Simply add a GPS field to the telemetry already being sent to Rolls Royce (or GE, depending on who made the engine). The signal is already being sent and all it needs is a GPS chip and a software change to allow for GPS lat/long/velocity (speed and vector) to be sent by both engines. For each pair of engines the GPS inputs should match and since they are both independent of the flight-station and each other... Should make for an easy fix which would be difficult to disrupt without direct access to the engines.