Signal Monitoring and Analysis: 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-f_s (f_s 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 f_d vs. Δf where f_d = 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.