A GPS is a device that receives signals from satellites
Three main components included in a GPS system is the receiver which reads the signals from the satellite and in turn calculates the position, the receiver does not transmit it only receives and GPS maps also have a space segment between a certain amount of satellites orbiting the planet. These satellites belong to the military and are highly accurate. The GPS systems are monitored by a control segment situated in the USA. When traveling in the bush in Australia your GPS needs to have a lock on at least four satellites in order to determine your position. You will get an inaccurate reading if there are less than four satellites. So when traveling in the Australian bush, do not rely on your GPS only, you need to have trail maps as well as a compass on hand, in case there is not sufficient satellite signal. If you intend walking or using your 4x4 in valleys, gorges, cliffs, thick tree covering, cold snowfall conditions or rainy conditions you may experience problems with your GPS readings. The best GPS systems for bush walking are those that have a high sensitivity system, as these can be used in all types of terrain. The GPS should also be water resistant, as there is a chance that you will have to cross rivers and other such terrain especially in the outback and in the Alps of Australia. Bear in mind your iPhone will never survive such an ordeal.
A GPS should not be used as a replacement for your compass and maps when you are bush walking, but it is a great additional tool to have as it can help to reduce any mistakes and can speed up decision making. Compass and ground map skills are still the most important tools when routing. When using a GPS in the bush make sure that the GPS map data matches the trail maps you are using otherwise you will encounter difficulties as well as lose you way if are relying on both methods.