Phase Coherency Multichannel RTL-SDR GNU radio

Hi !

I’m a beginner with GNU Radio and SDR like RTL-SDR, but I’m doing my best to understand how to use them !

I want to do goniometry / direction finding / passive radar with these equipments.
But first I need to focus on the synchronization of the receiver’s channels.

1/ I’m working with two RTL-SDR v3 dongles, which one of them is sharing its clock to the other. It is correcting the clock timing-drift and it works.
2/ With GNU Radio, I’m correcting the bulk delay (dongles are not sampling at the same time) and it works.
3/ Next, I want to correct the Frequency / Phase offsets by transmitting a reference signal (433 MHz) on my channel. This signal is an unmodulated sinus and I can see it with GNU Radio. The distance between my antennas is constant to see a constant phase difference between the 2 received signals. However, the phase difference of the 2 received signals from this reference signal is not constant.
So my system is not stable, and I want to understand why and how can I have a phase coherent system.

I did these 2 screenshots showing the phase difference is not constant through time between the 2 signals received by the dongles.

I studied a lot of online works, like the Pseudo-Doppler system of Michael Ossmann, the High Resolution system of Samuel Whiting, but I don’t clearly understand their phase offset correction.

I have some thoughts about it that I wanted to share with you :
a) With GNU Radio, I always see this message : [R82XX] PLL not locked
b) I always have difficulties to choose the right sample rate for the RTL-SDR dongles
c) I’m doing these experiences inside a house, so I’m thinking about the impact of multipath. I know that they will have an impact on the final direction of arrival estimation, but when I’m looking to the phase of the 2 signals, it looks like on of them is shifting constantly (like one of them is running “faster” ?).
d) Sharing the clock of one of the dongles can have an impact on this phase offset ?

Thank you in advance, I’m looking on solutions using Costas Loop, etc.