ATC IssuesA place where pilots may enter the 'lions den' that is Air Traffic Control in complete safety and find out the answers to all those obscure topics which you always wanted to know the answer to but were afraid to ask.
I live in the Northwest area of the UK, and it has come to my attention recently over the Radio Quality and Transmission Quality of some broadcasts being made by aircraft (and possibly stations) and I was wondering about your comments.
What I have noticed is how the quality of some G.A aircraft broadcasts; have in my view, deteriated and nothing appears to be getting done about them? I Regularly listen to the VHF frequencies of ATSU's in my area, and I have noticed over a period of time there are regular offenders that are very hard to understand, and one can only assume if I am having problems understanding transmissions then how do the people for who it matters understand? - E.g. other fellow Pilots, ATC etc etc
I am sure that a.c are reminded of there radio quality on initial 'call up' - but what I can not understand is that if this is the case, then why is nothing being done about it? I am aware that not every aircraft can be perfect, and I know some of older aircraft can suffer with problems in having newer av kits installed, but it does make me wonder how come a 1950's Chipmunk can have a better quality/sounding radio than a 80' Cessna 150 (for example).
Also, something else for you to consider before you write your replies. Should the old standard scale of 1-5 be used anymore? If so should it not be made more accurate - after all what is clear and precise to one person, may not be to another?
This problem is not unique to the UK, we have the same problem here in the US, and it isn't just aircraft, some of our air traffic facilities have problems with their transcivers and or ICSS equipment too.
Having once jumpseated over Africa I think the clarity of even the poorest quality transmissions in this country are perfect in comparison.
With regard to the transmissions by stations I presume you are aware of the slight off-setting of frequency for most ATC ground transmitters? If not (please don't let me teach you to suck eggs on this one) then tuning approx 5kHz either side of a ground station will improve quality no end. Of course if you do that aeronautical mobile stations will sound even more off frequency.
What you tend to find too is that after using radio in its many forms for a considerable period of time one acquires "RT Hearing" in that even the most unintelligible transmission gets decoded into something useful somewhere in the deep grey matter.
Much is to do with the quality of the microphone too of course - both in the air in your typical Cessna and on the ground. When LACC opened there were countless problems with clarity and volume of transmissions all of which were traced to the headset equipment.
In general there is nothing wrong with the quality of aircraft transmitters. Any which are faulty are pretty quickly put right. The standard procedure would be to ask the pilot to use another box.
I agree with you Deltaone that there are some cr*p radios out there, and as has been mentioned you do get accustomed to being able to decipher it. Some pilots with bad radios are better at getting their message across than some pilots with 5x5 transmission quality but who tell you everything except what you really need to know, but I digress...
I think the 1-5 readability scale is good as it saves people describing your transmission with a lot of words whereas the scale is at least concise. It is subjective though. After years of listening to HF in a remote corner of the globe, where there the best readability you gave anyone was 4, VHF even at its worst would probably get no less than a 3 from me, but maybe 1 from someone else.
As far as what can be done about it, well you give pilots a radio check and unless it is totally unreadable you are hardly going to send them back to the apron and spoil their day (talking about club flying here). Maybe we should phone the club/school concerned but I for one usually forget!!
Cor blimey I'm trawling my memory here but I'll try
Most ACC frequencies are transmitted from more than one Tx location. For instance at Manchester for one of the sectors we have transmitters in both Lancashire and deepest darkest Cheshire (exact locations withheld for obvious reasons).
If both of those transmitters were on precisely the same frequency any airborne station would just hear the horrible squealing cross-modulation of two stations transmitting at once.
To counter-act this each transmitter is adjusted so it's slightly off-frequency and thus each of them aren't transmitting over each other. So therefore the Cheshire Tx maybe be transmitting on 134.420Mhz and the Lancashire one 134.430MHz.
This slight off-setting tolerance is in-built into airborne radio equipment but anyone listening with a scanner who is closer to one of the stations would think that it was trnasmitting slightly off-frequency. Maybe then they'd need to alter the frequency they were tuned to by about 5kHz.
Someone could probably explain better than I have as it's been a looooong time since I had to learn all of this
Ah - never heard of that before, but it makes sense the way you describe it! Presumably only applies to larger stations, though? I can't imagine that, for example, Waltham Radio (chosen only because it's my home airfield) would have any need for two separate transmitters?
I don't know how it works over the Channel to be honest as we don't have 8.33kHz spacing in the UK yet as you know. I suppose though that the same aircraft use the UK 25kHz spaced frequencies that use the 8.33kHz spaced ones so I would presume that the tolerance on the new 8.33kHz airborne equipment has been made much much smaller. Either that or offsetting is not used - which is a distinct possibility.
If I can dig out my original documentation on the subject later on I will do so and post a more enlightened comment but my short answer is : I don't know
From memory, in the UK the frequency offset is 7.5 kHz. So, if three transmitters are required to give the required geographical coverage, one will be on the nominal frequency, one will be 7.5 kHz above nominal and the other 7.5 kHz below nominal. When the transmitters are keyed an audio howl (hetrodyne) will be produced, caused by simultaneous transmission on different frequencies. This is often heard when two aircraft transmit simultaneously on the same frequency. However, with frequency offset transmitters the howl is not heard by the aircraft as the frequencies produced (in this case 7.5 kHz and 15 kHz) are above the upper limit of the receiver's audio passband (normally about 2.7 kHz) and are thus filtered out.
Aircraft VHF radio equipment in the ICAO EUR Region will still be required to be able to tune to 25 kHz spaced channels and receive in an environment which uses offset-carrier systems (the so-called CLIMAX operation). Airspace users should take into account that these offset-carrier systems will continue to be used throughout Europe.
Hello All, Thanks for the replies and I hope your enjoying your new year!
Ok, I am pleased with the response that you have all given including the technical details provided by niteflite01 (I am sure we have met before? ) - Just goes to show after many years of doing the job you can still remember the knowledge!
The receiver i have used is of a good make, and I am sure its nothing todo with it being set incorrectly as after taking your advice i recently tuned into a local ATSU Providing FIS to hear the same aircraft that a normally hear having the same distorted radio quality! It appears, or should that be sounds to me, that the radio or headsets are ''over modulating'' the radio transmission
Anyway thanks for your comments and look forward to any more replies
Edited to remove identity of aircraft
Last edited by PPRuNe Radar : 4th January 2004 at 09:52.
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