How do those GPS systems work? The ones they put in cars now.?

Question

Becasue roads are always changing because of construction.. so how does the car know?

Answer 1

They use satellites to get your starting point and then you proceed where you want to go and as for the changing of roads and highways and businesses......you will need to buy the updates............

Answer 2

The car never knows,  it is just a machine that takes it's input from you.   The GPS is an entirely different thing,  it triangulates signals received from satellites and computes your position,  then positions the marker on a preloaded map.  

Even construction won't change the directions much,  and some units can be updated with more current mapping programs.