Actually, sounds like one of the polar ionosphere 'radars' with an unusually high sweep rate. Normally, as you know, they sweep a LOT slower, but, I have seen them fairly fast before well below 10 meters ... a couple years back I noted the 'sweepers' active between 27.5 and below 28 MHz ...
The ionospheric radars you describe most often use an FMCW waveform, typically LFM, although not all of them, the SuperDARN, for example, uses a simple pulse.
The slower sweep rates of the ionospheric radars are specifically chosen to work with the anticipated ranges targeted by the radar. The repetition rate of an uncoded sweep (or uncoded pulse, for a pulse type radar) sets the maximum unambiguous range.
However, stepping outside the area of ionospheric radars, to HF radars looking for missiles, ships, and aircraft, the LFM / LFMCW waveform is still the most common used. For a variety of reasons simple pulse type systems are a minority on HF.
This radar could indeed be swept vs pulsed, I do not have enough information to tell for sure. The shape of the spectrum does not support a rectangular pulse transmission, but it might support a shaped pulse envelope to reduce bandwidth (as is done with the pulsed SKiYMET HF Meteor radar, there is a video on my Youtube channel if you are not familiar with that radar). By the same token the spectrum does not really support an FMCW transmission either, I would expect that to be more flat topped, however every time I have received it the SNR has been too low for me to place a lot of confidence in that. I simply don’t know if it is swept or pulsed, but if it is pulsed it is a shaped pulse.
The ionospheric radars and “real” radars (looking for aircraft, ships, etc) can use identical waveforms, and it can be extremely difficult or impossible to tell them apart. Some are on continuously (such as the British PLUTO radar) and others jump around, frequency hopping and hitting different frequency ranges to tailor the illumination area. If you have not seen and identified a specific radar or sounder before sometimes the best you can do is make a “best guess” at what it is. In general if I see something hitting a specific frequency range once every 5 minutes or less often I think of it as a probable ionospheric radar. If I see it revisiting a frequency range every few seconds I think of it as a probable radar. And just to show there is always an exception, the SuperDARN radar is on nearly continuously, frequency hopping around a fairly confined range of frequencies (although were in the spectrum this range in varies with conditions). And the SuperDARN is looking at atmospherics, not man made targets.
As a practical matter, a bistatic CW RADAR can (and does) operate at a LOT lower RF power level and can do so on account of the BWs (bandwidths) involved, what with noise power being proportional to BW and all ... meaning short PW radars require a LOT more power on account of maybe a 1 MHz or 5 MHz occupied BW ... I would expect to see simple 'carriers' as opposed to sweeping for a simple Bi-static CW detection radar ... but then one enters into the area of FM or 'swept' radars and a two (or bistatic) site setup, where the range can now be calculated knowing when the sweep started ... unambiguous range being associated with the starts of successive sweeps ...
Pulsed radars with a BW of 1 to 5 MHz (3 dB points, approximation BW@3dB point ~ 1/PW, this is not exact at all, but gives a rough working number) would have pulse widths from about 200 nanoseconds to about 1 microsecond. While that kind of pulse width is common at microwave frequencies I don’t think I have ever seen an HF pulsed radar with such a short PW. Again using the SuperDARN or the HF TIGER as an example of HF pulsed radar those systems use a 300 microsecond pulse, or about 3.3 kHz of BW (3 dB). The old Russian Woodpecker used a 3.1 millisecond pulse, and should have had about a 322 Hz BW (3 dB), however the Barker coding used caused this pulse transmission to not exhibit the typical sin (x) / x shape of a simple rectangular (or trapezoidal) pulse.
Not 100% sure I understand what you are getting at with the rest of that paragraph, so if I have misunderstood forgive me.
Pure unmodulated CW radars are rare on microwave frequencies (outside of things like speed guns and as missile illuminators), and I don’t think I have ever seen or heard of one on HF, at least not inquite a while, maybe not post WW II. An unmodulated CW radar (either semi-monostatic or bistatic) can only yield two things, bearing (with a directional antenna and something like a monopulse feed) and radial velocity based on Doppler. It cannot yield distance or any target detail. They do bring to the table high average power on the target and the ability to notch out non-moving objects, a good form of MTI if you will. By using a source / transmitter with low phase noise, and a notch filter with steep skirts on the CW transmitted frequency, your ability to discriminate slow moving targets from the clutter is limited by the notch width and the spectral purity of the transmitted signal. You can detect anything with a radial velocity relative to the transmitter that is great enough to produce a Doppler shift outside the notch filter.
HF FM radars can be either bistatic or monostatic. Bistatic is most common because it is easier to do, easier to protect the receiver from the transmitted energy and to notch out the transmitted frequency, resulting in lower Doppler target detection. And although it is not very common, you can encode the swept FM so that you can detect nth time around targets, extending the unambiguous range while not requiring slower sweep rates.
Unless this radar is used constantly, it's pretty useless as a detection radar as well (right?). The angular resolution would be poor at 27 MHz ... multiple receivers could yield position much as a GPS receiver computes a position looking at the Doppler shift from GPS several birds and working the 'math' for a fix. What does not figure is the sweep, unless it is a simple two-site only bi-static detection FM (swept freq) radar which then gives range resolution with some sacrifice of Doppler speed determination (really useful only for known "to or from" radial courses anyway).
The radar can be seen for hours on end, and the burst rate seen is high enough for good detection of targets. It is a much faster burst rate than something like the US ROTHR (AN/TPS-71 radar). This suggests, to me, the Iranian radar is looking for something that moves faster than the ROTHR looks for.
The angular resolution does not have to be poor at 27 MHz, we have no idea of the size of the array. It is quite possible to get reasonable beam widths at such frequencies, it just takes a larger aperture. And it also depends on what their spec is, what is desirable or acceptable for resolution. A res of 3 or 4 degrees might be all they need for their application, which is possibly to queue another system.
What I find interesting about this radar is the combination of frequency and the repetition rate. As we both indicated above, the rep rate (be it pulsed or swept) limits the unambiguous range of an unencoded pulse. Although I have not been able to look at the signal in detail I have not yet seen anything leading me to believe it is encoded. If it is not encoded we can look at its PRI’s and calculate a maximum possible usable range. The two PRI’s in my video are roughly 872 Hz and 308 Hz (I did not actually measure them, just eyeballed the spikes on a spectrum display, but should be close). 308 Hz would yield a maximum possible range of about 487 km, and 872 Hz would be about 172 km. Both of these ranges seem odd to me when combined with the observed frequencies, I just do not normally think of the 25-30 MHz area as a good choice for 170 to 500 km paths, not when 4 to 10 MHz will probably do it better. I suppose this range would be good to detect objects high in the Earth’s atmosphere, so that propagation becomes less of a player, but then why not move up in freq and shrink the antenna? Or maybe I am missing something about this radar, having not really seen it very much at all.
T!
Topic: Iranian Radar 11 meters 27 Sep 2013 (Read 8256 times)
