To count as a precision approach (which ILS is), the equipment and procedure has to meet certain accuracy requirements (e.g. <x> feet error at <y> distance/heig

To count as a precision approach (which ILS is), the equipment and procedure has to meet certain accuracy requirements (e.g. <x> feet error at <y> distance/height from touchdown) as well as the ability for the equipment itself to alert the user that things are out of tolerance within <z> seconds of being detected faulty. I.e. the transmitter on the ground must be able to detect that something is wrong and immediately cut out the signal / report it as inop to the receiver on the plane. When you're seconds from hitting the ground, that is a requirement. Although GPS based approaches have some aspects of this, I believe that they just don't meet the exact requirements as written currently, although they are close.