Everyone knows the radio communications doesnt sound great altough they work perfectly.
But can the brand new ADS-B communications be used for carrying audio signal? Would it be clearer?
The ADS-B will be mandatory by 2020 (At least for anyone that wants to fly at the controlled airspaces) so why dont we ditch the old radio system full of parasties and use ADS-B instead for voice communications instead?
Why dont we ditch the old radio system full of parasties and use ADS-B instead for voice communications instead?
Well, why don't we use televisions for washing dishes? Because televisions don't wash dishes.
Likewise, ADS-B doesn't carry an audio signal, so it can't be used for voice communications.
Perhaps ADS-B could be modified to carry voice signals, but this is probably not a good idea. The reason for this is that, while aviation voice communications currently use a band which is 19 MHz wide, the band that ADS-B uses is only about 1 MHz. (The Signal Identification Guide says, "There are two types of ADS-B: one [...] with about 50 kHz of bandwidth; [and] one that transmits at 978 MHz [...] using a larger bandwidth of about 1.3 MHz".)
Transmitting voice signals digitally is a good idea, but I don't know of any reason why this would be done using ADS-B. It would make more sense to simply design a new protocol from scratch.
ADS-B is very low bandwidth and is completely unsuitable for carrying voice traffic. The answer to the "why don't we" question is quite simply "it doesn't work that way".
As an additional point... the reason ADS was given its own frequencies was so that other communications would not interfere with the needed Position and Identification information sent out via that method. ADS can in many places surplant the need (though I'm not at all suggesting that it should do so) for primary radar. There are too many things that can end up in the air that primary radar can see which don't have a any transmitters...flocks of birds come to mind, planes with malfunctioning radios, weather balloons or even idiots in lawn chairs with balloons.
By nature -- in radio -- a single station is either listening or transmitting if you have ATC transmitting on that frequency, 3 things will happen.
ATC wont hear all of the other stations transmitting on the frequency,
Any other Receiver wont hear anything except ATC's extremely powerful transmitter attached to good antenna.
Since ATC is stationary on the ground its position, altitude, and all of the other things reported in a ADS packet will be particularly useless to all of the other planes that can also receive ADS traffic.
The last factor is Time all of those ADS identifiers are really really short in time -- way under a second. This is why so many planes can use the same frequency.
Even if two stations step on each other, transmissions have a random delay between them from a given station. Good signal processing can even separate stations that transmit at the same time in some conditions, for instance if each transmitter is slightly off the center frequency differently.
There are a number of reasons using ADS-B's frequency as a carrier for voice communications isn't a great idea. There are a few issues and factors which I believe you're not taking into consideration which, once understood should illustrate why that is not a workable or desirable solution.
First, I'm not going to address this in terms of the UAT system, the fact that it's only used in the US would proclude it's use anyway.
Now, the frequency that ADS-B operates on for transmitting digital data packets is 1090 Mhz. When ADS-B was first designed, in order to minimize the complexities inherent to any work integrating a newer, more modern data transceiver system into an older existing system, eliminating any need for additional frequencies which are only compatible with the new gear, and this would require additional system components to translate or retransmit, is always a smart move.
In the case of ADS-B, the system engineers decided to use an existing frequency by simply building the entire ADS-B system essentially over the existing Mode-S transponder Secondary Surveillance Radar (SSR) systems. In fact, for a short time way back when, an ADS-B signal was actually referred to as Mode-S ES signal, or Mode Select - Extended Squitter signal. The Squitter being the colloquial term for a Mode-S data packet. Therefore, if one wants to fit more data to send, just extend that squitter. Tada!!!
While the Mode-S system does reduce the frequency congestion when compared to the previous generations of transponders (Mode-A/3/C), one of it's primary design goals, even without all the ADS-B noise being added to the frequency, trying to for a voice carrier onto the same frequency would certainly be less than ideal.
Additionally, adding voice coms, even in digital format, would inevitably begin to cause interference with the SSR and transponders exchanging data, so there's that.
When you consider that we haven't even broached the fact that all the air carriers, who are required to be equipped with and operating with a Traffic Collision Avoidance System (TCAS). Wanna take a shot and guess which system TCAS is built on top of? That's right, Mode-S.
I've never heard what it sounds like, but I've got to imagine that when 2 TCAS boxes start chatting feverishly about who goes high and who goes low so they don't collide, it's not a pleasant sound. And these are REALLY important communications which should not be interfered with, cause if you've got 2 TCAS talking to each other, best to let them finish the conversation!!
