[zb]

> This in turn is why air traffic still use AM where you can hear both overlapping transmissions at once (possibly garbled if carrier wave was off), and react accordingly rather than being unaware that it happened.

I’m not convinced this is the reason. The carrier wave is always off by a little. While you’re transmitting you hear nothing anyway. And when two parties are transmitting simultaneously, any third parties just hear very loud screeching. A 0.001% difference in carrier frequency would be more than enough to cause this effect in a VHF radio. Notably, this exact problem was a major contributing cause to the worst accident in aviation history. Using FM would have prevented it.

https://archive.ph/2013.02.01-162840/http://www.salon.com/20...

[p_l]

AM is used for two reasons - simplicity of transceivers

AND the fact that two simultaneous transmissions result in buzz instead of locking onto stronger signal. We WANT to know that there's a collision in transmission so that we know we need to retransmit. What would be the expected effect if two FM transmission on same channel were sent?

Fixing the "glitch" would result in way more problems than it solves. Interestingly, aviation authorities do not blame collission behaviour of AM radio for Tenerife, but instead corrected crew management procedures and pushed greater radio phraseology standardisation.

[BigTuna]

>We WANT to know that there's a collision in transmission so that we know we need to retransmit

Digital trunked public safety systems solved this problem decades ago. If you key up when the frequency is in use you get a distinct rejected tone. I'd think prevention is far preferable to sorting it out once everyone's finished walking on each other.

[p_l]

It also means you need to replace everyones radio at the same time because everyone needs to hear everyone on the channel.

Where new additional technologies are possible, they have been applied (digital packet networks, like with CPLDC - Controller-Pilot Data Link Communications).

Replacing A3E modulated VHF radio requires you replace it for literally everyone, because there are way more users at airport than you think.

[mindcrime]

> It also means you need to replace everyones radio at the same time because everyone needs to hear everyone on the channel.

In the public safety context it's not uncommon to phase in new systems (like digital trunked systems) incrementally. You accomplish that by simulcasting the dispatch audio over both systems, and monitoring incoming audio from both systems.

A common pattern for how this plays out would be something like this: all the fire departments and ems agencies in a given jurisdiction are dispatched using two-tone (eg, motorola) paging over a VHF frequency. New digital radios are introduced, and all the fire/ems personnel keep their existing pagers, and (some|most|all) are given the new digital radios. People without the new radios can still talk to dispatch using VHF. And of course systems can be configured to mirror audio around so that if one person is transmitting on VHF they can be heard on the digital system (usually on a channel in the 800mhz or 900mhz band). It's basically a fancy version of a repeater.

Dispatches are then given out over the same old VHF channel AND the new digital channel. In theory you can eventually replace all the old pagers and radios and quit with the simulcast deal, but IME, sometimes things stay in "parallel" mode more or less indefinitely for whatever reason[1]. That said, to your original point, you typically do want to get at least radios standardized as much as possible, even if you maintain the split for (paging|operational communications).

To illustrate, two jurisdictions I'm familiar with: Orange County NC, and Brunswick County, NC. Both followed the path I talked about above: all VHF dispatch for fire/ems, then adopted the NC VIPER digital trunking system, but continue to page on VHF and simulcast the dispatch information over both channels. I'm not sure exactly when Orange County adopted VIPER but it's been quite some time and they're still doing both. FSM only knows if/when they'll ever completely abandon the old VHF system.

[1]: and that reason is often as simple as "money". Plenty of volunteer fire departments in rural areas are skating by with barely enough money to keep their apparatus road-worthy. Replacing every hand-held and mobile radio they own in one fell swoop is often out of reach.

[Source: was a firefighter and 911 dispatcher in a previous life]

[p_l]

You're perfectly correct except one small thing.

You're writing about experience in a closed system - as far as I know all such dispatch systems for public safety etc are closed system where everyone who is ever going to be on the net is part of the system, and it might at most be a case of "we don't have money to replace every member's radio".

In comparison, aviation radio is an open system - not only you do not know who is going to communicate, the communication is also peer to peer, unlike many digital trunked systems which often depend at least on some level of cellular support system.

The only "access control" on the airband VHF and HF comms is of legal variety, with explicit carve out that the person actually flying the aircraft is way less bound by legalities in case of emergencies, and everyone has to be able to talk with everyone, especially on one of the standard common channels.

Examples from personal experience involved various combinations of small airfield ATZ, MiG-29, gliders, old ursus tractor (agricultural kind), busted up Opel Kadett, airliners, ultralights, small transport planes, private helicopters, and dunno who was responsible party but helicopter working as diplomatic flight.

All on one small airfield. And every one of those had to communicate independent of each other with everyone else on that list.

The only time we do "rebroadcast" is when we end up having to do a manual relay due to distance, which is also one of the rare cases where comms might switch over to a more modern system, because someone could ask ATC over VHF to pass something over CPDLC to airliner or using HF, and vice versa.

The poor A3E modulation on VHF airband is the lingua franca, the lowest common denominator, which allows random aircraft from anywhere in the world talk to another random aircraft, as well as ground.

[arghwhat]

> AM is used for two reasons - simplicity of transceivers

That is not a factor anymore. Capable wideband transcievers like the ones in Baofengs and similar supporting multiple types of modulation cost cents.

[p_l]

There's cost in simultaneous replacement for huge portion of the fleet.

Don't devolve into simplism, consider that you need to replace the radio for everyone sharing the same space, and that there might be way more planes sharing that space than you think.

[arghwhat]

The cost of replacing equipment in a fleet is large, but the modulation no longer has any impact on that cost if a replacement was to be made.

AM is not providing any benefit of simplicity, but not changing standards avoids the transaction cost of change.

[nradov]

And who would pay for the Supplementary Type Certificate for every single aircraft model out there, including many that were built by manufacturers that no longer exist? I don't think you understand how this stuff actually works.

[arghwhat]

Equipment certification for aircraft no longer in production is not at all related to the simplicity of AM. Certification complexity would have been exactly the same if FM had been selected as the standard back in the day.

[nradov]

You're not making any sense. FM radio wasn't a practical option at the time. Do you even know the history of how this stuff was developed?

[rlpb]

That article makes out as if transmission blocking leads to a safety problem if a transmission gets lost. It doesn't. What that article misses is that aviation radio communications require readback and verification of the readback, in safety critical instructions such as "cleared for take-off". Not just for radio transmission blocking reasons, but also to detect mistakes in mishearing instructions.

https://en.wikipedia.org/wiki/Tenerife_airport_disaster#Comm... tells a more accurate story: the root cause was that the captain assumed they were cleared for take-off without actually hearing their own callsign and the word "cleared".

Since then, the word "take-off" is avoided in any other type of communication (eg. you might hear "report ready for departure" but never "report ready for take-off"), and every pilot knows never to assume that a clearance has been given unless they hear those exact words together with their callsign.