Published by Australian Academy of Science
GPS and never having to say: 'Where am I?' Box 2 | Improving GPS

GPS provides a remarkable capability for all-weather navigation. However, as with many technologies, GPS has its weaknesses.

Signal interference or jamming

One limitation of GPS resides in the satellite signal. The GPS signal broadcast by the satellites is extremely weak by the time it reaches a GPS receiver on the Earth – far below the level of many radio transmissions. Consequently, GPS signals are vulnerable to interference be it accidental, (eg, swamped by electromagnetic interference around communication towers) or deliberate (it is possible to purchase inexpensive pocket-size jammers to throw out GPS receivers).

So concerned was the US military that their sophisticated GPS system could be derailed with simple hand-held jammers that it invested considerable resources in developing a GPS Jammer Locator. To test it they worked with Australia's Defence Science and Technology Organisation (DSTO) on land around Woomera in South Australia. (Australia is a valuable testing ground for GPS because we possess vast tracts of land empty of civilian infrastructure where there is little chance of outside GPS interference.) The tests involved the US Air Force using their Jammer Locator to pinpoint a range of GPS interferers (jammers) set up by DSTO. Lessons learnt through the joint trials mean that, in future, GPS users will be able to identify, and then avoid or deal with, GPS interferers.

Limited accuracy

And then there's the problem of the limited accuracy of the civilian GPS signal. While knowing your position to within 20 metres anywhere on the planet is a major technological feat, it's still not good enough to land a jumbo jet in the fog (400 tonnes of machine carrying hundreds of people at 250 kilometres per hour). To achieve the necessary level of precision for tasks like this you need to augment the GPS signal by making use of additional signals from ground stations. This increases the accuracy of the GPS position down to less than 2 metres and warns if the GPS signal becomes inaccurate or unsafe.

Airservices Australia is one of the world leaders in the development and flight testing of augmentation systems for GPS. Australia has established an operational 'augmentation test bed' of six satellite monitoring stations and a master control station. The GPS navigation signal is collected at the monitoring stations and processed at the master control station. A data signal is then re-transmitted around the country to provide aircraft with error correction and a warning should something go amiss in the GPS satellite signal. The Australian-designed augmentation system has been widely accepted as a viable, cost effective solution to improving the accuracy of the GPS signal.

US military could turn off GPS

Another major concern with GPS is its owner, or more specifically the fact that it has only one owner: the US military. While there's no reason to believe the US will limit the availability of the GPS in the future, we live in an uncertain world. The outbreak of war or some unforeseen event that changes international relations could see the US turn off GPS or alter its accuracy.

With so many industries increasingly being designed around GPS technology, the prospect of losing access to the GPS signal would be catastrophic. Consequently, a raft of global positioning satellites is being sent into orbit by a number of nations. Russia has sent up a constellation of satellites in a system similar to GPS. It is known as GLONASS. The European Union is planning to establish its own network, called Galileo. It will consist of 30 satellites and should begin operation in 2008. China is also developing plans for its own network.

Signals in the GPS format are now available from other communication satellites and this will effectively provide more satellites for the GPS network. The net effect is that GPS, or whatever name the parallel system goes by, is here to stay and it has you covered.

Related sites

• Australia-US defence trials point to more reliable GPS navigation (Defence Science and Technology Organisation, Australia)

• A Ground-based Regional Augmentation System (GRAS) – the Australian proposal (Airservices Australia)

Box 1. A short history of GPS