An Overview on RF Direction Finding with RTL-SDRs

Thanks to K2GOG of the Hudson Valley Digital Network for writing in a sharing with us his latest blog post which is a useful overview of some direction finding techniques that can be used with RTL-SDR dongles. RF direction finding is the act of using a radio to determine the physical location of a signal.

In his post K2GOG mentions our successfully crowd funded KerberosSDR which will be shipping in January next year. KerberosSDR is our 4x coherent RTL-SDR, and one possible application is to use it as a four antenna phase coherent direction finder. K2GOG explains the phase coherent concept in his post quite elegantly.

While looking over KerberosSDR, K2GOG was also reminded of another direction finding technique called heat mapping which can be performed with a single RTL-SDR. This process involves driving around with an RTL-SDR and GPS logger, measuring the signal power as you drive and combining it the current GPS coordinates. From that data a heat map can be generated, which shows where the signal is the strongest, and therefore where the likely source is. The RTLSDR Scanner application by eartoearoak makes doing this easy, and in his post K2GOG provide a short tutorial on setting it up.

A heatmap generated by K2GOG with an RTL-SDR, GPS and RTLSDR Scanner.
A heatmap generated by K2GOG with an RTL-SDR, GPS and RTLSDR Scanner.
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how do i change the nr of scans? its no way to change the default 20 in continius mode as far as i can se.


Is it possible to combine the phase coherent RDF capabilities of the KerberosSDR and the signal strength heat map on the same device (ei: laptop or Tinkerboard S) for super fast foxhunting? Can it all be combined on the KerberosSDR by logging the signal strength or will we need a ‘5th’ RTL-SDR dongle for the heat map method?


On paper, “time-synchronous averaging” will even give a higher SNR by using multiple receivers.
( ref: Understanding Digital Signal Processing, Second Edition by Richard G. Lyons – Chapter 11, Section 1 – coherent averaging )

4 phase coherent receivers should give an extra 6dB, if exactly aligned in the time domain, but that will take extra maths to do with the KerberosSDR because the samples are fully phase coherent, but not exactly aligned in the time domain. So extra computation is required to realign the samples phase at a smaller granularity than the sample rate which would probably be easier to do in the frequency domain than the time domain. It is possible to do, just needs a DSP programming expert to sit down with a pencil and paper and workout some maths and then write the code to implement it.

Well that would be my understanding of it anyhow, I could be wrong. But maybe some or most of the required code will be standard boilerplate code that is provided with the KerberosSDR, and all that will be required is some basic DSP code and GUI code that make use of it.


So when us kerberos guys start winning all the fox hunts in record time, what will be the new game?