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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 AMSA initially determined these antenna positions using the now superseded AGD datum, and transformed them to WGS84.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 - www.ga.gov.au) 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 [ PDF] 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 absolute accuracy of one to several meters (depending on the order of the survey mark used).

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. To get the AMSA DGPS reference station antenna positions (WGS84) click on the DGPS broadcast station name.

DGPS Broadcast Station Name	WGS-84 Absolute Accuracy	Survey Marks / Transformation Used
Albany, WA	0.1 metre	ITRF97
Brisbane, Qld	0.1 metre	Several 1^st order AGD84 (Higgins parameters) AGD positions GDA-adjusted
Cape Flattery, Qld	0.6 metre	Zero order GDA94 - B065 Piebald & high precision GPS solution
Corny Point, SA	0.1 metre	ITRF97
Crib Point	0.1 metre	ITRF2000
Darwin, NT	0.1 metre	ITRF2000
Exmouth, WA	0.1 metre	ITRF2000
Gladstone, Qld	Several metres	4^th order AGD84 - 117329 (unknown transformation)
Horn Island, Qld	Several metres	1^st order AGD84 - B086, BC4/023 & 102635 (Higgins parameters)
Ingham, Qld	Several metres	1^st order AGD84 - QGS2105, QGS2106 & PSM71735 (Higgins parameters)
Karratha, WA	0.1 metre	ITRF97
Mackay, Qld	Several metres	1^st order AGD84 - 115644 & 115645 (Higgins parameters)
Mallacoota, Vic	0.1 metre	ITRF97
Perth, WA	0.1 metre	ITRF97
Sydney, NSW	0.1 metre	Zero order GDA94
Weipa, Qld	0.1 metre	ITRF97

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last updated: August 2007