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TDoA solution considerations

This discussion was created from comments split from: Direction Finding and linking existing KiwiSDRs.

Comments

  • edited July 2018
    Hi Martin,

    The nice TDoA map you have attached [see first comment in thread "TDoA operating notes"] can be used to make an important point: only if the distance between the receivers and the source of approximately equal and the propagation modes are the same and there are no large gradients in the ionospheric election density (e.g. day-night) there are no large biases due to ionospheric propagation effects.

    73
    Christoph
  • And if your set of receivers has bad geometry, for whatever reason, you might get things like this. TFK/NRK is Iceland 37.5 kHz MSK. "Kiwi99" is in Ireland.

    image

    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/67/af9304536f1f0d28763f275b25fbf1.png
  • This TDoA solution is ambiguous which happens if the used KiwiSDRs are not well distributed. In this case two are very close by each other and the one in Ireland is far from them, so it is more like having only two independent measurements.

    It should be easy to see from the individual TDoA maps how the two red spots arise.
  • Hi Christoph,

    Just a bit more feedback.

    I recently used the TDoA extension to ID the likely source of a FSK signal that was intruding into the 40m Amateur band. This had been reported to the UK regulator OFCOM and also the German regulator.

    I had TDoA the likely source as being from St Eval in Cornwall on the South West tip of the UK, but this was not known when the signal was initially reported.

    The German regulator DF'ed it as likely to be from Plymouth in the South West of the UK about 50km South East of St Eval.

    So your TDoA combined with a much wider distribution of KiWi SDR receiver sites is probably at least as accurate as some professional kit, and in some cases even better :-)

    Regards,

    Martin - G8JNJ
  • I used TDOA professionally for a few years as Duty Engineering Officer at the Ofcom Baldock monitoring station before I retired 18 months or so ago.

    We only used it for VHF and up as it was developed and running ready for the Olympic Games to enable us to manage the spectrum for that event and the system continued to evolve from there.

    The findings I have read here and some playing with the system confirm what we experienced in that you need to have your receive sites spaced reasonably evenly around the signal you are locating and if this is not the case then you will get an indication of a likely direction but not one you can rely on.

    Even with good signals from 3 or more receive sites the results still varied in accuracy but usually would get you at least to the area from which the signal of interested originated and on a good day to the actual building.

    The system interface drew a line from each receiver to the next as you selected them allowing a good idea of the area covered to be seen so you could keep the suspected target are inside a ring of selected receivers. Maybe this would be useful on the Kiwi system. It also confirmed that each site could receive the signal by indicating received signal level/quality from each receiver.

    The result was then displayed as an overlay using google maps and also provided a lat/long of the centre point to be used on any other mapping of your choice.

    On HF we used conventional angle of arrival DF but had the advantage of then requesting bearings from other countries such as the German regulator that Martin mentions. Having obtained bearings from a number of countries these were entered into mapping software which plotted them and displayed where they crossed.

    Ofcom have 2 HF SRDF (Super Resolution Direction Finder) systems, one at Baldock and another in Scotland.

    They are made by Roke Manor Research and the manufacturers quote an accuracy of better than 1 degree.

    Regards

    John - G4HPW / KD8HEB
  • Hi John,

    Very interesting.

    I was aware of the 24 RMDF TDoA sites in the UK covering 20MHz to 6GHz (it was mentioned previously on this forum with a link to a video) http://forum.kiwisdr.com/discussion/940/direction-finding-and-linking-existing-kiwisdrs/p1

    However I hadn't come across the SRDF system before.

    I think some of the maps you mentioned can be seen on this website.

    https://www.the-hug.co.uk/opus2516.html

    Regards,

    Martin - G8JNJ
  • Hi Martin

    It appears that there is a new version of the map plotting software now in use after looking at the link to the maps you posted. Looks like it is much improved over the system that was in use at the time I left which had been in use for many years and was provided by the FCC.

    The number of TDOA sites varies as they are often moved around to suit various work in progress. The sites also have a further use in that they carry out spectrum occupancy measurements as well as TDOA functions.

    That often means they are on their own just doing occupancy measurements and are too far from the next site to be of use for TDOA.

    Regards

    John - G4HPW / KD8HEB
  • edited July 2018
    Hi John,

    On-rereading the information on the https://www.the-hug.co.uk/opus2516.html website.

    I now see that all it does is plot the great circle paths from the bearings that have been typed in.

    It's just a crude method to manually transfer and combine bearings from different sources and systems and not the actual SRDF system interface GUI.

    Regards,

    Martin - G8JNJ
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