Usually for tactical drones they get fitted with an IMU (Inertial measurement unit / gyros) for when GPS signal is lost. That way the drone can continue operation but do suffer from drift the longer the GPS signal is not available. But cheap IMU's fitted on smaller drones has very poor accuracy / drift.

A bigger upset is loss of communications, very few if any drones operate autonomously currently, and if there is loss of communications they may try to return to base. Off course it is more difficult to jam line of sight or drone to satellite communications than GPS.