VHF receiver sites at airports

[Mooncrest]

Technical question. In my limited experience the radio receivers are sited remotely from the control tower in order to minimise interference. My (probably silly) question is, how does the sound from the receivers reach the voice switch in the tower ? Is it automatically retransmitted on a discrete UHF frequency or is it simply underground copper or fibre-optic cables ? If the latter, are the cables in tunnels or simple conduits ? How are they accessed for maintenance ?

Genuine questions for a genuine interest folks. I'm not about to go on a wrecking rampage with a pick axe. Thankyou.

[HEATHROW DIRECTOR]

A lot of ATC transceiver antennas are mounted on the tower. In other cases, transmissions usually travel by cable to a remote transmitter same as received signals from remote receiver sites are sent to the tower. Transmitters and receivers for ATC centres are often hundreds of miles away and I believe that microwave links are employed but I stand to be corrected.

[Mooncrest]

Thankyou HD. At my local, EGNM, the distance between the receiver site and the control tower is probably no more than a quarter of a mile, if that. I am certain there are no microwave links involved - I don't remember seeing any parabolic dish antennae recently !

[chevvron]

I think you'll find that BT make lots of money renting telephone lines to the airport operators for use between the control tower and the Tx/Rx site. These might be copper lines but they may have been replaced by fibre.
At one time at Farnborough, our main Tx/Rx site was at Ewshot, about a mile south west of the airfield. For a short time, our techies tried a microwave link rather than RAE having to pay BT lots of money (they even rented cables to operate traffic lights on the airfield from BT) but it was quickly found that heavy rain, not even torrential, was sufficient to block the signal, so we reverted to cables.

[Mooncrest]

Thankyou chevron. So it seems that cable is the way and BT might profit from it. At EGNM, the receiver site is within the airfield boundary so I wouldn't have thought BT would benefit, unless they provide technical expertise. Do you know how the cables are maintained or replaced ? Access must be an issue; again, at EGNM, there is a runway between the receivers and the tower which rather limits scope for digging holes and trenches !

[Glamdring]

At my unit there are 2 separate fibre links, as well as a backup Microwave link for redundancy.

[HEATHROW DIRECTOR]

When I left school in 1961 I became a lowly-paid civil servant in the MoA. One of my tasks was to sort out Post Office bills for approval by Tels engineers. The bills were for various landlines at airfields including those twixt ATC and TX stations. They also included bills for lines from TWR to APC, just downstairs. I imagine things have changed now.

[chevvron]

During a radar re-fit programme back in '91, we wanted to use the RAF's mobile Watchman radar whilst our new one was being installed. The rental for temporary lines (which had to be fibre optic as the Watchman won't interface with copper cables) for 6 weeks would have been over £10K, so we used an alternative way of keeping going.

[Mooncrest]

Sounds like a licence to print money for BT. I wonder if these phone lines, whether fibre-optic, copper or co-axial would be direct point to point or routed via a telephone exchange.

Each question answered raises another!

[Mooncrest]

Forget co-axial. We're not talking about radar data here.

[JuicyLucy]

Simple copper wires on a loop to provide isolation and redundancy.
Can carry audio, control, data. If on site and owned by the airport then no bills, but off site a different story...
We were still using some cables laid in WWII at various airports when I worked as a Tels engineer !
Now being replaced with fibre and complex interfaces....

[KelvinD]

It all depends on where you are.
In Saudi Arabia, Jeddah FIR had transmitters and receivers located in the tower base for local work. There were also transmitter/receiver facilities as far north as Tabuk, Riyadh to the east and Abha in the south. As these were huge differences, tropo scatter was used for the long haul, with microwave links feeding the tropo sites.
As this was in the 70s & 80s, the microwave/mux systems were analog, no opportunity to use digital signalling, so equipment status and control was achieved by sending tones up and down the links. Radar data from the distant sites at the edges of the FIR used the same links with massive Racal modems beavering away at 9600 baud! (The really clever bit was the way it was all assembled in Jeddah so a complete radar picture of the whole Kingdom as available for the scope dopes.)