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Author Topic: OTHR signal 22774 KHZ USB 1537 UTC 12/05/2014  (Read 2700 times)

Offline ka1iic

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OTHR signal 22774 KHZ USB 1537 UTC 12/05/2014
« on: December 12, 2014, 1551 UTC »
OTHR signal 22774 KHZ USB 1537 UTC 12/05/2014





I've added this one because the audio was great because I got a number of different pulse rates on this single frequency with a low noise level.
« Last Edit: December 12, 2014, 1613 UTC by ka1iic »
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Offline Token

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Re: OTHR signal 22774 KHZ USB 1537 UTC 12/05/2014
« Reply #1 on: December 13, 2014, 0117 UTC »
While it can sometimes be very difficult to tell a sounder from a radar it is pretty safe to say this one is probably a radar.  But which radar is anyone's guess.

Let me step back a second, many sounders are indeed radars, for example the JORN radar has a "miniradar" (a sub set or sub waveform of the main radar) backscatter sounder.  But for the purposes of this discussion lets say a radar is intended to track things, ships, aircraft, missiles, waves, ocean currents, etc, and a sounder is intended to measure / map / plot the ionosphere.  And sounders can use identical waveforms (transmissions) as radars, although they are generally modified in some way.  For my description lets say that a radar tracks something, almost anything, other than the ionosphere or propagation.  And a sounder looks at / measures atmospheric aspects that might impact propagation, including path loss and ionospheric responses.

So, based on how often this signal transmits it is probably a radar.  The atmosphere changes slowly, so there is little need for a backscatter sounder to transmit as often as this signal.

From the recordings the signal is probably LFMCW, Linear Frequency Modulated Continuous Wave.  I say probably, because the recorded audio bandwidth is too narrow to be sure.  The bandwidth of the recording appears to be about 3.2 kHz, and in that 3.2 kHz the signal is linear, but this bandwidth is not wide enough to show the start and stop of each sweep.  LFMCW means that during each transmission the signals sweeps linearly in frequency (in this case low to high) and cycles back to the start freq without interrupting the transmission.  So it cycles low to high, low to high, low to high, etc, for the entire transmission.  If the signal is not LFMCW it is LFMOP.  LFMOP is Linear Frequency Modulation On Pulse.  This means that each sweep is actually a pulse, and that during the pulse the frequency is swept linearly.  So it would be low to high, stop transmission, pause, low to high, stop transmission, pause, low to high, stop transmission, pause, etc.

Most HF radars are FMCW, but a few are FMOP. 

Why does it have different sweep rates (changing pitch for each transmission)?  There are several reasons, the most probable in this case is to eliminate blind speeds.  Blind speeds are speeds (radial velocity) of targets that, for a given sweep or repetition rate, cannot be detected because of a mathematical relationship of that target radial velocity to the repetition rate.  For a specific sweep rate (and pulse rate for a pulsed radar) there are multiple blind speeds.  By changing the repetition rates of the radar you can make sure to always be able to see all speeds, eventually, even if not in one specific transmission.  With changing repetition rates the blind speed set for the first transmission will not be the same as for the next transmission, or the next after that.

As I said above, this is probably a radar, and not a sounder.  But to get more precise than that, which radar it is or where it might be located, we do not have enough information.  There are so many similar radars in use that narrowing it down a bit is pretty much not going to happen.  A bearing would help to point to a portion of the world.  Total swept bandwidth is a parameter that can be used to match the signal to known systems.

But even then the source would most likely be just an educated guess.  Many HF radars are very similar, and few of them are well documented with regards to specifics of the waveform.  There are a few HF radars that are very distinctive, but they are a minority.

« Last Edit: December 13, 2014, 0126 UTC by Token »
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