Do UAVs use ground-based navaids?
For UAS, the aircraft may be below the service volume of a VORTAC most of the time.  Furthermore in the US sUAS are normally line of sight to the pilot so in theory if GNSS is unavailable, the pilot will control visually.  Finally the variability and uncertainty of VORTAC is high enough so as to make it impractical for UAS stability, drift and position control