How GPS works – and where it fails

The GPS satellites orbit at roughly 20,000 kilometres altitude. On board they carry atomic clocks that are extremely accurate. Each satellite continuously transmits where it is and what time its clock shows. The receiver compares the time stamp with the moment the signal arrives. Since radio waves travel at the speed of light – about 300,000 kilometres per second – the travel time gives the distance.

Knowing the distance to one satellite only tells you that you are somewhere on a huge sphere around it. With three satellites, three such spheres intersect and a single point is left. The fourth is needed because the clock in a phone is nowhere near as accurate as an atomic clock. The receiver simply calculates its own clock error as a fourth unknown.

A simple calculation shows how sensitive this is: an error of one millionth of a second corresponds to about 300 metres at the speed of light. That is why all the accuracy depends on precise clocks and clean, direct signals.

The signals come from far away and are very weak at ground level. Rock, concrete, metal, the human body and wet foliage weaken them further. In a narrow gorge or a street canyon between high-rises the device sees only a thin strip of sky and therefore few satellites, which are also badly positioned in a line. The position then becomes uncertain.

Reflections are even more treacherous. If a signal bounces off a rock face or building, its path is longer than the direct one. The device thinks the satellite is farther away than it is, and the position often jumps by many metres. A typical result is a track that zigzags through blocks of houses although you walked straight ahead.

The altitude reading is usually less accurate than the horizontal position, because all the satellites are above you and none “measures” from below. For heights, the map, contour lines and a calibrated altimeter are more reliable. Many devices display an accuracy circle – it is the device's estimate, not a guarantee.

For a good fix, go to an open spot, hold the device up and away from your body, and wait briefly until the display settles. After a long pause or in a new place the receiver sometimes takes longer for the first position. Phones fetch helper data over the mobile network and are then faster – but positioning itself also works without a network.

What is often missing without a network is the map. Many map apps load their maps continuously from the internet. So download the maps for your area before the trip. Otherwise you see a blue dot, but on an empty grey background.

Always check the display against the terrain: do the stream, the slope, the path junction fit? And note: the satellite position does not tell you which way you are facing. The arrow direction while standing still usually comes from a built-in magnetic sensor that can be disturbed by metal and electronics. A direction from GPS alone only results when you move.

A device can fail at any time: flat battery, cold, wet, a fall onto rock, a software error. So the rule is: carry a paper map and compass and practise using them while everything still works. People who only follow the blue dot often do not notice where they actually are, and are suddenly without orientation when it fails.

Save energy wherever you can: dim the screen, use airplane mode with location still on, carry the device close to your body in the cold, and have a power bank with you. Check the battery level regularly, not only when it is already red.

In some regions satellite signals are deliberately jammed or spoofed. The device may then show a completely wrong position without any warning. If the display does not fit the terrain, trust the terrain and the map.

“GPS” is actually only the name of the US system. The umbrella term is GNSS, global navigation satellite systems. These also include the European Galileo, the Russian GLONASS and the Chinese BeiDou. Many devices use several systems at once and so have more satellites in view – an advantage especially in valleys and cities.

In an emergency, coordinates are worth their weight in gold. They can be given in different formats, for example decimal degrees or degrees, minutes and seconds. Read them out slowly and completely and name the format. In many countries a phone also automatically sends its location to the control centre during an emergency call – still, it is worth knowing where to find your coordinates on the device.

An astonishing side note from physics: because of Einstein's theory of relativity, the satellites' clocks run very slightly differently from clocks on the ground. The system compensates for this effect – otherwise the position would drift noticeably within a short time.