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.  

Chirp sounder measurements with KiwiSDRs (2)

Using the recent work on KiwiSDR waterfall recording (see kiwiclient/kiwiwfrecorder.py) it becomes possible to search for chirp sounder signals in KiwiSDR waterfall data, continuing the topic from this post.

kiwiwfrecorder.py connects both to the 'SND' and the 'W/F' websocket streams. The 'W/F' stream contains sequence numbers for each waterfall line which are used to synchronize the waterfall to the audio data. These sequence numbers can also be used to attach to each waterfall line a corresponding GNSS time tag obtained from the 'SND' stream. As the waterfall data can arrive before or after the audio data, both are combined in a third thread using python Queues for thread-safe communication.

About half an hour of waterfall data was recorded on the AB1LD KiwiSDR using the highest KiwiSDR waterfall speed which turns out to provide waterfall data about for each SND frame, i.e, each 512/12000 seconds. Thanks to the owner for setting up this KiwiSDR and allowing unrestricted access!.

Then the recorded waterfall data (saved in a .npy formatted file) was rebinned in time to 1024/12000 second bins and exported as a .png file. Switching from python to octave, a search for chirp sounders was performed for chirp rates from 80  kHz/s to 130 kHz/sec in steps of 1 kHz/sec: for each chirp rate and for each start time the content of the waterfall bins was summed up along the corresponding line (Hough transform).

Two chirp sounders were found, each having a repetition rate of 720 seconds (12 minutes) with chirp rates of 82 kHz/sec and 100 kHz/sec, respectively. It might be interesting that this list of chirp sounders contains entries for three chirps sounders with 720 second periods located in Norfolk, VA, Kingsville, TX, and in Puerto Rico.

The plots below show zoomed waterfall diagrams around the chirps and on the bottom panel the result of the chirp search, i.e, the sums of waterfall bins along lines with a given slope.

1^st chirp sounder detected using the AB1LD KiwiSDR

2^nd chirp sounder detected using the AB1LD KiwiSDD

Chirp sounder measurements with KiwiSDRs (1)

The plots below show measurements of the chirp sounders from Cyprus (RAF Aktrotiri). There are two chirps (100kHz/sec) separated by 5 seconds and repeating each 5 minutes. In G3PLX notation they are called 300:240 and 300:245. 

Using GNSS timestamped IQ samples from the KiwiSDR @IU8CRI, tuned to 13,200 kHz, a pair of chirps was analyzed in terms of instantaneous frequency and of propagation time delays obtained from the FFT of the de-chirped signals.

Top: instantaneous frequency; bottom: propagation time delay obtained form the FFT of the de-chirped signal

In this earlier blog post a time offset of about δt=0.3±0.03 msec w.r.t. start time at the beginning of a given UTC second of the chirp signals from Cyprus was found in the chirp monitoring data from the U Twente WebSDR. This offset is also found in the KiwiSDR measurements, i.e., without applying this offset, the first peak in the propagation time delays would be below the equivalent time of propagation along the great-circle distance at the speed of light.

It would be very interesting to implement a more systematic monitoring of chirp sounders using KiwiSDRs.