Nib asked
How does a GPS know there's traffic?
GPS satellites know nothing about traffic. Your phone works out where it is, and the map learns about jams because lots of phones report how fast they are moving.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the twist
GPS satellites broadcast timing and orbit signals to receivers on the ground. They do not receive data from your phone and know nothing about roads or traffic.
The satellites only say where they are and what time it is.
a common mix-upGPS satellites don't see roads or cars. They only broadcast signals; the traffic comes from somewhere else.
2finding yourself
A receiver compares timing signals from at least four satellites to work out its own position and the time. That position is computed on the phone itself.
GPS gives your phone its own position.
3phones as sensors
Traffic services learn about jams from phones that share their location. Many phones moving slowly on the same road means slow traffic.
A crowd of slow phones means a jam.
4the server
Phones that share location send position and speed over the internet. Servers combine many phones on each road segment and work out the typical speed.
5red and green
The map turns the average into colour: green where phones are moving, red where they crawl. Drivers' reports and road sensors add detail in some places.
a common mix-upThe map is not watching your car in particular; it is combining many phones into a speed for each road.
6the whole idea
Satellites give each phone a position. Phones report position and speed. Servers average the reports into a traffic colour for each road. The satellites play no part in knowing traffic.
Satellites say where; crowds of phones say how fast.
Next time the map shows red ahead, remember: it is not the satellites, it is the crowd of phones crawling there.