With a network of 400 software defined receivers in your browser, you can easily do TDoA Direction Finding on signals across low, medium and high frequencies. Here’s how.

Recently, I was reading about the TDoA features built into the KiwiSDR network at SDR.HU. So, I thought I should give it a try and provide you with an explanation.

TDoA stands for Time Difference of Arrival. Radio signals travel at the speed of light. If you receive a transmission at several different locations, you can measure the differences in how long the signal traveled to each receiver. Then, with a bit of math, you can figure out the location of the transmitter.

So, I picked out a known shortwave signal, namely Radio Marti on 13.605 MHz broadcasting from Greenville, North Carolina. Then, I performed TDoA using three receivers on the SDR.HU network. These receivers, shown above with yellow tags, were located in Alberta, Nevada and Bonaire (formerly Netherlands Antilles.)

These three receivers monitored 13,605 kHz for 30 seconds and then the network displayed the transmitter location as 35° North and 77° West. This result, shown in green above, is the exact geographic location of Greenville, North Carolina.

How about that? My first attempt with TDoA using KiwiSDR receivers worked perfectly. Of course, I checked all three receivers beforehand to make sure they were receiving Radio Marti at the time.

TDoA Direction Finding – How It Works

In order for TDoA Direction Finding to work, you need two things. First, three receivers with known locations. Second, the received data must be time stamped with a great deal of accuracy. Since the KiwiSDR receivers on this network use GPS synchronization, time and location data is well known.

Once you select three receivers, each records 30 seconds of I/Q data from the received signal. The data is saved as a modified WAV file (chunks of radio signal) with time and location embedded. After signals are captured, they are analyzed on a server. By cross-correlating the signals, the time difference of arrival is calculated.

Then, using non-linear statistical regression, a series of hyperbolic arcs are estimated, coming from each receiver. Where the three arcs intersect, you have the transmitter location.

In summary, you can do TDoA if you know the exact locations for three receivers and you can record decent signal data with a very accurate time stamp. After that, it’s just math.