Jetguy From , joined Dec 1969, posts, RR: Reply 1, posted (11 years 2 months 3 weeks 2 days 23 hours ago) and read 2282 times:
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.
Jetguy From , joined Dec 1969, posts, RR: Reply 3, posted (11 years 2 months 3 weeks 2 days ago) and read 2117 times:
Not really. The frequencies used for the localizer and glideslope signals are pretty much immune from weather interference - at least at the ranges that they are usable for lateral and vertical navigation. The main signals that are affected by weather are:
1. The ADF - It's primary job is accomplished using low frequency signals. However, most people just us it to listen to the AM radio band.
2. Weather Radar - If it wasn't affected by weather (rain) it wouldn't be of much use.
3. GPS - It's signal can be attenuated by heavy rain.
4. VHF Comm radios - Can be affected with precipitation static (p-static), but this there is usually an underlying problem with the aircraft bonding if this is an issue.
Jetguy From , joined Dec 1969, posts, RR: Reply 5, posted (11 years 2 months 3 weeks 1 day 23 hours ago) and read 2107 times:
Airplay, You're absolutely correct about p-static affecting the localizer frequencies, but I qualified my statement by saying "at least at the ranges that they are usable for lateral and vertical navigation." For all intents and purposes, once you're close enough to be using it for an approach, say within 15 miles or so of the transmitter antenna, it's going to have to be raining awfully hard to cause you a problem. (Probably so hard that you'd be on your way to the alternate.) Additionally, like I said, if you encounter a problem with p-static it's because there's probably a maintenance problem - poor bonding - going on at the same time.
Artsyman From United States of America, joined Feb 2001, 4745 posts, RR: 36 Reply 6, posted (11 years 2 months 3 weeks 1 day 21 hours ago) and read 2093 times:
If the radio beams are as strong as you suggest, is there contamination of the beam when you fly into somewhere like San Francisco ?, you would have signals from San Francisco, Oakland and San Jose all crossing each other.
Ups763 From United States of America, joined Nov 2001, 199 posts, RR: 0 Reply 8, posted (11 years 2 months 3 weeks 1 day 13 hours ago) and read 2032 times:
Jetguy said it all. When you select the localizer frequency(108.10-111.95) you receive both LOC and GS, except when flying the BC. In the helicopter I usually pick up the GS at approx. 10 nm from the runway. As for the localizer, usually at about 18 nm out from the runway. Around 10-18 nm from the runway the localizer is good for about 10 degrees on each side of the centerline. When under 10 nm out it becomes more sensitive and is usable for about 35 degrees on each side of the centerline.