SSB & SELCAL

[Hawker]

I've read the theory behind them in the AIM and Aerocourse books, I'm just wondering if someone could please explain a brief general description of what they are and how these are used in day to day operations!
Thanks!

[Rockie]

High Frequency (HF) radios are lower frequency than VHF which is basically line of sight with an effective range of about 200 miles. The lower frequency of HF allows much longer range communications pretty much world wide depending on conditions and power of the radio. Aircraft use it to communicate with ATC on oceanic routes or up in sparcely settled regions like the arctic/siberia etc. Satellite communications are slowly taking its place, but it is expensive so the HF is still extensively used.

HF is sensitive to atmospheric disturbances (thunderstorms) and especially solar activity which can render it almost useless. It usually has a lot of static on it, but in some regions like the pacific it is actually pretty clear.

SELCAL is a method used by a ground station to "dial up" a specific airplane via a discreet four letter code assigned and hard tuned to that specific airplane (like VA-SM), that sets off a tone or other device in the cockpit indicating to the crew that someone is trying to contact them on that radio. SELCAL is normally used during HF communications so the crew doesn't have to listen to all that background static.

[Dagwood]

SELCAL is also used on VHF company frequencies as well. Flight123 doesn't want to listen all day to how many dinners Flight234 needs or why pilot in Flight567 needs to talk to maintenance. Also, for HF with SELCAL, with the long range there would likely be a lot of talking going on as it picks up a lot of stations.

SSB from Wikipedia. Basically, the way I understand it works, only half of the signal is sent (either the upper side band, or the lower side band). This allows more power to be available for the transmission. The receiving radio puts the signal back together for the listener.

Maybe someone has a better explanation?

[Spokes]

Pretty close explanation. In AM radio, a voice signal - typicaly 3khz bandwidth is modulated onto a carrier, In HF say 5MHz or 5000KHz. When this is done, the electronics produce an output that includes the carrier (5000KHz) and both the sum and the difference of the carrier and the intelligence (voice in this case). So you would have a signal in my example that has a 6KHz bandwidth covering a frequency range of 4997 to 5003 KHz (or 4.997 to 5.003 MHz). It takes about 2/3rds of the transmitted power to send the carrier, with the rest of the available power devoted to the intelligence part of the signal. Since this sum and the differenceparts are the same information, it is only nessecary to send it once, hence only one of the the sidebands (usually the upper side band) is transmitted. Since the carrier itself has no information in it, it too does not have to be transmitted. That just leaves the single side band (SSB) ranging in frequency in my examply from just above 5000KHz to 5003 KHz. The carrier is added by the receiver for demodulation of the signal.

Now, wait you say. The human hear can hear from 20-20KHz so the voice signal should cover this bandwidth. Well this is true, and in Hi fidelity FM radio this is done. But in reality, an inteligable voice signal can be sent with a 3000Hz bandwidth and be easily understood by the human ear-brain combination.

The talk of noise on the HF is oh so true. Having spent several years as a radio operator on military Tracker aircraft, I can tell you- we sure wished we had selcal.

Hope this helps

[ettw]

SSB is predominatly used for passing time listening to CBC, BBC, VOA, and Radio Mumbai. Seriously though, we still use it in the north as do the oceanic flights. My preference was to use HF over sat phone cause the HF was free.

Best range I've heard of was about 8000 miles. From Patriot Hills, Antarctica to Iqaluit, Nunavut.

The SELCAL is a 2 tone code sent by the station to the aircraft. You can hear them quite clearly on frequency. You will generally also hear aircraft requesting a test of their SELCAL. They give the ground station the aircraft 4 letter ident and a moment later you hear to distinct tones. Those tones trigger the cockpit annunciation.

Cheers,

ETTW

[Hawker]

Thanks guys that gives me a better idea as to how it's used. One more question though, where does each aircraft's four letter ident come from for the SELCAL? I don't suppose the last four of the aircraft's reg would be used?

[Rockie]

Thanks guys that gives me a better idea as to how it's used. One more question though, where does each aircraft's four letter ident come from for the SELCAL? I don't suppose the last four of the aircraft's reg would be used?

For more than you'll ever need to know about SELCAL see here:
http://www.selcalweb.co.uk/faq.html

Question six is what you're looking for.

[Hawker]

That's perfect thanks Rockie!

[ahramin]

Major Kong, I just got a message on the CRM-114

[The Old Fogducker]

Spokes:

Good on you man ... not many on here have the slightest clue of what you described, but I sure did, and I say ...

73 de a VE4 that operates Collins, Icom, and Racal equipment.

[Spokes]

Thanks odf. I used to be alot better at explaning things.

I still cringe when I here about HF radio waves 'bouncing' off the ionosphere. Makes me want to cry.

/k

[niss]

Thanks odf. I used to be alot better at explaning things.

I still cringe when I here about HF radio waves 'bouncing' off the ionosphere. Makes me want to cry.

/k

Does refracted make you feel better?

DE VA3UBC

[Spokes]

Thanks odf. I used to be alot better at explaning things.

I still cringe when I here about HF radio waves 'bouncing' off the ionosphere. Makes me want to cry.

/k

Does refracted make you feel better?

DE VA3UBC

R+