4 ms·

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 f

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 ...
- RF_Savage 4y ago
  KiwiSDR's are GPS synced and have timestamps. There's a TDoA direction finding implementation for it. So people have used them for locating transmitters, but the accuracy is not amazing. Still very impressive capability available for free to everyone.
  - femto 4y ago
    Nice. I wasn't aware of KiwiSDR. It's pretty well what I was thinking about, if the samples can be streamed. The docs mention that the clock is GPS disciplined, but is it the case that the samples are timestamped in a way that can be traced back to an absolute time. (If not, it would be a matter of programming?)
- sciurus 4y ago
  There's definitely been some interest in using amateur HF activity for radar, e.g. https://www.rtl-sdr.com/nils-critiques-the-mh370-wspr-aircraft-scatter-theory/ https://www.rtl-sdr.com/nils-critiques-the-mh370-wspr-aircra...

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 ...
RF_Savage 4y ago
KiwiSDR's are GPS synced and have timestamps. There's a TDoA direction finding implementation for it. So people have used them for locating transmitters, but the accuracy is not amazing. Still very impressive capability available for free to everyone.
- femto 4y ago
  Nice. I wasn't aware of KiwiSDR. It's pretty well what I was thinking about, if the samples can be streamed. The docs mention that the clock is GPS disciplined, but is it the case that the samples are timestamped in a way that can be traced back to an absolute time. (If not, it would be a matter of programming?)
sciurus 4y ago
There's definitely been some interest in using amateur HF activity for radar, e.g. https://www.rtl-sdr.com/nils-critiques-the-mh370-wspr-aircraft-scatter-theory/ https://www.rtl-sdr.com/nils-critiques-the-mh370-wspr-aircra...