GPS & Sat-Nav Blog

How Does GPS Work?

I think everyone has a vague idea of Sat Nav working by picking up signals from satellites "Out There" somewhere. However, I got asked the question, "Why does a Sat Nav need to see 4 satellites to get a fix?". So I desided to find out.

The first surprise was just how many satellites are involved.

Currently there are around 30 satellites involved in providing the signals for the Global Positioning System used by all current Sat Navs.

Unlike the satellites used for Sky TV, which are in geo-stationary orbit above the equator so that they appear in a fix position in the sky, the GPS satellites are arranged in a number of lower orbits tilted relative to the equator, and each satellite orbits the earth twice per day.

The satellites orbit at a height of 12900 miles above the surface of the earth and the orbits are arranged so that at least 6 satellites are visible at any time from almost everywhere on the surface of the earth. Normally in the UK around 10 satellites are visible at any time.*

Each satellite broadcasts precise data about its own orbit, general information about the orbits of all the other satellites and a very, very accurate time signal derived from the atomic clocks on board each satellite.

Back on the ground, the Sat Nav on the windscreen picks up the signals from the orbiting satellites. These signals are actually quite weak so the navigator needs quite a good view of the sky, which is why they don't tend to work well indoors or in tunnels.

Once the Sat Nav reciever has information from a number of satellites the receiver calculates the time taken for the signals to arrive from a satellite and since radio signals travel at the speed of light**, the navigator can then work out the distance to each satellite. With the position of each satellite known from the data it sends and the distance known to 3 or more satellites, the navigator can work out its exact position in terms of latitude, longitude and height above sea level.

However, calculating the time taken for the satellite signals to reach the receiver assumes that the receiver also knows the time very accurately.

It would be far to expensive to include such an accurate clock in any navigator, so the system uses data from 4 satellites to work out its exact position in terms of Lat, Long, Height and Time.

So the time displayed on a GPS receiver is almost certainly the most accurate clock around.

Thats the basics of the system. Follow this link for a much more detailed description of the GPS system.

Rob

*The orbits are arranged as six orbital planes inclined at approximately 55 degrees to the equator and with 60 degrees right ascension to each other, that is how far around the equator each plane is spaced. Each orbital plane contains 4 satellites so there is a total of 24 active satellites in the basic system with some others also active and some spares in case of satellite failure

**Actually the speed of light changes a bit between the vacuum of space and when traveling through the atmosphere so the satellites also transmit some correction data to help the receiver make a more accurate position calculation