GPS and never having to say: 'Where am I?'
This topic is sponsored by the Australian Government's National Innovation Awareness Strategy.
The Global Positioning System (GPS) has been operating successfully for over 10 years, allowing anyone with a receiver to work out exactly where they are.
You're driving home from a party, it's late and you're starving. What's open and where is it? Whip out your mobile phone, tap in 'pizza' and '10 kilometres' and instantly you're given the location of 4 pizza parlours that are still open and less than 10 kilometres away. Then it's just a matter of asking the car which is the best way to get to one of them. The car computer responds by telling you that while the first pizza place is very close, there's been a car accident on the road that would get you there, therefore it's quickest to head for pizza place 2.
Believe it our not, the technology that would make this situation possible is not from the distant future, it's here with us now and is being used in many places around the world.
Where am I?
At the centre of these tasks is the problem of location: where are you and where do you want to be? It might be as trivial as wanting to know where to get a pizza when it's late, or as important as letting rescue services know where you are when you're caught out bushwalking in an unexpected snow storm.
Historically, the problem of location has been solved using local references. Be it your memory, a road directory or a topographical map, it's up to you to figure out where you are by looking for reference points and then navigating to where it is you want to be. Of course if you're in a strange area or local landmarks are difficult to see, the problems of location can be enormous.
But now, the problem of 'where am I?' has been effectively solved with the Global Positioning System or GPS. To plug into the GPS system you need a GPS receiver. In Australia, small hand-held units the size of mobile phones can be purchased from most electronics shops for around $400. What's more, as the GPS industry grows, GPS receivers will become increasingly more affordable.
The Global Positioning System consists of a number of car-sized satellites circling at about 24,000 kilometres above the Earth. There are a nominal 24 satellites, but there are additional operational satellites placed in orbit to replace satellites as they fail. Thus there are usually quite a few more than 24 satellites circling. Each satellite contains an atomic clock which is synchronised with a central clock located at the GPS control centre in Colorado, USA. Every satellite transmits a continuous radio signal, giving its position and the time the signal was sent. When your receiver on the ground picks up these signals it can work out your precise location (latitude, longitude and altitude) based on the time it took for the signals from four different satellites to reach your receiver.
Think of it like this: pretend you're out on a featureless plain with no point of reference. Around you, in the distance, are people at known reference points. They shout to you the exact time (to the second). You hear them and note down the time they're shouting. Sound travels at 331 metres a second. If there is a time difference of one second between the real time (the time you're keeping) and the time that was shouted you know the person is 331 metres away. If you note down the time from several people you can calculate the distance from each of them, and thereby work out your position (relative to the shouting people).
GPS is basically the same system except instead of shouting people there are satellites in orbit around the planet and the signal is radio waves instead of sound. Of course, radio waves travel faster than sound. Like all forms of electromagnetic radiation, the radio waves are travelling at the speed of light which is 299,792,458 metres per second (around 300,000 kilometres per second).
Now, if you do the calculations (the satellites are 24,000 kilometres away, the signals travel at 300,000 kilometres per second), to pinpoint a location on Earth down to a few metres, you'd need to use very accurate clocks in the satellites. These clocks would have to accurately measure not just thousandths of a second but right down to billionths of a second (nanoseconds) and keep exactly in time with all the other clocks on other satellites and back down at the control centre. And that's exactly how accurate the atomic clocks carried in the orbiting satellites are.
But it would be impracticable to have such accurate clocks in the GPS receivers, so the problem is overcome by the receiver using signals from four satellites when it is determining its position. This process synchronises the receiver's internal clock with the satellites' atomic clocks. Thus, precise time is available from each receiver. Industries that require multiple events to be synchronised, or to occur at exactly the right time (eg, electrical power distribution systems), are increasingly using GPS receivers.
The development of GPS
The US military initially developed GPS to help guide their submarines and missiles, and the navigation satellites are now operated by the US Air Force. To safeguard US national security, the satellites transmit two sets of signals one for military use and one for civilian use (Box 1: A short history of GPS). The commercial market for GPS services has expanded rapidly, with an increasing array of affordable receivers now available for individual use.
It didn't take long after the establishment of GPS for the commercial applications to follow. Shipping companies equip their tankers and freighters with GPS for navigation and to record and control the movement of their vessels. Trucking and transportation services use GPS to keep track of their fleets and to speed deliveries. Airlines have saved millions of dollars by using GPS to hone their flight plans.
It is used to keep track of government buses and trains in Genoa and Helsinki, and Britain is examining the possibility of equipping vehicles with GPS receivers to determine when a vehicle is on a toll road. A dashboard unit would calculate fees and send the information to the toll road operator.
GPS is being used in four major Australian cities to manage and track taxis. In some of these taxis, the GPS receivers are linked to an emergency response system which allows the driver to contact police with a precise location.
Engineers use GPS for surveying roads and planning buildings and bridges, and it can be used to map natural resources such as soil and vegetation. For example, farmers can use GPS, along with other technologies, to pinpoint low-yielding areas within a paddock and apply corrective treatments.
In 1995, Australia played a leading role in the milestone event of the first certification of a new GPS navigation system for large aircraft. This work was done with a Qantas Boeing 747-400 aircraft, together with special equipment from Airservices Australia, and led the way for widespread international use of GPS for many aircraft. So, every time you fly across the Pacific, your aircraft is being guided by GPS.
GPS receivers for aviation have inbuilt circuitry to detect if there is a faulty signal from satellites. Indeed, the receiver can even decide which satellite is faulty, exclude it from determining position, and thus allow the flight to proceed using the remaining 'healthy' satellites.
The future of GPS
It's easy to see that GPS is quickly becoming indispensable to our everyday lives. Consequently, the technology needs to be robust and ultra reliable. In many ways it is, however GPS does come with some weaknesses. For example, like all radio navigation systems, the signals coming from the satellites are vulnerable to interference. Research into these areas of weakness is leading to improved performance of GPS receivers (Box 2: Improving GPS).
Once the use of GPS in cars and mobile phones becomes widespread, 'location services' will be able to offer roadside assistance, traffic updates, route planning and shopping guides. How to protect the information gathered on your habits and whereabouts is a major privacy issue that will have to be addressed.
So the next time you here a discussion on GPS, don't just pass it off as an interesting bit of technological trivia about satellites many thousands of kilometres away. It's actually a technology that will affect us all.
Page updated July 2006.