Monday, 28 October 2013

Dual antenna passive radar interferometry

Last week I tested passive radar interferometry with two USRP N200 devices and new K&R filters (this could in theory be done also with thee rtlsdr dongles with a shared clock). Three channels were recorded: the transmit waveform with a directional antenna pointed towards the FM radio transmitter, and two SKA (Square Kilometer Array) prototype log periodic prototype antennas pointed towards zenith to record echoes. 

To analyze the signals, I performed deconvolution of the transmit waveform, and estimated a cross-spectrum between the two channels at each range gate. I then plotted the range-Doppler-intensity of the echoes, using hue to encode relative phase difference between antennas. The results mainly show airplanes moving about in the two antenna interferometer with a ~18 lambda spacing, but there are occasional specular meteor echoes too. The phase gives some idea of where the signal is arriving from, but there is still ambiguity that can only be resolved by adding more antennas.    

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Here are some plots of some specular meteor trails that I spotted in the results:

Specular meteor echo at 200 km range.

Specular meteor echo at 150 km range.
Meteor at 240 km.
Meteor at 150 and 200 km.

Meteor at 110 km.

Meteor at 90 km

Meteor at 230 km.
Once we get more cables to the antennas, we should be able to start testing passive radar imaging and radio source calibration. There are already plenty more antennas awaiting cabling:

The rapid antenna field. The xmas tree like log period antennas are the SKA Cambridge prototype antennas, the loop is for HF, and the small white dome is the NASA space debris radar.

The ground clutter has been removed from the above pictures, to avoid self-noise. Out of curiosity, I also plotted that:
Ground clutter phase difference-range-time-intensity picture. The clutter phase difference is fairly stable, indicating that the ground clutter angle of arrival distribution is also fairly constant over time.  
The bottom line is that the 16 channel usrp receiver works fine for passive radar with the K&R filters to remove aliasing.

Update: added a picture of the antennas, and more meteor echoes. Fixed typos.

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