One should know the error rate of AINS, but it is a variable that cannot be checked until a ground based aid at a known fixed point is interrogated.
A ground based radar will have a system whereby its integrity can be ascertained with certainty, and will be regularly calibrated. The actual radar at McMurdo was normally only used for approaches. the maximum elevation was from memory only 30 degrees. If it was to be used as a surveillance radar it involved a considerable amount of adjusting.
In saying this I admit my exposure to AINS was limited to a couple of demonstrations of its capabilities whilst in the jump seat of a DC10 on a couple of long haul runs. Only the elite fleets were supplied with such state of the art equipment at that time.
It was the visual positive fix that was wrong., thus illustrating why the descent requirements were as they were.