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).
How accurate are GPS?
- Normal GPS can be up to about 20 m 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 picking up Differential GPS (DGPS) — a network of stationary GPS receivers with known locations that constantly compare their 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.
- No satellite signal, no guidance. Car navigation doesn’t work when you’re in an underground car park or tunnel. And even high-rise buildings in a city centre can block the view of the sky to an extent that leaves navigation systems lost or at least handicapped.
- The systems can only be as good as the mapping data allows. When we let the units search for destinations or plot routes in other states, our randomly selected addresses and points of interest showed weaknesses in the mapping data and points of interest, especially in rural areas.
- Don’t blindly trust your navigation system. There were a few occasions during our test when the voice instructions were wrong but the displayed information correct. And there may be traffic situations that are simply too tricky to handle for a navigation system — for example, if some quick lane-change manoeuvres are required.
- Some models have a ‘walking’ option that lets you plot a pedestrian route ignoring one-way streets and using, for example, walkways through parks. However, none of them is suitable for guiding you on bushwalks.