the only means the crew of F901 had (at the time of their descent) to verify their position with the aid of ground instruments would be messages relayed by ATC based on their radar returns.
The only aids available to the crew were,
TACAN - using the distance read out you get a circle of probability ie if it reads 40 miles you don't know which direction you are from the station and need back up information to determine direction. You can carry out a what is known as a DME homing procedure which determines a coarse direction and ultimately the procedure will bring you over head the station, where upon the distance displayed will be your altitude.
RADAR - if the aircraft was above 30° elevation a radar fix would not have been available. With that limitation a radar fix would be available to within a little under 5 NM by my back of the envelope calc if the aircraft was at the 16,000 LSALT.