KrakenSDR and GNU Radio to track airplane transponder signal

Hello there.

I’ve been struggling to find direction to an aircraft transponder using GNU Radio. Everything works fine in a lab environment with prerecorded and replayed signal, but outside, I just get noise bearings. I have 3D printed antenna array to use with specific 1090 MHz dipole antennas to receive aircraft bearing from my position.

So I know that aircraft transponder signals are very short pulse signals, mode A/C 20,75 or 25,20 microseconds and mode S 64 or 120 microseconds, both which consist 0,45 to 1 microsecond pulses. I have tried to put decimation in GNU Radio blocks to 1 and cpi_size to 32, so it should make about 12,5 microsecond coherence processing intervals and maybe should see transponder signals?

Basic settings in GNU Radio: freq 1090 MHz; samp_rate 2,56 MHz; gain 49,6 dB; decimation 1 to 1024 used; cpi_size used from 2^5 to 2^20.

In lab environment I’m using R&S PR200 prerecorded transponder signal with directional antenna and replaying it with R&S SMBV100A, with the same characteristics that it should come out from an airplane and this does show me the direction. Even with as low power as -20 dBm at 23 meters.
I also have another RTL-SDR dongle that’s decoding transponder signals, and after I filter out ones that have their coordinates with them to visualize them on the chart where DOA is seen, but I just see basically straight lines of DOA.

Closest airplane has been about 4.4 kilometers from the KrakenSDR, but still no DOA. I can see the transponder signal power spikes on the chart also, so I know that Kraken is geting the signals.

Would appreciate any tips or tricks, thanks. Ask for more specifics.

With ADS-B are you certain that you are receiving information from ONE aircraft only? If you have busy skies the ADS-B channel is likely to to be full of pulses, so any sampled chuck of data is going to contain ADS-B pulses from multiple aircraft. And the Kraken will not be able to lock onto a single specific pulse because of this.