ATCC Question

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ATCC Question

Postby Xr3iste » Mon Feb 04, 2019 10:58 pm

Im new to this fascinating pastime, and with guidance from the members on here, I am slowly getting to grips with the basics.....However im slightly confused with the numerous ATCC frequencies.
As far as I have learned.... ( in lamens terms) the ATCC controls airspace between a lower and higher altitude, and it has various different frequencies. There are TWO main centres, Prestwick being the Northern and Swanwick being the Southern.
My initial question is Are these frequencies/broadcasts "repeated" across a network of transmitters such as KELSALL ?

Am I anywhere near correct in my thoughts :embarrased: :embarrased: :embarrased:
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Re: ATCC Question

Postby Chris P » Tue Feb 05, 2019 10:15 am

As I understand it whilst the controllers are in Swanwick and Prestwick the transmitter sites are located in the areas that they cover and each transmitter site will have transmitters on the relevant frequencies for example Chedburgh has transmitters on the frequencies for the north sea and Clacton sectors whilst Kelsall will have transmitters on the Wirral and lakes sectors . As far as I am aware only one transmission on one transmitter is used simultaneously by one controller but the controllers can use different transmitters if they require to eg if there is a transmitter failure an alternative may be used.
Only broadcasts ( that is transmissions intended for anyone who can hear them not just an individual aircraft )will be transmitted simultaneously on more than one transmitter . Examples of this are volmet transmissions which were transmitted on several transmitters albeit on slightly offset frequencies ( it used to be with plus or minus 5Khz or 7Khz offsets ) at the same time. The offset reduces the likelyhood of interference but with 8.33Khz channel spacing receivers now in use I am not sure whether this may cause problems now or even if it has improved the arrangement as the new receivers will see the offsets as being different and only one transmitter will be received .When the wider bandwidth Rxs (25khz spacing) were in use the two transmissions would not cause beat ( heterodyne) interference as the interfering signals were out of the normal audible range of the listener and so could be "read" .

The reason why two transmissions on the same frequency simultaneously are not used is that if both transmissons can be heard by the aircraft using AM (Amplitude Modulation ) it will cause an audible beat ( heterodyne) as the two transmissions will be on slightly different frequencies due to transmitter inaccuracy or doppler shift (if one transmitter is moving) , severe distortion and interference making the signals unreadable . EG if two aircraft transmit at the same time the ground station will not be able to read either or both which often results in the controller requesting one station or both to repeat the message as he was "stepped on" .

on FM or NBFM systems only the strongest signal will be heard which is known as the capture effect and is used to allow multiple simultaneous transmissions on physical / geological separated transmissions without detriment to reception by the intended receiver . examples of this can be heard on the VHF marine band where simultaneous transmissions on the same frequency are used to give greater coverage of say a weather information broadcast .
Regards Chris aka G8FFF nipper or tazmin88
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Re: ATCC Question

Postby Minus1 » Tue Feb 05, 2019 10:20 am

Some are transmitted from multiple sites in order to cover a large area.
A good example in London Info West on 124.75.
It definitely comes from Clee Hill and Ventnor; and probably Davidstow Moor.
In such cases, two sites will be offset by +5kHz and -5kHz.
Three sites will be offset by -7.5, 0, +7.5 kHz.
I've not yet found out what the equivalent offsets might be for an 8.33 kHz channel.

Others come from one site only.
Frequencies covering upper airspace (because the aircraft are much higher and can receive from further away) tend to be transmitted from a single site.

Sometimes one controller can handle multiple sectors (especially if they are not busy sectors), and those sector frequencies re linked by 'bandboxing' them at the control centre.
I reserve the right to ignore people who have made no attempt to the read the manual, and expect others to do it for them.
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Re: ATCC Question

Postby Chris P » Tue Feb 05, 2019 11:09 am

On military bases that use both UHF and VHF airbands the controllers often band box the two bands, for example either the approach or radar or director VHF and UHF frequencies under one controller will be band boxed which has the additional advantage in that aircraft on the UHF band will know when the controller is in contact with aircraft on the VHF band and will therefore know there are other aircraft in the vicinity even when they may not have TCAS or be able to see the other traffic .
Regards Chris aka G8FFF nipper or tazmin88
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Re: ATCC Question

Postby rozbiff » Wed Feb 06, 2019 11:32 am

You mention Kelsall so I guess you are 'close' to that.

An example of an offset frequency would be 134.425 which is transmitted from Kelsall at a frequency of 134.430 (a +5kHz offset as mentioned by Minus1)
(NB. Frequency, not channel designator)
Another Kelsall example would be 277.625 which is transmitted from there at 277.630

The control centres are offices. Swanwick houses the London Area Controllers, the London Terminal Controllers and the Military Controllers, Prestwick houses the Oceanic Controllers, Manchester Area Controllers and the Scottish Area Controllers. All radio traffic is routed to the appropriate transmitter site for the aircraft

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Re: ATCC Question

Postby radiostationx » Wed Feb 06, 2019 11:58 am

Quite often same NATS controller will be on kelsall 135.580 ,132.860 and Preston 133.705 and simultaneously.
For some reason the 135.580 is the best reception for my location by far even though the 2 kelsall freqs 135.580 and 132.860 are txd from same site. Directional antennas perhaps ?
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