What "sources" would you like Genghis to quote??
You answered the question yourself when you said
I understand how GPS works out its present position.
It keeps working out the present position. If you walk to the other side of the room - you are at a different lattitude/longitude co-ordinate.
It knows you have moved - however short a distance or however slow a speed - you have a new co-ordinate.
Clever, isn't it!!