HF Over-the-horizon radar processing using GNSS timestamped KiwiSDR IQ samples

OTHR parameters: 

• Chirp repetition time Δt=20 msec
• frequency slope=1 MHz/sec.

Its bandwidth is 20 kHz which fits nicely into the 20.25 kHz bandwidth of KiwiSDRs in 3-ch mode. 

Instantaneous frequency vs. mod(gpssec, 20 msec)

In the following we use GNSS-timestamped IQ samples for performing bi-static radar processing:

• Dechirping and framing into 20 msec long GPS-time-aligned frames
• interpolation in each frame (512 samples/frame)
• compute the 1st FFT along rows -> relative range
• compute the 2nd FFT  along columns on the result from the previous steps -> relative Doppler shift

Shown below are maps with relative Doppler frequency vs. relative range for 30 1-minute long periods, i.e., averaged over 1500 chirps each:

• the main signal comes at two different Doppler shifts and two different ranges, corresponding to two different ionospheric propagation paths.
• The pattern for the main signal can be found in several other places as well: this should correspond to reflections off some targets where the reflected signal then propagates with similar paths than the main component.
• It is interesting that besides the main component there are several instances of another pattern present, having two different Doppler shifts and ranges.
• It might also be that the secondary peaks are artifacts created by the signal processing.

Animation showing relative Doppler frequency vs. relative range.