GPS stands for Global Positioning System. It was developed by the US military, but is available free of charge to the general public. It has many commercial uses, from land, sea and air navigation to land surveying and map-making.
GPS consists of 24 satellites that orbit Earth exactly twice a day at an altitude of about 20,200 km. The orbits are aligned so that at least four satellites are ‘visible’ at any time from most places on Earth.
The satellites carry highly accurate atomic clocks and constantly send coded time signals to Earth. A GPS receiver can read these signals and use the time delay between the send and receive times to calculate its distance from the satellite (assuming the signal travels at the speed of light).
A receiver uses a method called three-dimensional trilateration to then calculate its position on Earth:
- It calculates the distances to four of the satellites.
- Each calculated distance is the radius of a sphere around each satellite. The four spheres intersect at exactly one point — the receiver’s location.
- Theoretically, three satellite signals are enough: while three spheres intersect at two points, only one of them is likely to be on Earth.
- If your GPS receiver doesn’t get a signal from at least three satellites (for example, if you’re in a tunnel, or if the signals are blocked by high-rise buildings), it can’t calculate your position.
- Normal GPS can be up to about 20m out – the satellites' signal may be slowed down by the atmosphere, the signals may bounce off high-rise buildings or other structures, or the receiver itself may have its limitations (quality of the antenna, etc).
- More expensive GPS receivers can correct at least for atmospheric inaccuracies by using Differential GPS (DGPS) — a network of stationary GPS receivers that constantly compare their known position with the position calculated from satellite signals. They can then calculate a correction factor, which is sent to DGPS-capable receivers in the area. DGPS increases accuracy to a few metres.
If you want to read more about GPS, there's lots of information on the web - for example:
- Go to Wikipedia and search for ‘GPS’.
- VicRoads has a guide that explains the basics.
- If you need help with unfamiliar routes only occasionally, there are websites that let you plot a route with turn and distance instructions, and print it out for you to take it on your trip. Try Whereis or NRMA Travel.