Wednesday, January 17, 2018

OTHR on 6950 kHz

TDoA maps 6950 KHz 20180112T2138Z 

Cross-correlations 6950 kHz 20180112T2138Z 
The bandwidth of this signal is larger than the 12kHz seen by the KiwiSDR. The auto-correlation for this signal has peaks at multiples of about 0.0462938428 sec but it is interesting that the pulses look like this:
abs(z) vs. time 6950 kHz 20180112T2138Z
so an interesting form of modulation is being used.

Sunday, January 14, 2018

Cyprus OTHR

Yesterday I found the OTHR from Cyprus operating with 25 Hz pulse repetition rate and μ=500kHz/s sweep rate. The most likely geo-location is just north of the known location:

TDoA plots 17030 kHz 20180113T1126Z

Cross-correlations 173030 kHz 20180113T1126Z
I was thinking about an alternative way of geo-location using an extended Kalman filter, and found this reference: www.dtic.mil/dtic/tr/fulltext/u2/a285972.pdf. It is written very well and I was able to implement it quickly in octave using dfpdp from the optim toolbox for the Jacobian. More on this later.

Friday, January 5, 2018

De-chirping applied to a HF over the horizon radar signal (2)

This is a follow up to this blog post. Again the signals of the OTHR in Cyprus were analyzed using GPS time-stamped IQ samples from several KiwiSDRs.

In this case the OTHR signal was found between 13965 and 13985 kHz. The slope of the FMCW signal is μ=1MHz/s, and there are 50 sweeps per second. This results in a bandwidth of about 20kHz.

As is well known, the frequencies Δf of the spectrum of the de-chirped signal correspond to time delays τ=-Δf/μ in the case of a stationary target.
  • for this one has to multiply the received signal with the original waveform
  • tuning offsets w.r.t. center frequency of the OTHR signal correspond to a time delay of the original waveform
  • when Δf is positive the delay corresponds to the frequency Δf-fs (fs is the sampling frequency), i.e. the ranges wrap around the x-axis.
Applying a 2nd Fourier transform to the time direction of time vs. Δf data one obtains the Doppler frequency fd vs. Δf where fd = v•λ/2 (for velocity v and wavelength λ). Here one has to be careful to adjust the phases in each bin to correct for the fact that the sampling times are not exactly aligned to multiples of 0.02 seconds.

Plots for 5 KiwiSDR are shown below. The splitting of the ionospheric reflection is most likely due to O- and X-mode propagation in the F layer.

13975 kHz 20180105T1114Z @CS5SEL; the band at fd=0 is caused by an interference

13975 kHz 20180105T1114Z @DF0KL

13975 kHz 20180105T1114Z @G8JNJ

13975 kHz 20180105T1114Z @Izhvesk

13975 kHz 20180105T1114Z @Julisdalen
Thanks to all KiwiSDR owners who make their receivers available and do connect a GPS antenna.