6.7390 USB 2330 5 June 2013

[Labviewguru]

Interesting - today at 2330 UTC 6.7390 USB S9 signal, young-ish male Midwestern US dialect.
As I was tuning by he made an announcement but what made me stop was that he had such a great signal with a very pronounced long path echo. He had to be running a LOT of power.

at 2330 he gave a rather lengthy series of alpha numerics, paused, then repeated them. I couldn't write it down at the time.

Frank, South Carolina

[Token]

This station is not a numbers station, but rather it is a transmission, possibly called an EAM (Emergency Action Message), on the HF-GCS (High Frequency Global Communications System) network.  These transmissions are frequently misidentified by people who have never heard them before as a numbers station.  This network is maintained by the US AF, but any US force has access to and can use the HF-GCS.  The “young-ish male” is probably a US Air Force enlisted person.

There are multiple frequencies used by these periodic messages, and typically each message is transmitted on all frequencies simultaneously.  Other freqs to monitor are 4724, 6739, 8992, 11175, 13200 and 15016 kHz (there are others also, those are probably the most active and easily heard in your area).

The signal strength you heard does not necessarily indicate high power.  The network has multiple transmitters around the World, and all transmitters, or selected sub-sets, can be used at the same time.  There are 4 transmitter sites in the continental US, as well as others outside the US, any, or several, of which you might have been hearing.

And that leads us to the long path echo.  It was almost certainly not long path echo.  Two things happen, the same audio is transmitted from multiple locations around the World at the same time.  You might hear more than one site at your location at one time.  So, the propagation time difference between the stations you are hearing will make an echo much like long path.  However, the echos on the HF-GCS network can be greater than the maximum long path time possible, i.e. longer than a complete path around the World.  The longest possible time delay around the World is roughly 134 msec, time delays on the HF-GCS can sometimes be in excess of 300 msec.

So, how might this longer than possible delay occur?  While I am not sure of the entire network infrastructure, I suspect that the audio that is sent to each transmitter station around the World is sent via a network (that part is indicated by documentation in the public domain).  Probably something similar to a VOIP type of a setup.  Network lag contributes to normal propagation distance delays, making longer delays than should be possible.

T!

[Labviewguru]

Thank you for the tutorial. I knew that it was not a "numbers" station, but I didn't see anywhere else to put it. The multiple transmitters site makes sense - I read many years ago that we maintained multiple transmitters for some of this stuff to not only provide wide coverage, but by switching between them to prevent a weapon from "locking on" to them should the need arise.

Do these messages vary in length or the number of messages per day?

Anyway, it was a very neat discovery. I've been a SWL since 1967 and have never heard them. Always something new!

[Token]

In general these messages probably best fit in “Utilities”, like most military comms will.

The multi TX site configuration here is only to give World wide coverage on HF, not for any ECM or anti targeting capability.

Yes, the messages do vary in length, I have heard them with as few as less than 10 characters to as many as over 300.  There is another kind of message referred to in the hobby community as a “Skyking”, and they can be very short.  Also, not all coded messages are “EAM” or “Skyking”, other messages can be similarly coded and sound very much alike, so hobbyist have pretty much just adopted “EAM” for any of these messages that do not contain the phrase “Skyking”.

Also the number of messages a day varies.  They do not occur in any set intervals, and you might have 10 of them back to back or you might have one an hour.

These messages, or similar to them, have been around a long time.  I don’t remember when I heard the first, but I want to say it was back in the Main Sail days, probably late 70’s or early 80’s.  Even the freqs then were pretty close to the same as today.  In the 60’s I remember signals something like this, but different.

T!

[BoomboxDX]

Ah, yes, the 1980's, when the Cold War was still on. 

I remember tuning my DX-440 religiously to 6761 and 11239 (?) and hearing all sorts of "Skyking, Skyking, do not answer" stuff, and imagining some guy in a B-52 somewhere copying a bunch of numbers down.

Also a lot of 'skybird' traffic, routine comms.  It seemed I'd hear something on one of those two channels at least every 5 or ten minutes, some sort of transmission.  I don't know if those sorts of channels are as busy today, as I haven't monitored the military much since they changed everything in the early 90's.  Just lost most of my interest in it.