Is this the "differential" technique in which you have a number of ground stations observing each satellite, so you can compute the positions of the satellites,

Is this the "differential" technique in which you have a number of ground stations observing each satellite, so you can compute the positions of the satellites, and the device that wants to know its own position both observes the satellites and communicates with the ground stations so that it knows where the satellites are? (Obviously the communication with the ground station doesn't have to be low-latency or line-of-sight. If you had line-of-sight communication with the ground stations you could just use them instead of the satellites!) Or does this work without the "device" having to communicate with ground stations?