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

Here is another look at the HF OTHR signal from Cyprus already mentioned in this blog post.

This OTHR signal is (at least to 1st order) FMCW modulated (period 0.02 sec, bandwidth 20kHz), as can be seen in the FM demodulated phase which follows a sawtooth pattern with period 0.02 sec.

Although the bandwidth of the KiwiSDR IQ data stream is smaller than the bandwidth of the OTHR signal it turns out that de-chirping can be done: using KiwiSDR GPS time-stamps t, de-chirping can be implemented by multiplying the IQ samples by exp(-2i*pi*f(t)) with f=@(x) 0.5*1e6*(mod(x,0.02)-0.01).**2-50;  (this is octave notation)

The de-chirped signal looks like in the bottom panel below and is flat as expected:

OTHR FM demodulated phase; bottom: de-chirped

The relative range (which includes a contribution from the doppler effect, see, e.g., here) is then obtained by performing a FFT transformation on the de-chirped signal in each 0.02 sec period (~240 samples):

OTHR de-chirped spectrum vs. time

• There is a main reflection in the ionosphere which has an interesting, time-varying structure to the left of it, i.e., towards smaller ranges
• In the light blue regions there is a kind of digital pattern visible; this might be due to the signal processing and limited bandwidth of the KiwiSDR but it could also be caused by some additional modulation of the radar signal