I missed this posting when it was first up in August.
This is a signal I have been seeing and paying attention to for 6 or 8 months. I see it very seldom, I mostly only see it during the week, and I mostly only see it during what would be considered the work day in North America.
6970 kHz is just one of the frequencies it hits. Most commonly I see it start low (say around 4 MHz) and work its way up in freq, spending anything from a few seconds to several minutes on each freq. I have seen it go as high as 28 MHz.
The transmissions are not all the same, but follow a few basic formats, with variations.
Often it starts with a simple carrier, sometimes there are amplitude variations on the carrier that look like a signal being peaked or tuned. Sometimes a very shallow AM modulation follows the carrier, say 4.2 kHz and 10% or less modulation depth. It typically goes to the pulsed signal just before it leaves a specific frequency. The time it is in this pulsed mode can be very short, several seconds, to several minutes.
During the pulsed portion of the signal the pulse duration is generally around 830 microseconds in length and the PRF (pulse repetition frequency) is generally about 303 Hz. Because of these numbers, particularly the pulse width, it is unlikely to be a radar of any kind.
Such a pulse width would give a range resolution of about 125 km (since there does not appear to be any pulse compression), or two targets would have to be 125 km apart before they could be distinguished as two targets, and not just one. While the maximum range that PRF allows (since there does not appear to be any pulse coding) is about 495 km. So, if this were a radar it would be one that could only see 495 km, but not tell two targets 120 km apart from each other. Not very likely.
It just so happens I have several pictures and a video online of the signal on 6970 kHz, and just a week or so before your report. I have other frequency SDR recordings and pictures, but the 6970 kHz is the ones I put online.
Video here:
http://www.youtube.com/watch?v=3AIZLc-ZqW0Image here, annotated to show the spectrum of the pulsed transmission. The spectral purity is pretty good indicating a pulse with a good rise and fall time.
Unannotated image here:
I have not seen this signal in a couple of months, and I look for it regularly. Tomorrow (today is my Friday, so the start of my weekend is tomorrow) I will have to look for it again.
T!
