Multiple ATC transmitters
 

Question for ATC:

It seems that when a controller broadcasts on all his transmitter sites all we hear is garbage when we're within range of several transmitters.

How often do you guys de-select transmitter sites so as to only use the site closest to the aircraft your talking to? Is it a simple operation or is advanced equipment required? It seems the Chinese never bother and just use all their transmitters, which makes them unreadable most of the time.

I've always wondered how this works on the other end... thanks

In the UK controllers have no control over which transmitter(s) are being used for their transmissions. Some ATC sectors are transmitted on several transmitters - called multicarrier - but the frequency of each station is tuned slightly off the nominal frequency to prevent interference so you should not have any problems. I believe the offset is about 7kHz, but my memory is not too good so I might be wrong. The system has been in use in the UK for an extremely long time and it there were any problems with QRM they would have surfaced by now.

However, it is possible for controllers to select a particular frequency and during very quiet times the controller might be monitoring more than one frequency so if he transmits on two frequencies at once it might give rise to problems unless dedicated equipment is used to prevent interference.

Multi-carrier Offsets are used for large sectors and engineers usually manage that. Audible result is a kind of 'phasing'. ATCOs have control of cross-coupling of transceivers on different freqs ('re-transmit') used when combining sectors ('bandboxing'). It often does not have the desired effect, and the side-tones used to be awful. I hope they have improved because I found them to be a deterrent to bandboxing.

thanks for the insights... don't remember ever having the problem in the UK.

I wonder if Chinese ATC uses similar multi carrier offset technology. Wouldn't surprise me if they did not. As an example, Chinese ATC will transmit, and it's all garbage but we think we might have heard our callsign in there, so we told them that was unreadable. They'll come back with a 5x5 transmission for us... I figure he's just using one nearby transmitter on the second transmission.

I've also heard the "bandboxing" being used in other airspace.. in the Canadian Artic region for example. Seems to work ok there, no problems.

A system similar to bandboxing is permanently in use (in the UK anyway) for airfield vehicles. Their radios operate on UHF, around 450mHz, and the frequencies are "paired" with airband VHF frequencies such that when a vehicle calls the Tower,ite transmits on UHF and this is immediately re-broadcast on the appropriate VHF frequency. Equally, traffic on VHF is re-broadcast on the UHF frequency. Somebody can probably say why they don't simply use VHF radios in vehicles but I was told it was due to licensing problems...?

Could it have possibly been a crossed transmission with the controller transmitting at the same time as another aircraft?

Not in the case of Chinese ATC, because it happens all the time. This is not just an occasional occurrence. There are times when nobody on the frequency can make out what the controller is saying (I'm not talking about any language problems here; that's a separate issue). It sounds a lot like a garbled transmission that's been blocked / interfered with by the same controller on a different transmitter.

But many times, when someone finally tells them they are unreadable, they are able to come back with a more readable transmission that's not garbled / blocked.

This may be true for most units but at mine at least we are able to switch between our primary and secondary transmitters/receivers via the VCCS panel.

Same at my unit.

Glamdring: while this is true for airfields (which I assume is where you operate) we're talking about area control services with transmitters 50 - 100 miles apart. I suppose it's possible in China that once the controller has established where an aircraft is in his sector, he can select the nearest suitable transmitter.

UK Area unit; we can select different legs on our com panel and we have a choice of frequencies on each leg. We can "bandbox" [or cross couple] if necessary and so the pilot would not notice.

Same unit as 055166K - but in the other room. We can't select different legs - we have to get engineering to do it for us and they do so only after investigating any reported issues...

It's too bad ATC tech is so far behind the times. It's now possible (using GPS as a frequency standard) to put all transmitters on exactly the same frequency, so there is no audible beat between transmitters. This is done by some EMS systems and several Amateur repeater networks in the US.

MarcK.. But is the technique used for ATC communications?

I only know US ATC, and I don't think I've ever heard multiple transmitters on (nearly) the same frequency. Around here there are multiple frequencies in use for a single ATC position. My comment was just directed at the use of technology that hasn't been changed since deForest invented the audion "valve".