and an ils signal (loc + g/s) is controlled and calibrated very regularly by an aircraft loaded with equipment. you never see those flying the ils's from 100 nm away.
and the difference between safe and able, cited above, is the same as certified and demonstrated. a good 'ole 707 was able to do a good barrel role but wasn't certified and safe to. and the young'uns have (i think) as good an idea of what's possible with an aircraft and what's safe with one (we're carrying pax remember), as the 'ole ones when they were young'uns.
seb