How does the GPS system work?

How does the little handheld box know where we are?

How does it know the right time?

Well, the system works by using several low earth orbitting satellites. Each satellite transmits a time signal to receivers back on earth. Because they orbit the earth the receivers, the little handheld boxes, can be located almost anywhere. If the receiver can see a minimum of three GPS satellites then a processor inside the receiver box can measure the differences in the three received time signals. Using triangulation between the three satellites, the position on earth can be pin pointed to within a few metres.

Triangulation works like this:

Each time signal from the passing satellites is compared with the next, the difference in time between satellites is then used to determine your distance from each satellite. If we know the distance from each satellite we can calculate our position on the earth.

This is all done very fast and in realtime by a small computer in the handheld box.

Larger GPS systems exist, that are 19" Rack mount units or PC based systems, but they all do the same thing.

Some systems can plot your location on a moving map others simply tell you the time.

The time is very accurate because the systems use the distances from each satellite, that were determined to find our position, to calculate the time taken to send the signals to us. If we know that then the true time can be derived from the received time.

The more satellites we receive signals from the more accurate all of these calculations are.

For more information about GPS Satellite Systems goto the SSHP Website.

The New European GPS System

The existing GPS system is American owned, now we in Europe are getting our own. This excellent explanation of why comes from the European Space Agency.

Why Europe needs Galileo

Satellite navigation users in Europe today have no alternative other than to take their positions from US GPS or Russian GLONASS satellites. Yet the military operators of both systems give no guarantee to maintain an uninterrupted service. Satellite positioning has already become the standard way of navigating on the high seas: in the near future, its use will spread to land and air. If the signals were switched off tomorrow, many ships’ crews would be hard pressed to revert to traditional navigation methods using almanacs and sextants. In a few years' time, when the use of satellite positioning has spread, the implications of a signal failure will be even greater, jeopardising not only the efficient running of transport systems, but also human safety.
As far back as the early 1990s, the European Union saw the need for Europe to have its own global satellite navigation system. The conclusion to build one was taken in similar spirit to decisions in the 1970s to embark on other well-known European endeavours, such as the Ariane launcher and the Airbus. The European Commission and European Space Agency joined forces to build Galileo, an independent system under civilian control which will be guaranteed to operate at all times, bar the direst emergency. Galileo is now at the point of moving from definition to full-scale development.

European independence is the chief reason for taking this major step. However, other subsidiary reasons include:

By being inter-operable with GPS and GLONASS, Galileo will be a cornerstone of the Global Navigation Satellite System (GNSS). This system will be under civilian control and will allow positions to be determined accurately for most places on Earth, even in high rise cities where buildings obscure signals from satellites low on the horizon. This is because the number of satellites available from which to take a position more than double.

By placing satellites in orbits at a greater inclination to the equatorial plane than GPS, Galileo will achieve better coverage at high latitudes. This will make it particularly suitable for operation over northern Europe, an area not well covered by GPS.

With Galileo, Europe will be able to exploit the opportunities provided by satellite navigation to the full extent. GNSS receiver and equipment manufacturers, application providers and service operators will benefit from novel business opportunities. Under request from the European Commission, PricewaterhouseCoopers (PwC) prepared a study on the business plan for the Galileo programme.

Source: © ESA 2002