See a complete view of Mount Gabriel here!

Do you know how many times does a plane changes frequencies when flying from London to New York? How do planes fly over the Atlantic when there is no radar in sight? What is the maximum number of planes that can cross the 'pond' in a day?

It is questions like these that have made me take an active interest in learning about the airwaves. And I am not talking about VHF air comms but HF air as well (I bet you didnt know that planes in oceanic airspace must carry HF comms, did you?).

Secondary Surveillance Radar and                  Backup SSR and transmitting antennas and

receiving aviation antennas and equipment      equipment.

Now when you couple an interest in aviation together with ham radio you have a winning formula. In fact there is nothing better than sitting in a small cottage in the West Cork town of Schull, tuning through the HF and VHF bands with your scanner, and following a familiar flight like Concorde, whose schedule is to the minute and speed is Mach 2, en route from London to New York. And then hear the sonic boom, as it heads out into the Atlantic.

There are 350 flights a day crossing the Oceanic area of Shannon, which stretches from Ireland all the way into the 50th West line.

Over Europe, the average change of frequency from Frankfurt to London is 25 and on the average there is a change every 10 minutes throughout the route on VHF, plus even more changes on HF. There are a lot of frequency changes over the route. Mount Gabriel station, just above this town, is the site of the final radar site before the planes fly into the blue yonder and then they are on their own. For example, a flight coming from London would listen to the weather on 135.375, then get information on 133.075, request ground services on 121.900, clear takeoff with tower on 118.700 and switch to departure radar frequencies of 119.200, 119.500, 127.550 and 120.400, before handover from London to Shannon for Oceanic clearance. Using a scanner and airline timetables, you can monitor how planes cross the Atlantic and how one can monitor the entire trip from start to finish, right from a cottage in the wilds of Ireland.


Planes fly in selected airways, much like highways in the sky, so the flight starting in London would follow the highway called UG1, which would put it over Shannon, Ireland. From here, it would continue to the compulsory reporting point called either GAPLI, BURAK or OMOKO(they all have these 'weird' names), and then switch to HF.

Mount Gabriel tracking station, is the final radar site for the planes heading off or returning to the Americas, controlling airspace from an imaginary line stretching from 49N15W to 54N15W. There are two radars on the mountain. These are called Secondary Surveillance radars. The send pulses which are picked up by the plane's transponder and resent back, much like a repeater sending and receiving carriers. The returning radar pulse has a code indicating the plane's identifier plus the altitude information. Together with the position of the rotating antenna, the exact location of the aircraft can be identified. Because the system is a secondary Radar, called SSR, and because both parts are emitters, planes can be tracked out to a maximum of 250 miles and sometimes 300 miles, which translates to a longitude of about 18 degrees West.

The Radar systems are made by Tompson CSF of France and include powerful parallel computers to analyse the data received. The type of plane, flight plan progress, fuel remaining and destination and speed as well as altitude are all processed by the radar systems in real time. They correlate with the intended flight plan disseminated by the airlines.

In addition, Mount Gabriel Radar site is also the main repeater hub for VHF communnications. Using high quality landlines voice information comes from Shannon, London and air traffic controllers feeding these VHF transmitters.

They are 50 watt, remotely controlled transceivers using 65 db attenuation duplexers to avoid interference with each other. On top of VHF, planes also have HF communications facilities, which are controlled from a site near Shannon Airport. The name of the control center is Shanwick, a conglomeration of Shannon (where it is situated today) and Prestwick (where it used to be).

Over the Atlantic, the only way to QSO is with HF, and due to the unstable nature of weather, frequent reports are made on HF which is a must and will always be a necessity(voice at present which will change into data comms in the future). When a plane reports at BURAK, it is told to switch to a given frequency on HF. This frequency varies according to the time of the year and the time of the day. Much like hams, propagation affects these users as well. Frequencies range from 6622 and 6628, changing to 5649, 5598 and 5616 or 8831, 8864 or 8879 or even 2899, 2871 or 3018 (mainly for the Winter). All are in USB mode.

The last VHF radio control, which is repeated by the battery of Mount Gabriel repeaters, would be Shannon. Frequencies in use are (all AM) 135.22, 131.15. 135.60, 127.90, 121.50 (this is the worldwide emergency frequency) and 135.230.

Recently airlines started using CPDLC (controller to Pilot Data Link Control), a fancy name for satellite data although they all need to communicate to Gander or Shanwick on HF at 30 degrees West.

If you are ever scanning aviation in West Cork and have access to the Mount Gabriel signals, you can track a plane from the time it lives Gander using HF, listen to it at 30 degrees West, pick it up on Shannon aviation frequencies and then follow it to about 150 miles after it leaves the Mount Gabriel sector (all the way to Wales). Pretty good eh?


In a given day, a typical QSO between the control centers and a plane, in this case, American 74, would go as follows:(Shannon/Shanwick control is in Italic, plane in bold)


Shannon, American 74.-
American 74, good day. Your primary frequency will be 5616, secondary 8864. Please call Shanwick now for a selcal check.

Primary 5616, secondary 8864. Thank you and good day.

American then switches to HF, and tunes the loaded vertical whip antenna (which is found at the base of its vertical stabiliser in the back of the plane) on 5616. Shanwick is the last transatlantic HF control station and it is mirrored at Gander in Newfoundland and Long Island, New York.The frequencies are shared depending on the plane's path and can also include Santa Maria in the Azores. All of these stations can be used as propagation beacons, as well as the weather reporting stations at 13264 (London), 13270(New York), 10051(New York), 8957(London), 5505(London) which run 24 hours per day (see sidebar on Weather stations). They all run 10 KW power into omnidirectional, vertical antennas.

Shanwick, Shanwick, this American 74 for a SELCAL check on 5616.
American 74, what is your SELCAL please?
Shanwick, Selcal is Alpha Golf, Bravo Delta.-
(two dual-tone signals are emitted - see sidebar on SELCAL)
Selcal checks, American 74.
American, report position at 15 degrees West, Good day.

As the plane moves and hits the 15 degree West parallel, as indicated by a GPS unit in the cockpit, the pilot would say:

Shanwick, American 74 position on 5616.-
American 74, Shanwick.-
American 74 position: 51 degrees North, 20 degrees West at 0200, flight level 350, estimating 53 degrees North, 30 degrees West at 0256, 40 degrees next, fuel remaining 72.5; American 74.-

Shanwick, copy Gander, American 74, 51 degrees North, 20 degrees West at 0200, flight level 350, estimating 53 degrees North, 30 degrees West at 0256, fluer remaining 72 decimal 6, 40 degrees next.

American 74, checks.

Please call Gander at 40 degrees next, Shanwick.-

At 40 degrees west, the plane will be controlled from Gander, Newfoundland. HF will give way to VHF communicatiions as soon as it gets near to 55 degrees West and continue on VHF, all the way to its New York destination.

Normally, planes are given 5 mile separation over land and 20 miles over the sea. Vertical separation is 1000 feet per alternate directions, 2000 feet for the same direction. With a plane flying at, say 500 miles per hour, there is maximum of 25 planes on the same corridor at similar altitudes when flying the Atlantic, in a given hour. Normally planes fly at either 31000, 33000 or 35000 feet meaning that a maximum of 75 planes can cross the Atlantic at a given time (lower altitudes translate to higher fuel costs). The slot which is used by a plane when crossing the Atlantic is calculated from the moment a plane departs. If there is a delay, the plane would have to wait for an available slot, but as most airlines use conservative schedules, this rarely happens. Normally, you can see planes flying above or below, while looking outside your plane window (this certainly makes you hope the controllers know what they are doing!).

The air communications is not the only thing the Mount Gabriel station offers. It is also the site of a digipeater and the site of a voice repeater. In addition, it is the site of a TV MMDS repeater, Cellular telephone, TV and Radio transmitters Fire, Ambulance, doctor, veterinary and a repeater devoted to the organization involved in the artificial insemination of cattle!

The Digipeater links into the digital packet network of Ireland and can be accessed at 1200 bps on 144.600 (to use it connect to: EI3PKT-9). It can be accessed from an aeronautical mobile location, as far out as 16 degrees West (about 220 miles) from a height of 26000 feet. For those flying Lear Jets (the lucky very few) at 45000 feet, it can be accessed out to about 19 degrees west(more than 300 miles). Maritime mobile stations will have to do with 40 miles out maximum. The digipeater is composed of a Paccom packet unit interfacing to a Yaesu FT-2500M VHF 2 meter radio connected to a 5/8 vertical antenna (see the black tube in the center of the picture with the white tip).

The digipeater will be joined (hopefully by the time you read this) by a voice repeater, at 145.775 (-600KHz), the Westermost repeater in Europe, having a 10 watt output, also feeding two 5/8 antennas, one on top of the other (to give the minimum of interference). The voice repeater, will have caller report whereby users with registered DTMF ids, will have their ID recorded and made available to other users, similar to the MHEARD command on packet. The repeater will also be able to be accessed from about 19 degrees West. In common with many other Irish repeaters, this repeater is open (no 1750Hz tone needed) - only Ireland and Greece have open (ie no 1750Hz needed) repeaters. One of the features (switched usihg DTMF tones), would be to link this repeater with the repeater in Cork city, about 60 miles away, thereby giving wide coverage.

On a good day, the Mount Gabriel site has unprecedented views all round the peninsula. The infastructure in place is being used to offer Aeronautical mobile and Maritime mobile hams facilities intended to make their hamming more enjoyable than ever. We hope it will be a useful addition to the ham traveler's arsenal. Enjoy!


Selcal (Selective Call) was developed in the late sixties as a means of indentifying aircraft automatically when flying on HF (where the USB nature of communications makes automatic selcalling difficult). Even today, HF is used over the oceans, over Africa, Asia and parts of the Americas.

Selcal works by sending two, two-tone signals, one after the other, chosen from a list of frequencies in the audio range. The frequencies (see list) are not harmonics thereby reducing interference. The SELCAL is send using Single SideBand, Full Carrier AM. Every aircraft radio set contains an USB transmitter as well as an AM receiver in the same package. The audio signal from the AM detector is connected to a decoding network that uses PLL (Phase Locked Loops) to identify the signals coming in.

The original tones chosen for SELCAL were from a list which would indicate the tones available just from their separation! This means that if the separation was say, 145.7 Hz, the only two tones that could do this, would be Bravo and Delta. Many modern SELCAL decoders use DSP circuits to decode the codes, without the need for PLL.

The use of frequency independent selcal (since the separation of the two tones is all you need to identify them) is opening new ways of featuring automatic code transmission on HF. You could link an autopatch via selcal on HF (with a ham standing by to comply with FCC rules) and translating code letters to numbers. For example to send the number 555-1212, you could use Alpha Foxtrot, Alpha Foxtrot, Alpha Foxtrot, Alpha Bravo, Alpha Charlie, Alpha Bravo, Alpha Charlie, Alpha Papa.

SELCAL Frequencies:

•Alpha 312.6Hz

•Bravo 346.7Hz

•Charlie 384.6Hz

•Delta 426.6Hz

•Echo 473.2Hz

•Foxtrot 524.8Hz

•Golf 582.1Hz

•Hotel 645.7Hz

•Julliet 716.1Hz

•Kilo 794.3Hz

•Lima 881.0Hz

•Mike 977.2Hz

•Papa 1083.9Hz

•Quebec 1202.3Hz

•Romeo 1333.5Hz

•Sierra 1479.1Hz


Airplanes flying long distances often make use of Weather (WX) stations trasmitting information, 24 hours per day, on HF frequencies. When you fly London to New York New York weather is brought to you, courtesy of HF comms. These can be excellent propagation beacons for radio hams in the bands of interest.

WX stations operate on the 13, 10, 8 and 4 MHz bands, 24 hour per day, worlwide. Power is nominally 1, 5 or 10KW depending on location. They share frequencies but use straddled timing. The following is a list of the main weather stations around the world, broadcasting on HF frequencies using USB:

Info is given as transmitter location and minute past hour when broadcast starts. All broadcasts in English unless noted

PACIFIC - Frequencies: 2863 6679 8828 10048 13282


H+00 and H+30

Tokyo Radio

H+10 and H+40

Hong Kong Radio

H+15 and H+45


H+20 and H+50

ATLANTIC - Frequencies: 3485 6604 10051 13282

New York

H+00, H+05, H+10, H+15 and H+30, H+35, H+40, H+45

Gander, Canada

H+20 and H+50

EUROPE - Frequencies: 3413 5505 8957 13264

Shannon, Ireland

H+00 to H+25 and H+30 to H+55

MIDDLE EAST - Frequencies: 2980 5575 11391

Ben Gurion, Israel

H+05 and H+35

SOUTH AMERICA - Frequencies: 2881 5601 10087 13279

Rio de Janeiro

H+00 and H+30

Lima, Peru

H+10 and H+40

Brasilia, Brasil

H+15 and H+45

Buenos Aires

H+25 and H+55

SOUTHERN AFRICA - Frequencies: 2860 3404 5499 6538 8852 10057

Johannesburg, South Africa

H+00 and H+30

AFRICA - Frequencies: 3047 6716 9026


H+00 and H+25


H+05 and H+35

Brazzaville in French

H+30 and H+55

WEST ASIA - Frequencies: 3001 5561 8819


H+00 and H+30


H+05 and H+35


H+10 and H+40


H+15 and H+45


H+20 and H+50


H+25 and H+55

EAST ASIA - Frequencies: 2965 6676 11387


H+00 and H+30


H+05 and H+35


H+10 and H+40


H+15 and H+45


H+20 and H+50

CARIBBEAN - Frequencies: 2950 5580 11315

Port of Spain, Trinidad

H+00 and H+30


H+10 and H+40


H+25 and H+55