GPS Technology Tutorial & Basics

- summary, overview or tutorial about GPS technology basics detailing its operation, what it is and how it works.

GPS, or Global Positioning System, is also sometimes called NavStar. GPS is a satellite based global navigation satellite system, GNSS that is used to provide accurate location and time information anywhere on or near the Earth.

GPS is run and maintained by the US government, although access to it has been opened up so that it is freely available worldwide when used with suitable GPS receivers.

Typically GPS is able to provide position information to within a few metres, allowing accurate positioning to be made. It is also possible to extract timing information that enables frequencies and time to be very accurately maintained. Frequency stability performance figures of systems using GPS timing are far in better than crystal or many other accurate frequency sources.

The performance and ease of use of GPS has meant that it is now an integral part of everyday life, with many portable or car-based "satnav" systems being used, as well as many mobile phones incorporating them to enable them to provide location information superimposed on the maps from the phone or satnav.


GPS basics

The basic concept behind GPS is that signals are transmitted from the satellites in space and these are received by the receivers on or near to the surface of the earth. Using timing it is possible to determine the distance from each satellite and thereby using a process of triangulation and a knowledge of the satellite positions the position on Earth can be determined.

The satellites all send timing information so the receiver knows when the message was sent. As radio signals travel at the speed of light they take a very short but finite time to travel the distance from the satellite to the receiver. The satellites also transmit information about their positions. In this way the receiver is able to calculate the distance from the satellite to the receiver. To obtain a full fix of latitude, longitude and altitude, four or more satellites are required, and when the receiver is in the clear, more than four satellites are in view all the time. A fix of just latitude and longitude can be obtained from three satellites.

GPS satellite orbits
GPS satellite orbits

The fully operational GPS satellite system consists of a constellation of 24 operational satellites with a few more in orbit as spares in case of the failure of one. The GPS satellites are in one of six orbits. These are in planes that are inclined at approximately 55° to the equatorial plane and there are four satellites in each orbit. This arrangement provides the earth user with a view of between five and eight satellites at any time from any point on the Earth.

Using economic ground based receivers GPS is able to provide position information to within a number of metres. The economic costs have also meant that it is now fitted to many motor vehicles, while separate GPS receivers can be bought for a few hundred pounds or dollars. As a result it is widely used by private individuals, as well as many commercial and professional users. In fact the primary use for GPS is as a military navigation system. The fact that it is used so widely is a by-product of its success.


GPS satellites

The satellites are orbiting above the Earth. Their orbits are tightly controlled because errors in their orbit will translate to errors in the final positions. The time signals are also tightly controlled. The satellites contain an atomic clock so that the time signals they transmit are very accurate. Even so these clocks will drift slightly and to overcome this, signals from Earth stations are used to correct this.

The GPS satellites themselves have a design life of ten years, but to ensure that there are no holes in service in the case of unexpected failures, spares are held in orbit and these can be brought into service at short notice.

The satellites are provide their own power through their solar panels. These extend to about 17 feet, and provide the 700 watts needed to power the satellite and its batteries when it is in sunlight. Naturally the satellite needs to remain operation when it is on the dark side of the Earth when the solar panels do not provide any power. This means that when in sunlight the solar panels need to provide additional power to charge batteries, beyond just powering the basic satellite circuitry.


GPS receivers

A large number of GPS receivers are available today. They make widespread use of digital signalling processing techniques. The transmissions from the satellites use spread spectrum technology, and the signal processors correlate the signals received to recover the data. As the signals are very weak it takes some time after the receiver is turned on to gain the first fix. This Time To First Fix (TTFF) is of importance, and in early receivers it could be as long as twelve minutes, although modern receivers use many more correlators are able to shorten this considerably.

When using a GPS receiver the receiver must be in the open. Buildings, or any structure will mask the signals and it may mean that few satellites can be seen. Thus the receivers will not operate inside buildings, and urban areas may often cause problems.


GPS Applications

The primary use for GPS is as a military navigational aid. Run by the American Department of Defense its primary role is to provide American forces with an accurate means of navigation anywhere on the globe. However its use has been opened up so that commercial and private users have access to the signals and can use the system. Accordingly it is very widely used for many commercial applications from aircraft navigation, ship navigation to surveying, and anywhere where location information is required. For private users very cost effective receivers are available these days and may be used for applications including sailing. Even many motor vehicles have them fitted now to provide SatNav systems enabling them to navigate easily without the need for additional maps.

It can be said that GPS has revolutionised global navigation since it became available. Prior to this navigation systems were comparatively localised, and did not offer anything like the same degrees of accuracy, flexibility and coverage.


GPS summary

There are a number of salient features for GPS. These are tabulated below:


Parameter Details
Number of active satellites 24
Number of satellite orbits 6
Number of satellites in each orbit 4
Orbit altitude Approx 20 200 km
Orbit relative angles 55° to each other
Satellite velocity 2.6 km/s
Main satellite transmission frequencies 1.57542 GHz (L1 signal)
1.2276 GHz (L2 signal)
Signal format CDMA
Max transmitter power 50 watts
Satellite diameter Approx 5 metres with solar cells extended
Satellite weight Approx 1 tonne
Anticipated lifetime of individual satellite Approx 10 years

By Ian Poole


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