FM Immunity

"Gerry Caron" wrote in message
ng.com...

"Bob Noel" wrote in message
...
In article ,
Tman wrote:

What is that -- and does it matter to GA avionics? Heard it referenced
a few times in the sale of an older GIII. But Google does not turn up
much.

(It's not applicable in the USA.)

It has to do with the lack of frequency protection for ILS. High
power FM stations (in Europe) can bleed enough RF into ILS frequencies
that ILS receivers (and technically VOR receivers) need to do a
better job rejecting out-of-channel frequencies.

It's an ICAO Annex 10 rule that requires ILS receivers installed after Jan
2001 (?) to have improved FM Immunity. As Bob said, it requires a tighter
spectral mask on the receivers to better reject bleed over from adjacent
channels. It can be a real issue for CAT II/III ops. Since Part 25 a/c
are normally certified for world-wide ops, they've had to meet it even tho
it wasn't a real problem in the US.

As it turns out, it is needed in the US; but not for the original reason.
Since then our FCC approved HD radio standards that allow significantly
more out-of-band emissions than the old analog signals. Just hope the FAA
hasn't installed an ILS on 108.1 MHz near a 107.9 MHz HD station.

Proper frequency management would probably prevent a problem. Even if it
didn't, it wouldn't take long for the FAA to figure out there was a problem.
Even if the problem wasn't discovered by a PIREP, all ILS systems are
periodically flight checked.