Satellite (drag it)Receiver: the real position (drag it)Where the receiver thinks it isUncertainty area
A satnav finds the vehicle's position by GPS, shows it on a stored map, plans a route and gives turn-by-turn directions, recalculating when a turn is missed. Apps add live traffic over mobile data. Drawbacks: out-of-date maps, losing the signal in tunnels, and drivers trusting it over road signs.
Location data (latitude, longitude, sometimes height and time) is added to a file's metadata, such as a photo's EXIF data or a social media post. Photos can be shown on a map or grouped by place. Risk: a shared geotagged photo can reveal where someone lives or goes to school, so it can be turned off.
Couriers and bus companies track their fleets; parents and walkers use trackers; phones can send their position to emergency services automatically when someone calls for help. Stolen vehicles and devices can be found.
Tractors steer themselves along rows and spread fertiliser only where it's needed (precision agriculture); surveyors fix boundaries; maps and geographic information systems (GIS) are built from GPS data; drones fly set routes.
Every GPS satellite carries atomic clocks, so a GPS receiver also gets very accurate time. Mobile networks, power grids and banks use it to keep their systems in step.
GPS (USA) is one of four global systems (GNSS): with GLONASS (Russia), Galileo (EU) and BeiDou (China). Phones use several at once. Australia and New Zealand's SouthPAN sends corrections that make positions more accurate.
| Satellite positioning (GPS) | Cell towers (the mobile network) | |
|---|---|---|
| Where it is | About 30 satellites, about 20,200 km up (medium Earth orbit) | Masts on the ground, from a few hundred metres to tens of km apart |
| How it finds you | The receiver times signals from at least 4 satellites | Which towers the phone is in touch with, and their signal strengths and timings |
| Accuracy | A few metres outdoors | From about a hundred metres (cities, many towers) to several km (the country) |
| Where it works | Anywhere with a view of the sky; poor indoors, in tunnels and between tall buildings | Only where there's mobile coverage, but indoors too |
| Direction | One way: receive only, so unlimited users and the satellites don't know where you are | Two way: the network always knows roughly where a phone is |
| Cost | Free to receive; the receiver uses battery | Part of the mobile service |
Phones use both, with Wi-Fi: assisted GPS downloads the satellites' details over the network, so the first fix takes seconds rather than a minute. Other satellites do other jobs: geostationary satellites (35,786 km up) relay TV and weather pictures, and low Earth orbit satellites (a few hundred km up) give satellite internet.