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.

Passive radar with $16 dual coherent channel rtlsdr dongle receiver

My previous post describes the $16 dual channel rtl_sdr dongle hack. In the last few days I've done some more testing and it turns out I can use the system for passive radar! I didn't expect this, because the receiver only has 8 bits and passive radar requires a lot of dynamic range.

Airplanes and occasional specular meteor echoes. 

I hooked up the two channels into yagi antennas that we have used with Echotek and USRP receivers for passive radar. One of the antennas was measuring the transmit waveform, and the other was measuring the echoes. I ran a measurement, and to my great surprise, it worked just fine.

I did tweak the signal levels a bit in order to ensure that I optimally use the dynamic range. I also had the bandwidth set to 2.4 MHz, giving me about 4.5 bits extra dynamic range after filtering the signal to 100 kHz in single precision floating point.

Two log periodic antennas used to passive radar with the dual coherent RTLSDR R820T dongle.

This really does give us a glimpse of the future where high end digital receivers will cost $10 per channel. The low end ones are already in that price range. Think of all the potential science that can be done!

$16 dual-channel coherent digital receiver

I have been playing around with the cool RTL dongles (more on rtl-sdr dongles on superkuh's web page or rtl-sdr.com) that you can buy on e-bay for about US$8 (including shipping). These are very capable 8-bit digital receivers that have up to 2.4 MHz bandwidth and can tune anywhere between 24 MHz and 1850 MHz.

I recently came up with a trivial hack to build a receiver with multiple coherent channels using the RTL dongles. I do this basically by unsoldering the quartz clock on the slave units and cable the clock from the master RTL dongle to the input of the buffer amplifier (Xtal_in) in the slave units (I've attached some pictures).

I originally drove the master crystal with both dongles, which also worked. However, Ian Buckley pointed out to me that a more typical way of doing this is feeding the signal into Xtal_In (in the pictures below). So I tried that too, and it also worked. I'm still not sure what the optimal setup is, as there is no schema for the dongle, but both methods I've tried so far have worked in practice.

This is how you make a dual coherent channel digital receiver with $16.  The clock drive probably won't be enough for many of these, but this can be fixed with a buffer or some other active splitter. 

The oscillator is wired using a piece of 75 Ohm antenna coax that came with the dongle. It's like they designed the dongle for  multi-channel coherent applications. 

This has some implications for low cost geophysical instruments. It will be possible to use this receiver for the 150/400 MHz beacon satellite receiver, as this only requires that the receivers have clocks that are locked with each other. Interferometry and passive radar are other application examples. With more than two locked channels, applications such as imaging start to become possible.

I've made some relative phase noise measurements, and the systems don't have detectable sample drift over two hours, and their relative phase is also pretty stable.

Spectrum at 1 Hz sample rate of the relative z_1/z_2 phase signal going into two receivers. 

IQ plot of the z_1/z_2 relative phase signal over ~6000 seconds at 1 Hz sample rate. 

And oh, by the way, I found this nice usb hub, which I'm going to use to hopefully get a 7 channel coherent rtl system.

Hub with the right usb port orientation for rtl dongles. 

Stay tuned for more results. I already have some pretty nice passive radar results using the system, which I'll be posting in a few days.

Update: Apparently three dongles will also run fine from one master clock. I know the clock isn't split correctly, but adding any components would increase the total cost and the whole point of this exercise is to determine what the lower bound is for software defined radios.

Three channel coherent RTLSDR receiver. 

Passive Radar Update

This week, I spent a few more days trying to understand the errors related with strong radar targets in the analysis of weak targets in passive radar. As a consequence of this, I wrote a linear least squares estimator to remove this contribution more effectively (and consistently). I also optimized my code a bit, and got a massive speed-up. Now the signal processing and plotting all runs in real time.

This video shows a lot of aeroplanes around the New England area detected using a simple passive radar setup, consisting of: one USRP and two yagi antennas, a quad core Linux PC. Every now and then an occasional specular meteor echo is observed too. Because this is FM radio, the waveform is not always optimal for inverting the echoes, which results in a blow up of the solutions when the waveform is narrow band (eg., somebody talking, or silence). This is not a problem with digital modulation schemes, which will be explored next. I updated my code. I came up with a way to efficiently estimate ground clutter and transmitter self-noise using a linear least squares matrix equation, which works better than the previous attempt. I also made some optimizations that allow me to run this in real-time using a normal PC. Next up: imaging using the midas-mini cube.