Guest

Atc Comm, Am Or Fm?

Mon Apr 19, 1999 2:48 pm

I have a doubt: I know the comm frequencies between the tower (ATC) and pilots is in the range of 108 MHz up to somewhere 150 MHz, right?

But what type of modulation (AM or FM) do their transmitters use?

And the ATIS (automatic wheather information system), what frequencies and modulation use? Is there another acronym for such a system? (the tower frequencys are sometimes refered as TWR).

Regards

Jesus
 
Guest

RE: Atc Comm, Am Or Fm?

Mon Apr 19, 1999 8:03 pm

VHF comm between tower and aircraft is in FM, for ATIS i can't be 100% sure but it is also FM to i think and you find it in the VHF band, sometimes even but in rare causes being broadcast on a VOR ident.
 
mirage
Posts: 3612
Joined: Mon May 31, 1999 4:44 am

It's In Am

Mon Apr 19, 1999 8:34 pm

It's in AM.
The VHF aero band extends from 108 to 135.975Mhz, while the UHF bands falls between 225 and 400Mhz.
AM is used for both VHF/UHF traffic for two reasons. FM signals coming and going to an aircraft would be subject to the Doppler effect, wich can shift them off the established communications channels. The grater the aircraft speed, the greater would be the shift. Secondly, AM mode permits all stations working the facility at the same time to all be heard simultaneously (in FM mode, only the station with the strongest signal would be heard).

With any FM scanner you can hear AM communications with no problem but with a AM scanner you can't hear FM communications.

ATIS and digital communications (ACARS) are also in AM mode.

HF communications are in SSB mode but that's other story.

Luis, Faro, Portugal
CT2GLT
 
FedEx
Posts: 706
Joined: Wed Aug 25, 2004 5:07 pm

RE: It's In Am

Tue Apr 20, 1999 2:30 am

Mirage hit the nail on the head !!!
I got some ATC with my walkman once on 106.3!
 
David L
Posts: 8547
Joined: Tue May 18, 1999 2:26 am

RE: It's In Am

Tue Apr 20, 1999 4:35 am

I'm also with Mirage on the AM/FM front.

FedEx: I used to pick up Shanwick North Atlantic clearances on 102.5, where the commercial station Radio Clyde is (can't remember the real frequency - 133.x I think).
 
mirage
Posts: 3612
Joined: Mon May 31, 1999 4:44 am

RE: It's In Am

Tue Apr 20, 1999 5:05 am

I also copy FAO approach (119.4) on my FM radio at 98Mhz. It's some kind of weird interference.
Radio waves still have some misteries....

BTW, I copy traffic crossing the Atlantic Ocean, on HF band, Santa Maria control (Azores Is.).
Cool transmitions.

Luis, Faro, Portugal
CT2GLT
 
David L
Posts: 8547
Joined: Tue May 18, 1999 2:26 am

RE: HF Band

Tue Apr 20, 1999 5:37 am

HF... I must find out more about that from you some time. I've used a Sony "world" radio (SSB) to pick up a few ATC conversations from the other side of the Atlantic (the west side) and once from India but it's very hit and miss and only at night (something to do with the ionosphere, I believe).
 
mirage
Posts: 3612
Joined: Mon May 31, 1999 4:44 am

RE: HF Band

Tue Apr 20, 1999 7:32 am

You're right, it depends on good propagation conditions. We have good propagation when the ionosphere is "full" of ions (spelling?).
The ionosphere is full of ions when there's good solar activity.
The solar activity depends on the solar cycles and we are now almost close to the high point of this solar cycle wich will be about 2000 and 2001 (solar cycles have 11 years).
Here in my side, we have better conditions also at night but this is not an exact science.
HF communications are fascinating.

Luis, Faro, Portugal

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