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mrskyguy
Topic Author
Posts: 312
Joined: Thu Aug 28, 2008 7:13 pm

### Why Is 110.3 Such A Common ILS Freq?

Don't ask me to spit out data that proves it, but for some reason I've been noticing that 110.3 is a more common ILS frequency at airports worldwide. Flying abroad, I've noticed that this frequency has popped up on more than just a few Jepp charts and I was beginning to wonder if perhaps there was a correlation (perhaps somewhat loosely historical in nature) that tied it together?

Or pure chance and I'm loosing it?
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air

777ord
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Joined: Sat May 15, 2010 12:04 pm

### RE: Why Is 110.3 Such A Common ILS Freq?

I, too, have noticed this. If not 110.3, then frequencies close to it (not completely inclusive). The only conclusion I can come to are these:

1) It's a set frequency range selected for the type of operation. Although, I have not read any firm facts on the matter
2) Maybe similar thought process to why a majority of us airports have "ground.8" as 121.8 for example. For a sort of commonality type of thing.

Perhaps just coincidence, but you got me thinking! This'll give me something to do at work.

doug_or
Posts: 3244
Joined: Sat Mar 18, 2000 9:55 am

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting 777ORD (Reply 1):1) It's a set frequency range selected for the type of operation. Although, I have not read any firm facts on the matter

ILS frequencies are 108.10 through 111.9 I believe. It does seem that some occur more commonly, but that might be happenstance and perception based on the airports I operate into most often.

 Quoting 777ORD (Reply 1):2) Maybe similar thought process to why a majority of us airports have "ground.8" as 121.8 for example. For a sort of commonality type of thing.

.7 .8 and .9 are the common ones.
When in doubt, one B pump off

411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

### RE: Why Is 110.3 Such A Common ILS Freq?

110.3 was the first ILS frequency selected, when the system was developed in 1938. The second frequency was 109.9, and these two frequencies were used extensively until additional frequencies (within the present ILS band) were allocated.

mrskyguy
Topic Author
Posts: 312
Joined: Thu Aug 28, 2008 7:13 pm

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting doug_Or (Reply 2):ILS frequencies are 108.10 through 111.9 I believe. It does seem that some occur more commonly, but that might be happenstance and perception based on the airports I operate into most often.

You may be right, Doug. I just can't tell you how many times I've pulled the appropriate chart from the bag once given the expected runway for landing and turned to dial in the freq in to the pedestal only to do a double-take and chuckle to myself that it's 110.3 and already dialed from the last run.
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air

ThirtyEcho
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### RE: Why Is 110.3 Such A Common ILS Freq?

The airport where I learned to fly didn't have an ILS until1960 but I recall it being 109.9. The airport 60 miles away was 110.3.
We also had, 60 miles in the other direction, one of the last 4-course ranges in the country and had a few Red airways, as well.

Northwest727
Posts: 379
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### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting doug_Or (Reply 2):ILS frequencies are 108.10 through 111.9 I believe.

The localizers operate on, according to AIM 1-1-9(b)(1), "40 ILS channels within the frequency range of 108.10 to 111.95 MHz"

http://www.faa.gov/air_traffic/publi...tpubs/aim/Chap1/aim0101.html#1-1-9

RaginMav
Posts: 335
Joined: Tue May 25, 2004 5:22 am

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting doug_Or (Reply 2):S frequencies are 108.10 through 111.9 I believe.

That is correct. Also, in the USA, ILS and LOC frequencies always end with an odd number. You will never 110.2 as an ILS frequency, although it may well be a VOR frequency.

KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting Northwest727 (Reply 6):Quoting doug_Or (Reply 2): ILS frequencies are 108.10 through 111.9 I believe. The localizers operate on, according to AIM 1-1-9(b)(1), "40 ILS channels within the frequency range of 108.10 to 111.95 MHz" http://www.faa.gov/air_traffic/publi...1-1-9

Just for kicks and grins, anyone know what the corresponding glideslope frequencies are?    I know that, in a modern NAV receiver, you never have to hand tune the G/S, and that there is a fixed localizer to glideslope frequency pairing.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

b78710
Posts: 311
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### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting RaginMav (Reply 7):That is correct. Also, in the USA, ILS and LOC frequencies always end with an odd number. You will never 110.2 as an ILS frequency, although it may well be a VOR frequency.

yup. same here. ILS are odd ending 108 - 112 MHZ and VOR's are the evens.

mmedford
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### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting KELPkid (Reply 8):Just for kicks and grins, anyone know what the corresponding glideslope frequencies are?    I know that, in a modern NAV receiver, you never have to hand tune the G/S, and that there is a fixed localizer to glideslope frequency pairing.

Times the LOC freq by 3...you'll usually get the GS freq..
ILS = It'll Land Somewhere

FredT
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### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting KELPkid (Reply 8):Just for kicks and grins, anyone know what the corresponding glideslope frequencies are?

The complete table is in ICAO Annex 10 Volume I. Unfortunately, ICAO does not publish its publications online. However, check Wikipedia if you are satisfied with an un-official source. Haven't double-checked it against Annex 10, but should be good enough.
I thought I was doing good trying to avoid those airport hotels... and look at me now.

Revelation
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### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting 411A (Reply 3):110.3 was the first ILS frequency selected, when the system was developed in 1938. The second frequency was 109.9, and these two frequencies were used extensively until additional frequencies (within the present ILS band) were allocated.

And the reason why was in those days (of which 411A is so familiar with), radios typically used crystals, especially when high frequency stability was needed, so each new frequency needed a new crystal.

 Quoting Northwest727 (Reply 6):The localizers operate on, according to AIM 1-1-9(b)(1), "40 ILS channels within the frequency range of 108.10%u2009to 111.95 MHz"

Sure, now that phase locked loop synthesizers can be implemented in the area less than a grain of sand, it's no problem, but we are talking about a system that dates back to 1938, when radios used vacuum tubes, discrete resistors, capacitors, coils, etc as well as crystal controlled oscillators.
Wake up to find out that you are the eyes of the world
The heart has its beaches, its homeland and thoughts of its own
Wake now, discover that you are the song that the morning brings
The heart has its seasons, its evenings and songs of its own

flight152
Posts: 3502
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### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting KELPkid (Reply 8):Just for kicks and grins, anyone know what the corresponding glideslope frequencies are?

329.15-335.0 mhz

navion1217
Posts: 8
Joined: Sat Jul 24, 2010 10:13 pm

### RE: Why Is 110.3 Such A Common ILS Freq?

I think it has to do with the wavelength of the radio signal and the physical length of the reciever's antenna. It has probably changed with modern technology and tunable antennas, but the length of the antenna determines which frequency it will recieve "best". Notice that 110.3 is almost in middle of the LOC frequencies, the reciever's antenna is physically "tuned" to the middle of the range and performance degrades towards the ends. Like most popular FM radio stations are near 100, in the middle of the FM broadcast range.

Of course thinking about it, most GA airplanes use the same antenna for LOC and communications. Hmmm. Broadcast strength can make up for a lot too. When I was at the ERAU dorms, we would often get aviation band transmissions from the tower breaking in when listening to FM radio. I would think that the voice broadcast is a lot higher power transmission that the LOC. I'm starting to argue with myself. Anyway, the frequencies in the middle of the range will have the best performance.

b78710
Posts: 311
Joined: Fri Mar 24, 2006 4:21 am

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting navion1217 (Reply 14): but the length of the antenna determines which frequency it will recieve "best".

I seem to want to say the aerial should be 1/4 of the wavelength. Been a while since I did any radio theory though.

Revelation
Posts: 26133
Joined: Wed Feb 09, 2005 9:37 pm

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting b78710 (Reply 15):I seem to want to say the aerial should be 1/4 of the wavelength.

Yes, and at 110 MHz, wavelength = (speed) / (frequency) = (3 x 10**8) / (110 x 10**6) = 2.73 meters (rounded)

So a quarter wave is about 68 cm or 27 in long.

No surprise it's about the length of a car aerial, which is trying to pick up a similar frequency band (88-108 MHz).

 Quoting navion1217 (Reply 14):I think it has to do with the wavelength of the radio signal and the physical length of the reciever's antenna.

Wiki says:

 Quote: A localizer (LOC, or LLZ until ICAO designated LOC as the official acronym) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas.

Since you need several pairs of antennas, smaller is better (easier to build, easier to maintain, more survivable in harsh weather, etc).

But as above, this was being done a long time ago:

 Quote: Tests of the ILS system began in 1929,[7] and the Civil Aeronautics Administration (CAA) authorized installation of the system in 1941 at six locations. The first landing of a scheduled U.S. passenger airliner using ILS was on January 26, 1938, as a Pennsylvania Central Airlines Boeing 247-D flew from Washington, D.C., to Pittsburgh and landed in a snowstorm using only the Instrument Landing System.

And as I've mentioned, it was very hard to get a stable signal at those frequencies in those days using the kinds of devices they had available to them, or perhaps they'd have gone even higher in frequency.
Wake up to find out that you are the eyes of the world
The heart has its beaches, its homeland and thoughts of its own
Wake now, discover that you are the song that the morning brings
The heart has its seasons, its evenings and songs of its own

mrskyguy
Topic Author
Posts: 312
Joined: Thu Aug 28, 2008 7:13 pm

### RE: Why Is 110.3 Such A Common ILS Freq?

 Quoting 411A (Reply 3):110.3 was the first ILS frequency selected, when the system was developed in 1938. The second frequency was 109.9, and these two frequencies were used extensively until additional frequencies (within the present ILS band) were allocated.

Hoorah! Im not crazy afterall!
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air

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