This is a very good question. As you know, the ILS is comprised of two separate radio beams, the localizer and the glideslope. Both signals are highly directional directional and line of sight. Most glideslopes are set up for a 3 degree descent. Simple math will show that 3 degrees = 318' per nautical mile. If you're at 10,000 feet you will intersect the center of the glideslope signal approximately 31.4 miles. If you're at FL450 and the airport's at sea level the distance is 141.5 miles. Do glideslope signals have enough "oomph" to travel those distances? The answer is probably, but they aren't usable at those distances. I often fly from Klamath Falls, OR to Sacramento, CA. It's a short flight, something like 208 NM. The unique thing about those two airports is that after taking off from runway 14 at Klamath Falls it's an absolutely straight shot to either runway 16L or 16R at Sacramento. Out in that part of the country, the traffic is so light that you seldom get much if anything in the way of vectors. All things being equal, we would normally climb to FL330 on a trip of that distance. Sacramento is 27' MSL, so based on 318'/NM you would center the glideslope at about 104 NM out. I have done that several times. (We typically get "pilot's discretion" descents" going into there.) On that particular leg, the localizer signal has always been strong enough to be picked up at the "top of climb", which is usually about 160 miles out of Sacramento and we "fly into the glideslope around 100 miles out. This corresponds nicely with what the VNAV usually computes for us, so in essence, we fly what amounts to a 150 mile ILS profile. The next question is, are the signals usable at those distances? No. The signals are like spokes in a wheel and at those distances there is so much lateral and vertical "slop" in them, that the are practically useless for anything other than saying, "Hey look, we've got the localizer at 150 miles." We may be flying the profile with both the localizer and glideslope signals being, more or less centered, but that's just a fluke because we're navigating with the FMS. The localizer signals don't become "focused" enough to be really usable until you're 15 or 20 miles out from the airport. I hope this answers your question.