Mike writes:

The number of passes they make depends on what type of ILS it is and what
check they are doing. I've seen them take several hours to do a CAT III
ILS. For a typical CAT I ILS they will check it once per year making about
3 approaches and an arc. Every other year they will do a more extensive
check. The equipment is also checked extensively on the ground at regular
intervals.

How do they fly the plane with sufficient precision to make the check valid?
Or do they use methods that are independent of the precision of the aircraft's
movements (if so, what are they?)?