Monday, March 11, 2024

Measuring oblique ionograms using KiwiSDRs (1)

Oblique ionogram obtained from a KiwiSDR using GNSS timestamps

How it is done

The receive bandwidth of KiwiSDRs is limited to 12 kHz (20.25 kHz in 3 user channel configuration). In order to synchronously tune to a chirp sounder GNSS-based timestamps are used:
  • For the current GNSS timestamp T0 tune the KiwiSDR to the frequency on which the chirp is expected at a time T0+DT.
  • Record 10240 IQ samples, corresponding to 20 512-sample long buffers
  • Repeat
Timing diagram

In this way, the chirp is always seen at a specific offset. Note that the tuning frequencies are different for each different complete chirp sweep because the KiwiSDR sampling rate is not GNSS-disciplined (the GNSS timestamps are exact).

abs(IQ) vs. frequency

For a chirp rate of 100 kHz/s and sampling rate 12 kHz, the frequency steps are ~85.33 kHz and the group delay resolution is 25 km.

The time offset DT, which depends on the delay between issuing a frequency change command and its effect, is measured at the beginning. It depends on internal KiwiSDR delays and on the network delay. If the KiwiSDR is on a local network this delay is about 2 buffers long, for KiwiSDRs on the internet delays between 8 and 10 buffers were seen.