Antenna Array radius and DOA resolution

Hello. This may seem a dumb question, but I have worked a little with the other DOA estimation equipment (although it may have used a pseudo-doppler instead of a phase-coherent method as KrakenSDR does), and it did not require readjusting the antenna radius depending on the frequency. So is it possible (at least in theory), programmatically, to make Kraken DOA estimation resolution more precise on lower or the opposite, higher than 0.5 spacing multipliers to resolve ambiguities somehow? For instance, if my antenna radius is 1m (~128MHz), but I set the desired frequency to 446 MHz (~29 cm radius), I know that the antenna radius is actually 100/29 times bigger than it should be, and with that knowledge, I can adjust my DOA estimation, or at least provide +1 possible estimation with, let’s say, less certainty. I.e. I can assume that the wave has changed the phase 100/29 times (less, to be particular since the number of antennas is not even and there are no opposite ones) while traveling between the antennas and calculate DOA accordingly.
I hope you understand what I mean. The idea is not to re-adjust the static antenna array that is set on top of the 10m mast every time I want to change the frequency. Thank you

For pseudo-doppler you also need to adjust the radius in exactly the same manner. For both there will always be the hard limit of 0.5*lambda on the upper frequency for avoiding ambiguities. Then the limit on the lower frequency will be the resolution limit.

There is always a range of frequencies that a single radius will work decently for though. I suggest using the Excel calculator and calculating a radius that works for the frequencies you are interested in.

@krakenrf_carl , this question brings up something I have been meaning to ask, does the Kraken software / hardware measure the phase of arrival at each individual element and do calculations from that? Or are element sets combined in software as a phased array to form beams, and those beams cycled in virtual space? I realize those two questions are very similar.

Is there a theory of operations someplace that describes basic Kraken functions?



I found this video about the different RDF methods to provide a helpful, high-level overview. An Introduction to Direction Finding - YouTube
Correlative interferometry starts at How correlative interferometry works if you want to skip the basics and other DF techniques.

The software receives 5 raw sets of IQ data from the Kraken, and makes phase comparisons between them in software, with CH0 being a reference. So it’s all done in software.

Does the software consider some inaccuracies in cables, dipoles, boom ends having ±5 mm distance to each other etc? I mean, maybe when the frequency is set, the software somehow makes a calibration (for instance the first antenna sends a signal, and the rest 4 antennas receive it, and software checks that the phase in antenna 1 should have been X knowing the antenna radius, but it was X+2 degrees, so during this session it will subtract 2 degrees on each DOA measurement from antenna 1)

Everything after the KrakenSDR is not included in the calibration, which is why the lengths and construction of the cables and antennas should be identical.

Including the antennas and cables in the calibration would be difficult as we’d need to pass noise through the antennas into the Kraken. Obviously it’s not legal in most places to broadcast wideband noise into the air though.

1 Like

Is legal issue of broadcasting noise the only problem, or there are some other issues in this idea of calibration?

Also you’d have to manually accurately position the noise source at at least one angle of the array. Possibly more to improve calibration.

So you’d be performing a new manual calibration on each frequency retune and startup.

EDIT: Actually it might be possible to isolate the antenna and cables calibration from the Krakensdr calibration with this method. But it would change when the array size changes, or is moved etc.

I’ve thought of the 1st antenna as a source and the rest 4 as receivers, then 2nd antenna as an emission source and the rest as receivers, etc.

Can it be done programmatically (in theory) via Kraken software? This kind of calibration/adjustment. Or KrakenSDR hardware can not emit anything?

The KrakenSDR hardware for regulatory reasons is physically incapable of transmitting anything, so that calibration method won’t be possible with the KrakenSDR as is. You would need to design some sort of external calibration circuit if you wanted to do that.