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Guest RonB49

ILS/GS Frequencies

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This is a question I am raising out of pure curiosity.When we tune an ILS navaid, the frequency we dial-in is only the LOC frequency. The GS is another equipment which works in a frequency that is about three times that of the LOC. In the LOC frequency range (108.1 to 111.95MHz) there are 200 channels, while in the frequency range of the GS (329.15 to 335.00MHz) there are 40 channels.So there must be a fixed correspondence between the LOC frequencies we choose and those of the GS, which is not a one to one correspondence. Does anyone know how these frequencies are tabled against each other?Regards,

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You're partially correct. There are 40 glide slope channels, but there are also 40 localizer channels, not 200. The range of localizers is 108.1-111.95, but only certain frequencies in that range are allotted for localizer use, thus limiting it to 40 channels. (See AIM 1-1-9, table 1-1-4 for a listing of the pairings) Of course, knowing the actual channel pairings is pure trivia as most, if not all, recievers will automatically dial in the UHF frequency for the glide slope when you put in the VHF localizer channel.Hope that helpsRob

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EDIT -- Looks like that as I was pondering over my ignorance, Rob stepped up with better information plus the pertinent manual reference. Thanks Rob. R-Let me start by stating that I've never understood the relationship of frequency to channnels when applied to localizers. I do recall however that not all frequencies between 108.10 and 111.95 are used for ILS; I believe only odd-numbered tenths are used. Thus, 108.10, 108.15, 108.30, 108.35 . . . up through 111.75, 111.90, and 111.95 are used for ILS but 108.20, 108.25, 108.40 . . . through 111.80, and 111.85 are used for VOR.Another question that I would have is, Are the frequency increments the same in the VHF localizer band (108.10 - 111.95) and the UHF glide slope range (329 - - - BUT WAIT !!!!!!!!!!!As I typed, something else occurred to me.If I am correct that, ". . . odd-numbered tenths are used. Thus, 108.10, 108.15, 108.30, 108.35 . . . up through 111.75, 111.90, and 111.95 are used for ILS," and I'm correct that the NAV frequency increments are 50 KHz (not 25 KHz like COM frequencies), dosn't that mean that only 40 "channels" are used for ILS localizers? Or have I just proved my ignorance (again)? If someone else knows for sure, then please respond, 'cause now I'm curious too. R

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Thanks for your replies. In fact only the odd tenths of LOC frequency are used. The reason I mentioned "200 channels" for the LOC receiver, was due to the fact that equipment manufacturers tell that their receivers have 200 channels, while for the GS receivers only 40 channels are used.Where can I find the document (AIM) where the frequency pairs are tabled?Best Regards,

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>Thanks for your replies. In fact only the odd tenths of LOC>frequency are used.I have to take exception to that. To mention just a few freqs that use the hundredths digit, KLAS 25L @ 111.75, KSFO 28L @ 109.55, and KORD 32R @ 110.75.> The reason I mentioned "200 channels" for>the LOC receiver, was due to the fact that equipment>manufacturers tell that their receivers have 200 channels,>while for the GS receivers only 40 channels are used.Do the math. Localizer freqs 108.00 - 117.95 = 10 MHz. Each increment is 50 KHz = 20 increments per MHz. 10 times 20 = 200 channels.>Where can I find the document (AIM) where the frequency pairs>are tabled?>I bought my FAR/AIM (Federal Air Regulation/Aeronautical Information Manual) at the local pilot's shop (with the clever name "The Pilot Shoppe") here at San Antonio Int'l (KSAT) for about 15 bucks. R-

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You are right! The 200 channels are the "possible" channels and not those used by ILS.As for the AIM, I am just reading it from the website. By the way, I am a little bit far away from San Antonio, as I am in Lisbon, Portugal!Regards,

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Wow Domingos, let me congratulate you on you use of the English language; its much better than my use of Portuguese:-) I suppose it is not likely that your local pilot shop would have a U.S. FAR/AIM manual in stock, is it? I hope that your questions have been well and fully answered. R-

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I usually use the Jeppesen charts to get the approach information. When a full ILS is available, the approach chart shows the reference ILS or ILS DME Rwy xx. When only the localizer is available the reference is LOC or LOC DME Rwy xx. As for the frequencies used when only a LOC is available, I do not think 111.xx is used in all cases. For instance, in the case of LPPD (Ponta Delgada, Azores) we have LOC DME Rwy 30 and the frequency is 109.50. On the other hand, in LPLA (Lajes, Azores) we have ILS DME Rwy 33 and the frequency is 111.50.Did this help in any way?Regards,

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