I was at an airshow yesterday and occasionally they would put the pilot’s radio chatter on the speakers for the audience to listen in on. Problem was, it was so static-filled that you could barely understand what was being said. Then I realized that it seems like anytime aircraft radio chatter is heard (airshows, TV news, etc) it’s terribly hard to make out the speech over the static.
Why is that? Do the pilots really hear such a horrible transmission or is it some function of crappy equipment that is used to broadcast the sounds to the masses or something?
I have to base this on FCC Frequency Allocations tables, until somebody with actual firsthand experience in aeronautical communications chimes in. Aeronautical communications seems to be restricted to AM broadcasting only. AM is notoriously susceptible to stray interference, and the nature of amplitude modulated signal makes it essentially impossible to clean the interference up like we can easily do with FM.
Thus, every little static dicharge, EMI pulse, cosmic ray & sun spot leaves its mark on an AM wave until the transmission is full of spikes & distortion.
The transmissions are indeed amplitude modulated.
There is probably some historical reason for this, but I’m guessing (I really SHOULD know this) that AM is less prone to “picket-fencing”. One hears this all the time on 2m FM(narrow) amateur radio transmissions. IIRC it is due to alternating constructive and destructive interferance between ground wave & sky wave (think of one signal doing direct, the other bouncing off the Earth).
I have listened to airshow stuff (from the PA, not directly), and it difinitely sounds more staticy than what I hear when flying (with headphones) (in general).
Brian, PP-ASEL