3 ms·

Out of curiosity might it be possible to exploit signals generated by Over-the-horizon radar in a passive manner, for detection of planes etc? Or does receptio

Out of curiosity might it be possible to exploit signals generated by Over-the-horizon radar in a passive manner, for detection of planes etc? Or does reception of such signals have to be done near to where the transmitter is located.

retrac 4y ago
That's not too far off how radar was accidentally discovered, over and over. There is no one clear unambiguous first claim to radar, as far as I can tell. The general story is that plain ol' mediumwave/shortwave was bouncing off planes and ships as they passed, and operators couldn't figure out where the little signal spikes were coming from. Actual over-the-horizon radar systems have used transmitter and receivers at a significant distance from each other -- the Soviet Duga radar system in the 70s had its transmitter and receiver separated by ~50 kilometres or so. The receivers (and in active systems, also the transmitter) need to be in very precise time sync, and being physically close enough to run a direct cable makes that a lot easier. But theoretically, assuming the system is sensitive enough and time-synced enough and you have enough computational oomph available, an arbitrary number of receivers, at any distance, can be used to synthesize an arbitrarily large aperture. It's actually a similar matter to the synthetic aperture radio telescopes used in space astronomy, where telescopes with an effective aperture ~100 million kilometres wide have been created, using space-based radio telescopes linked with ground-based radio observatories. Such systems may not have the sensitivity of a 100 million kilometre wide telescope mirror (no matter to catch the photons since the "telescope" is mostly empty space) but it does have the equivalent angular resolution of such a 100 million km wide telescope mirror. It's mind-boggling when you consider the consequences of this as computation power improves. It will soon be possible to do this at such high speeds that it will allow frequencies into the far-infrared spectrum, not just microwave, for example.
- Trouble_007 4y ago
  Soviet Duga Radar : https://en.m.wikipedia.org/wiki/Duga_radar https://en.m.wikipedia.org/wiki/Duga_radar Soviet Duga-2 Radar Tower : https://abandonedplacesmap.com/huge-secret-soviet-duga-2-radar-tower-in-the-woods-of-chernobyl/ https://abandonedplacesmap.com/huge-secret-soviet-duga-2-rad...
- anfractuosity 4y ago
  Thanks a lot for your reply, that's really interesting re. the mediumwave/shortwave spikes. I also hadn't realised that the receiver of the Duga radar was so far away from the transmitter. I assume now you could probably use GPS/atomic clocks to keep them in sync rather than a cable.
  - femto 4y ago
    The rx/tx separation in over-the-horizon radars (OTHR) serves to increase isolation between the transmitter and receiver. The received signal is tiny, as it has not only bounced off the target but done a double bounce off the ionosphere. Doppler shifts are also very small, so one needs to minimise the clutter at zero Hertz, and a direct signal will appear as zero Hz clutter. You can get an idea of the precision of the timing required by considering how far a radio wave travels in a unit of time corresponding to the uncertainty in your time source and comparing that to the repetition period of the waveform. OTHRs typically have large ranges and use long period waveforms to minimise range ambiguity, so the timing requirements probably aren't too bad. Summary: A passive OTHR should be doable, especially as a cheapish SDR+computer can digitise and process the entire HF band. Front end sensitivity might be a concern? Low phase noise is critical. Phase noise is where many SDRs fall down? It's always been in the back of my mind that a distributed passive radar would be a cool project. Something like the existing network of hobbyist ADS-B receivers but pumping out timestamped HF samples which could be used for all manner of purposes. GPS could probably achieve a precision of single digit nanoseconds, which might be small enough to consider the receiver outputs as coherent, allowing synthesis to be done. If a node's network connection couldn't support continuous transmission it would still work to do burst mode, whereby each node might capture the first second of each minute (for example) then have a minute to transmit that data over the network.
    - krapht 4y ago
      I was working on something related for work many years ago... commercial gps receivers aren't sufficient. We kept time with GPS disciplined rubidium clocks, but that was only good enough for about 10 seconds of coherent operation before the phase wandered off too far @1GHz. (Not HF exactly, but...) It would be cool to try again with caesium or hydrogen maser ...

retrac 4y ago

That's not too far off how radar was accidentally discovered, over and over. There is no one clear unambiguous first claim to radar, as far as I can tell. The general story is that plain ol' mediumwave/shortwave was bouncing off planes and ships as they passed, and operators couldn't figure out where the little signal spikes were coming from. Actual over-the-horizon radar systems have used transmitter and receivers at a significant distance from each other -- the Soviet Duga radar system in the 70s had its transmitter and receiver separated by ~50 kilometres or so. The receivers (and in active systems, also the transmitter) need to be in very precise time sync, and being physically close enough to run a direct cable makes that a lot easier. But theoretically, assuming the system is sensitive enough and time-synced enough and you have enough computational oomph available, an arbitrary number of receivers, at any distance, can be used to synthesize an arbitrarily large aperture. It's actually a similar matter to the synthetic aperture radio telescopes used in space astronomy, where telescopes with an effective aperture ~100 million kilometres wide have been created, using space-based radio telescopes linked with ground-based radio observatories. Such systems may not have the sensitivity of a 100 million kilometre wide telescope mirror (no matter to catch the photons since the "telescope" is mostly empty space) but it does have the equivalent angular resolution of such a 100 million km wide telescope mirror. It's mind-boggling when you consider the consequences of this as computation power improves. It will soon be possible to do this at such high speeds that it will allow frequencies into the far-infrared spectrum, not just microwave, for example.

Trouble_007 4y ago
Soviet Duga Radar : https://en.m.wikipedia.org/wiki/Duga_radar https://en.m.wikipedia.org/wiki/Duga_radar Soviet Duga-2 Radar Tower : https://abandonedplacesmap.com/huge-secret-soviet-duga-2-radar-tower-in-the-woods-of-chernobyl/ https://abandonedplacesmap.com/huge-secret-soviet-duga-2-rad...
anfractuosity 4y ago
Thanks a lot for your reply, that's really interesting re. the mediumwave/shortwave spikes. I also hadn't realised that the receiver of the Duga radar was so far away from the transmitter. I assume now you could probably use GPS/atomic clocks to keep them in sync rather than a cable.
- femto 4y ago
  The rx/tx separation in over-the-horizon radars (OTHR) serves to increase isolation between the transmitter and receiver. The received signal is tiny, as it has not only bounced off the target but done a double bounce off the ionosphere. Doppler shifts are also very small, so one needs to minimise the clutter at zero Hertz, and a direct signal will appear as zero Hz clutter. You can get an idea of the precision of the timing required by considering how far a radio wave travels in a unit of time corresponding to the uncertainty in your time source and comparing that to the repetition period of the waveform. OTHRs typically have large ranges and use long period waveforms to minimise range ambiguity, so the timing requirements probably aren't too bad. Summary: A passive OTHR should be doable, especially as a cheapish SDR+computer can digitise and process the entire HF band. Front end sensitivity might be a concern? Low phase noise is critical. Phase noise is where many SDRs fall down? It's always been in the back of my mind that a distributed passive radar would be a cool project. Something like the existing network of hobbyist ADS-B receivers but pumping out timestamped HF samples which could be used for all manner of purposes. GPS could probably achieve a precision of single digit nanoseconds, which might be small enough to consider the receiver outputs as coherent, allowing synthesis to be done. If a node's network connection couldn't support continuous transmission it would still work to do burst mode, whereby each node might capture the first second of each minute (for example) then have a minute to transmit that data over the network.
  - krapht 4y ago
    I was working on something related for work many years ago... commercial gps receivers aren't sufficient. We kept time with GPS disciplined rubidium clocks, but that was only good enough for about 10 seconds of coherent operation before the phase wandered off too far @1GHz. (Not HF exactly, but...) It would be cool to try again with caesium or hydrogen maser ...