Issue 23
"Differential GPS in Australia "

Jon Fairall

For high accuracy use of the GPS, differential services are mandatory. There are now a significant number of services available in the region.

The GPS is, without doubt, the most cost-effective method of providing positioning information in existence, whether for locating assets for entry into GIS databases, navigation, cadastral surveys or any one of a dozen other applications.

But it is not a magic positioning device. Typically, a single, unaided GPS receiver will give position within 100 metres. That really is magic if you are lost at sea on a stormy night, but for a surveyor, trying to establish the boundaries of an urban housing block, it is next to useless.

To fix this problem, a number of differential GPS (d-GPS) services have been established. Broadly speaking, there are two types: real-time and post-processing.

The advantage of post-processing is that it is generally the most reliable and cheapest method of doing the job. However, it requires that the operator have two receivers and be able to log data from the satellites at both of them. This data must be stored, taken back to the office, and processed, which sometimes takes a considerable period.

The only way around this is to use a radio link which will communicate the error signal directly from the base station to the mobile. This gives instantaneous access to d-GPS positioning, and is clearly mandatory in navigation applications, and desirable in many others.

The principal disadvantage of a radio link is its generic unreliability. There are unpredictable range limitations, line-of-sight limitations and atmospheric effects. As a result, a radio link can never be guaranteed until it is tried, which adds considerably to project costs.

The solution is to use one of a variety of permanent differential services on offer across the nation. One type uses conventional radio communications. The most prolific service of this type is supplied by the Australian Maritime Safety Authority. It is establishing a string of stations around the country for mariners. The first of these covered Bass Strait and the oil fields of north west Australia. Now, Torres Strait and the Great Barrier Reef are also covered, with plans in hand to provide the service in all major ports.The GPS receiver manufacturer Sagem runs a base-station at its Sydney Rosebery office that is available on request.

The Victorian Department of Natural Resources has set-up base stations at Bendigo, Ballarat and Melbourne in a system called GPSNet. Currently, it is intended primarily for post-processing. However, it is investigating establishing sites on the top of the hose-drying towers of the Country Fire Authority as bare-base stations. The sites will be accurately surveyed, then supplied with all the requirements for establishing a base station whenever required.

An alternative communications channel is to use a sub-carrier on regular FM radio transmissions. The best example of this kind of coverage is the AusNav system. AusNav is an alliance between AusLIG, who developed and operate the service, Differential Corrections Incorporated (DCI) in the US, which developed the technology, and the Australian Broadcasting Corporation, which provides the broadcasting medium.

Pseudo-range corrections are broadcast as a sub-carrier on the ABC's TripleJ FM radio network. The RTCM SC-104 Version 2 differential data is encoded and transmitted using the Radio Data System (RDS) protocol developed by the European Broadcasting Union.

AusNav uses hardware provided by DCI in the US. The RDS 1000 is a portable paging receiver designed for hand-held applications. The RDS 3000 is a receiver designed for vehicular applications.

Another method of broadcasting the GPS differential corrections is by using a geo-stationary communications satellite. Unlike terrestrial radio transmissions, this system is independent of distance and is available over the whole Australian continent, using a small omni-directional antenna via systems available from Fugro's Starfix or Racal's Landstar system.

The Landstar d-GPS system can be subdivided into two distinct segments. They are the control segment, operated by Racal Survey, and the user segment. The control segment consists of the eight reference stations around the coastline and one at Alice Springs.

The Network Control Centre in Perth is the central hub which co-ordinates all the data from the reference stations, does data validation and integrity checking and computes the differential corrections to the pseudo-ranges of all satellites in view at each reference station.

At each reference station there is 100 per cent redundancy of all components in case of a failure. This means there is two geodetic quality GPS receivers with two antennae, backup data storage, unbreakable power supplies and dedicated telephone lines. The reference stations are monitored 24 hours a day at the Network Control Centre for quality assurance and data integrity.

From the Network Control Centre the GPS differential corrections are uploaded from the Optus centre at Lockridge, WA, to the Optus satellite, and then broadcast, in the RTCM SC-104 format, Australia-wide, to all landstar equipped users.

The user segment consists of an unlimited number of people equipped with landstar receivers, antennas with which to receive the differential corrections from the satellite and d-GPS-capable receivers. The user simply selects the nearest reference station and receives a set of differential corrections for that selected reference station only.

For the d-GPS assumption to hold true -- that the GPS errors are common between the known reference station and the unknown mobile -- the separation distance between the reference station and the mobile must be kept to a minimum. The location of the Landstar reference stations has been chosen so that no user anywhere in Australia will be further than 1000 km from a reference station. Landstar's specification says that on this basis, positioning accuracies are always better than 5 metres and are typically 1--3 metres, anywhere in Australia.

All these systems operate because the GPS industry has developed an industry standard format for the broadcast of differential corrections that all GPS manufacturers appear to have adopted. This is the RTCM SC-104 (Radio Technical Commission for Maritime Services Special Committee 104) standard.

The implication of this standardisation is that any GPS receiver capable of accepting differential corrections can operate independent of the medium by which the error signal is transferred, as well as independently of the type of receiver at the reference station.

With all these services operating, there would seem to be about fifty reference sites on the Australian continent. That will not be too many, as more and more users tune in to the advantages of really accurate GPS positioning.

 How it Works

D-GPS requires that two GPS receivers acquire the GPS signal simultaneously. One receiver must be on a point that has previously been surveyed, and the other receiver at an unknown location. At the known point, the instantaneous GPS total error can be found by comparing the observed GPS position with the known position.

The underlying assumption here is that the major error sources in GPS are external to the GPS receiver, are systematic and highly correlated between separated sites.

Table 1 gives a typical error budget of the various GPS error sources. These are typical values and serve to illustrate the general nature of the errors. The receiver clock error, multi-path and the reference station co-ordinates are specific to each site and are therefore not correlated and so do not cancel out in the differential process.

Table 1. typical GPS and d-GPS Error Budget. The User Equivalent Range Error is the square root of the sum of the individual errors squared and shows the total GPS error budget.

 Current AusNav Coverage

Area	State	Radio Station	Freq. (MHz)
Adelaide	SA	Triple J	105.5
Brisbane	QLD	Triple J	107.7
Canberra	ACT	Triple J	101.5
Darwin	NT	Triple J	103.3
Dubbo	NSW	Triple J	102.3
Geraldton	WA	Triple J	98.9
Gold Coast	QLD	Triple J	97.7
Griffith	NSW	Radio National	98.9
Hobart	TAS	Triple J	92.9
Launceston	TAS	Triple J	94.1
Melbourne	VIC	Triple J	107.5
Moree	NSW	NOW FM	98.3
Newcastle	NSW	Triple J	102.1
Orange	NSW	Triple J	101.9
Sunshine Coast	QLD	Triple J	89.9
Sydney	NSW	Triple J	105.7
Wollongong	NSW	Triple J	98.9
Perth	WA	Triple J	99.3
Toowoomba	QLD	Triple J	104.1

This is a list of each ABC station that supplies the AusNav correction signal. In most cases this is the TripleJ youth network, although in some sports, where the network is not available, other stations are used. In general, coverage is available wherever the FM signal from the transmitter can be received. More detailed coverage maps of each site are available at http://www.auslig.gov.au