Receiving HF Air Band With An RTL SDR Receiver - SELCAL

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Here I take a look at receiving HF Air Band broadcasts from Ground Stations to International Bound Aircraft.

We also discover a data packet which is a SELCAL test. More information on this below.

Sorry for the video being mainly me talking, but I had to provide an explanation of what we were listening to.

I did NOT cover this software in this video but may do so in a future video.

[new_comment]

The controllers are actually in an, office if you will. Usually remote from any airfield. I listen to the HF comms on my Amateur HF base radio, while live tracking the flights usually using Flight Radar 24. It's really cool to be watching the tracked flight on screen, out over the Atlantic Ocean. While also listening to them communicate with the center, on your transceiver. I get to hear most of the traffic on 3.455 mHz USB. So far, every frequency I've monitored on, they're always using USB.

[bulldogbrower]

For all of those radio heads out there. Back in the late 60’s and 70’s Johnson the manufacturer of fine CB radios incorporated a select call system on a couple models. It delivered a tone and lit a call light on the radio. There were a number of different frequencies that could be selected so you didn’t get another persons call. It was kind of unique, and not widely used. Very similar to the aircraft HF system in that it was audible and lit an indicator to alert you someone was calling you individually.

[nvca]

Thanks for sharing - great signal to noise for an 8 bit dongle. I always thought the ATC had the selcall audio generator connected to the same SSB transmitter he is talking on, but as you can see the selcall seems to come from another transmitter that intentionally also sends out a strong carrier as a reference to compensate for any RF frequency offsets. Or maybe the operator does not use a standard radio, he uses a special one where you can temporarily disable the carrier suppression while sending out the two pairs of audio selcall tones.

[JohnHill-qohb]

You are close with your idea of operation. The HF radio volume is turned to the comfortable listening level, the radio's squelch is adjusted to turn off the radio audio. The squelch is connected via the SELCAL decoder. When the correct tone sequence occurs, the SELCAL decoder opens the squelch so the crew can hear who is calling. Only that aircraft receives the message or those aircraft with the same SELCAL code. Air band HF frequencies are limited, ie not enough band space for all the AC in the sky, SELCAL is a way of allowing more AC in a limited HF band space.

[CD-ortv]

All correct. However, in the age of increased reliance on the satellite based communication network, we no longer maintain a listening watch over assigned HF frequencies, instead relying primarily on datalink (a requirement when operating within the upper levels of the North Atlantic route structure). Anything else is in the interest of communicative redundancy.

Once contact with the respective oceanic sector is made, the assigned frequencies are retained, but relegated to standby in favor of quieter VHF air-to-air frequencies. If HF voice is again desired, a SELCAL query is sent and yes, both an illumination and audible tone are received on the flight deck.

Since ground to air is primarily direct, those of you further inland will likely be unable to creep on oceanic communications. Sorry.

[linuxro]

So,
SELCAL alerts aircraft's crew that a GRS wants to communicate with aircraft.
SELCAL uses a ground-based encoder & radio transmitter (VHF or HF) to broadcast audio tones picked up by decoder & radio receiver on aircraft.
HF radio has high levels of background noise and is difficult to listen to for long periods of time. Crews keep radio volume low. A SELCAL notification informs crew they are about to receive a voice transmission, so they have time to raise volume.
Each airline assigned SELCAL codes. Operator enters 4-letter SELCAL code at SELCAL encoder which converts it to audio tones, broadcast on frequency in sequence: the 1st pair of tones are transmitted simultaneously, lasting 1 sec; silence of 0.2 secs; followed by 2nd pair of tones, lasting 1 second.
When SELCAL decoder on aircraft receives a signal containing its own assigned SELCAL code, it alerts crew by sounding a chime, activating a light, or both. Crew turn up volume to hear the incoming voice transmission.
Each letter designates a specific audio tone frequency. Letters within pair are transmitted in alphabetical order (e.g., AB-CD is an allowable distinct SELCAL code, as is CD-AB, but CD-BA is not). A letter is used only once in a SELCAL code; (e.g., AB-CD is allowable, but AA-BC & AB-BC are not).

[SpaceCaptainDR]

As an overwater pilot and ham radio guy, this is the most interesting thing

[BritishBeachcomber]

I've often heard HF air band in the uk from as far away as Africa.

[MatthewHearne]

Found this really interesting, thanks. Would love to know your antenna setup for receiving that? I am really limited on space, so not sure what sort of antenna I could rig up for HF listening.

[badthebone]

hi need help i see may local airport tower on the waterfall but on audio it i can here the statice but no voice

[brianbrown]

i used to hear it on 5.56 mhz usb Shanwick Ganda etc. bit not heard it for a long time now

[NicksStuff]

The squelch doesn't make it bearable, though?

[nowaymuller]

You can receive it maybe becouse you are from UK? I am here in the Middle of Europe sourroundet by Mountains and receive nothing on the HF Air Band.

[Marg]

I would love to become an aeronautical HF radio operator, but I'm afraid that such radio would no longer be used and the radio offices would no longer be hiring.

[crazynordlander]

That was interesting. Not sure that I could pick that up here in Minnesota or not. What do you use for an antenna when you are listening with a RTL SDR dongle?

[paulbaker]

Do I need a virtual cable to listen for any communications, I Know it is a late question, and I am just getting back into ham and these SDR LTD sticks .

[brianbrown]

you nust ahve a good antenna to hear this

[electronicsandradio]

As much as I disagree with it, I dont believe you're allowed to disclose or rebroadcast any radio received transmissions that were not intended for you. It's part of the UK wireless telegraphy act. Probably why we can't log in and listen to aircraft radio traffic as they do in the US.