system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come you have to pay for data on your phone, but the GPS in your car is free?
gpt
GPS is a receive-only technology. 

A GPS unit does not need to transmit any signals to determine its location, it need only listen for satellite signals or 'pings' that are constantly sent down to earth from satellites in orbit.

The GPS unit uses knowledge of each satellite's precise location in orbit plus the timing of the pings plus the speed of light to determine how far away each satellite is from the unit. If the unit can receive pings from enough satellites, it can pinpoint its exact location through triangulation.

The reason why you often need data for GPS on your phone is that the maps are loaded from a server in real-time as you move to a new location whereas with a standalone or built-in GPS unit for a car, the maps are all pre-loaded into the system.