SDRDF

Introduction

Radio Direction Finding (RDF) is the act of locating the position of a transmitter. This can be done in a number of ways, using different hardware configurations depending on the properties of the signal-of-interest and the chosen RDF technique.

This page is based around a presentation I gave on the subject. You are encouraged to watch the video to learn about RDF, and how it can be done (mostly) in the software domain (using Software Defined Radio, such as a USRP). I refer to this as Software Defined Radio Direction Finding (SDRDF).

Traditional Hardware-based RDF

Pseudo-doppler DF setups usually perform the antenna switching, filtering and 'direction finding' all in hardware. Only the AF (Audio Frequency) output of a separate radio is required as input.

Here is a great homebrew example:

• http://www.pi4wag.nl/index.php/pseudo-doppler-radio-direction-finder
• http://members.chello.nl/~w.hofman/pa8w/dopplerRDF.htm

There are many others. One common design is known as the Roanoke Doppler.

References

To learn more about the fundamentals of RDF, please consult the following sources. If you can think of any other helpful ones, please feel free to contact me.

• "Transmitter hunting: radio direction finding simplified" By Joseph D. Moell, Thomas N. Curlee

http://books.google.com.au/books?id=RfzF2-fHJ6MC&lpg=PA126&pg=PA120#v=onepage&q&f=false

• http://silcom.com/~pelican2/PicoDopp/ABOUT_DOPP.html

• http://en.wikipedia.org/wiki/Direction_finding
• http://en.wikipedia.org/wiki/SIGINT

Historical perspective:

• http://en.wikipedia.org/wiki/Adcock_antenna
• http://en.wikipedia.org/wiki/Y-stations

Video

http://www.youtube.com/watch?v=NSC4Y8yA-jY

Theory

Measure phase difference of pseudo-doppler signal introduced by electronically rotating antenna array against a reference signal of the same frequency as the switching rate.

Implementation

Antenna clock must be in lock-step with reference signal on computer.

SDR allows for same clock to be used to drive array, as that which is being used as external time reference for software DSP.

USRP

Modify firmware to output switching signal directly from FPGA. Same clock is used to time samples to computer, so a software signal source can be used as reference (orientation requires phase calibration).

References

Google Books