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Box 2. Improving GPS
Further reading
Useful sites
Glossary
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.
GPS basics
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.
GPS applications
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.
| Related site:
How it works: Supergoose in space Explains how GPS is used to track the migration of geese. (Wildfowl and Wetlands Trust, UK) |
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.
The space age was born in October 1957, when the Soviet Union launched Sputnik – the world's first satellite. It took just one day for US researchers to work out the precise orbit of the satellite by observing how the frequency of its radio signal appeared to increase as it approached their receiver and decrease as it departed – an effect known as the Döppler shift. (The Döppler shift is also used by police radar to estimate the speed of approaching vehicles.) The researchers reasoned that they could also reverse this procedure. If they knew the exact position and orbit of a satellite, they could use its radio signals to precisely locate the position of a receiver on the ground.
The US military were particularly interested in developing satellite navigation systems to help guide their submarines and missiles. In 1965, the US Navy established the Transit system consisting of six satellites circling the Earth continuously in polar orbits. Submarines would receive radio signals from the satellites and analyse them to work out their Döppler shift. From this, submariners could determine the distance to the satellites and they could then determine their own location. The process took 10-15 minutes.
GPS was a natural next step. In 1973, the US Department of Defense proposed GPS as an instantaneous satellite positioning system that would be operational at all times from anywhere on the globe. The essential components of GPS are the Navstar satellites that orbit the Earth every 12 hours in a formation that ensures that every point on the planet will always be in radio contact with at least four satellites. The first operational GPS satellite was launched in 1978, and the system reached full 24-satellite capability in 1993. Additional satellites are now in orbit as operating spares.
At first, the US military had exclusive use of GPS through a secret signal and an encrypted code. When it was obvious that GPS had huge commercial potential, the Americans provided a separate, civilian signal, freely available worldwide for non-military use. The civilian signal can be used by anyone with a receiver to pinpoint their location to an accuracy of 20 metres. The error can be reduced down to centimetres by special techniques in complex receivers. The military signal remains encrypted and offers accuracy down to a few metres.
Related site
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
Useful sites
GPS Primer (The Aerospace Corporation, USA)
A good introduction to GPS.
http://www.aero.org/education/primers/gps/
All about GPS (Trimble, USA)
Another good introduction. Explains, with animated diagrams, the principles behind GPS. Presents the information in a logical sequence of sections. Includes explanations of how GPS works and examples of real world applications.
http://www.trimble.com/gps/index.html
GPS: A new constellation (Smithsonian National Air and Space Museum, USA)
Based on an exhibition at the National Air and Space Museum, this site presents a basic introduction to GPS and its uses. Well-illustrated.
http://www.nasm.si.edu/exhibitions/gps/index.htm
From stones to satellites (Javad Navigation Systems, USA)
An interesting history of navigation, from the stone age where the points of reference may have been markings on trees, to today's use of satellites.
http://www.javad.com/jns/gpstutorial/Chapter1.html
Australian Broadcasting Corporation (transcripts)
Notes on basic GPS positioning and geodetic concepts (Satellite Navigation and Positioning Group, University of New South Wales, Australia)
These notes were prepared for a short course on GPS, with particular reference to land-based applications such as mapping and surveys. There are three chapters available as PDF files: Introduction to GPS, GPS enhancements, and Mapping issues.
http://www.gmat.unsw.edu.au/snap/gps/gps_notes.htm
Special issue on Global Positioning System (Proceedings of the IEEE, January 1999, Institute of Electrical and Electronics Engineers, Inc. USA)
Provides a technical look at GPS – its development, measurement of the Döppler shift of
signals, the accuracy of its location estimates, and examples of civil applications.
http://ieeexplore.ieee.org/iel4/5/15872/00736338.pdf?tp=&arnumber=736338&isnumber=15872&arSt=3&ared=15&arAuthor=Enge%2C+P.%3B+Misra%2C+P.%3B
How GPS phones work (Howstuffworks, USA)
Reviews how cell phones and GPS receivers work and how phones combine these technologies.
http://www.howstuffworks.com/gps-phone.htm
Global Positioning System: The role of atomic clocks (Beyond Discovery, National Academy of Sciences, USA)
Describes how basic research into the nature of time and ways to measure time accurately contributed to the
development of GPS. The complete article is available.
http://www.beyonddiscovery.org/content/view.article.asp?a=458
Global positioning system (GPS) technology and cars (CSA Illumina, USA)
Describes the uses of GPS technology in new cars.
http://www.csa.com/discoveryguides/gpscars/overview.php
atomic clock. An extremely accurate clock that is based on the vibrations of an atom or molecule. For more information see How atomic clocks work (How Stuff Works, USA), The most accurate clocks in the world (Miami University, USA) and How atomic clocks work (Science Museum, UK).
Döppler shift. The change in the perceived frequency of waves (electromagnetic or sound) when the source of the waves moves in relation to the receiver.
The phenomenon is named for Christian Döppler, who realised that the pitch of a sound emanating from a moving source varies for a stationary observer, depending on the speed of the source and the direction in which it is moving. Imagine you are on a train in a station and you can hear the signals ringing at a rail crossing just down the track. Since both you and the signals are stationary, the signals sound normal. They continue to ring at the same rate as the train starts to move, but now because you are travelling towards them they seem to get faster.
In effect, the time between arrival of pulses of sound is being compressed (or shortened) and the apparent frequency is increasing. The result is that the signals sound higher-pitched. This change in frequency is called a 'Döppler shift'.
For more information see Introduction to the Döppler effect for sound (Fear of Physics, USA) and How radar works (How Stuff Works, USA).
electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We can imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. The highest frequencies in the spectrum of electromagnetic radiation are gamma-rays; the lowest frequencies are radio waves.
For more information see Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA) and Measuring the electromagnetic spectrum (High-Energy Astrophysics Learning Center, Goddard Space Flight Center, USA).
frequency. A measure of how frequently an electromagnetic wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).
Galileo. A satellite radio navigation system initiated by the European Union and developed for non-military applications. The final system will be based on a collection of 30 satellites. For more information see What is Galileo? (European Space Agency) and Galileo: European satellite navigation system (Europa, European Union).
GLONASS. Global Navigation Satellite System, operated by the Russian Federation Ministry of Defence. When completed, it will have a constellation of 24 satellites, and is intended to service maritime and aviation users throughout the world. For more information see GLONASS – summary (Andrews Space and Technology, USA).
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