The users GPS receiver applies the corrections, and 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 are achievable.

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

As the distance between the user and the station increases, the signal from the satellite is forced to take different paths to each.

Due to atmospheric variations, the user’s receiver experiences a different delay compared to the reference station.

The greater the separation, the more difficult it becomes to correct errors.

As a result, the further the user is from the reference station, the more inaccurate the navigational data can become.

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)
  • the Australian Hydrographic Office produces charts, navigational publications and digital products in WGS84.

AMSA initially determined antenna positions using the now superseded AGD datum, and transformed them to WGS84.

This has produced uncertainty of up to several metres in some stations due to the transformation process, but is being rectified.

This table shows 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).

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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