Sect_12.1.5

12.1.5 Constraining GPS Networks

THE IGS and the NEW GEODETIC DATUM FOR AUSTRALIA

The International GPS Service (IGS) and the International Terrestrial Reference System (ITRS) have provided the impetus for a renovation and redefinition of many geodetic datums.

Let us look at the example of the Australian Geodetic Datum (AGD). By datum "renovation" is meant the strengthening of the geodetic network in Australia through the establishment of a superior GPS-only network at an approximate 100-500km spacing, with the highest internal accuracy (far higher than that possible through the use of commercial GPS surveying techniques). Datum redefinition is achieved by tying the GPS-only network to several "fixed" stations of the IGS/ITRs network, and hence effectively making (a) the datum geocentric, and (b) for all intents and purposes coincident with the GPS satellite datum WGS84.

The new datum is known as the Geocentric Datum of Australia (GDA). The New Zealand datum also has undergone a redefinition to make it compatible with the GDA. It's correctt designation is GDA94. The datum is compatible with ITRF92 at epoch 1994.0.

To understand how the AGD was transformed into the GDA it is important to recognise the roles of the various GPS networks (MANNING & HARVEY, 1994):

There are three stations in Australia that are a subset of the IGS core tracking network (Figure 4 §6.2). These are Yaragadee, Tidbinbilla and Hobart (Figure 1 below), and their coordinates can be considered "fixed" in the ITRS. They therefore provide the fundamental link to the global geocentric datum.
The Australian Regional GPS Network (ARGN) consists of up 15 permanently operating GPS receivers, 8 located on the continent (Figure 1 below) with additional sites on Cocos Island, Macquarie Island, in New Zealand, and in Antarctica. They are operated by the Australian Survey and Land Information Group (AUSLIG). These will allow continuous connection to the ITRS to be maintained, as well as support Australian initiatives such as a GPS Integrity Monitoring Network (for GPS navigation users), special GPS campaigns for crustal motion studies and for the monitoring of the stability (in height!) of the high precision tide gauge network around Australia, as well as for any GPS surveys by state or federal geodetic authorities.
The Australian Fiducial Network (AFN) is a subset of the ARGN, consisting of the 8 continental stations.
The Australian National Network (ANN) is a network of GPS points surveyed by AUSLIG, together with state geodetic authorities, in order to provide an approximately 500km density of "zero order" stations across Australia (Figure 2 below). These points were surveyed during several observation campaigns in 1992, 1993 and 1994. The data was reduced using the GAMIT scientific software in a rigorous multi-baseline / multi-session phase adjustment. To achieve the 0.1ppm relative accuracies the GPS orbits had to be adjusted as well as local tropospheric parameters, and only the IGS core stations were constrained to their apriori ITRF coordinates.
The ANN forms the "backbone" for each state's geodetic network. Each state is responsible for holding the ANN sites fixed and readjusting current (and future) geodetic observations in their state in order to create t heir local realisation of the GDA. However, in 1997 a combined national adjustment of all GPS baselines observed over a period of more than 10 years, as well as the terrestrial observations were adjusted in a single procedure to derive the current GDA94 coordinate set.

The Geocentric Datum of Australia (GDA94) is a bold step in the evolution of the geodetic datum from one that is locally defined to one truly global in context, and from one adequate for survey technologies at the 10ppm to one able to support GPS surveys at accuracies better than 1ppm. The GDA is both a timely response to the challenge of accommodating GPS surveying technology, and is absolutely reliant on it for it's definition and high internal accuracy.

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Figure 1. The Australian Regional GPS Network.

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Figure 2. The Australian National Network.

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