[g_p]

GPS isn't required to schedule handovers in a mobile network.

Handovers can be signalled by the network or the handset, but they use the received signal quality and strength (and handsets can send their signal quality and strength to the network to facilitate better handover scheduling) to arrange handovers.

You can also triangulate devices effectively using just the cellular signal to them, if you are the mobile network operator, using signal strength, or other techniques like time difference of arrival, if you have good clock sync across your base stations.