Stephen Cass Wakes the KrakenSDR and a Raspberry Pi for a Speed-Finding Passive Radar Project
Using the Empire State Building as a radio signal source, Cass' passive radar can pick a plane out of the air and tell you its speed.
IEEE Spectrum special projects editor Stephen Cass has turned a KrakenSDR software-defined radio into a passive radar — "borrowing" radio-frequency illumination from other transmitters to plot the speed of airborne objects.
"Unlike conventional radar," Cass explains, "passive radar doesn't send out pulses of its own and watch for reflections. Instead, it uses ambient signals. A reference antenna picks up a signal from, say, a cell tower, while a surveillance antenna is tuned to the same frequency. The reference and surveillance signals are compared. If a reflection from an object is detected, then the time it took to arrive at the surveillance antenna gives a range. Frequency shifts indicate the object’s speed via the Doppler effect.
For the receiving device, Cass turned to the KrakenSDR — a device unveiled by RTL-SDR.com back in 2021 as a successor to its earlier KerberosSDR. Like the KerberosSDR, the KrakenSDR's primary selling point is its packing multiple receivers — five, in the KrakenSDR — into a single device, allowing for a range of neat projects from multi-band reception and range-finding to high-accuracy passive radar.
To this, Cass added two low-cost antennas originally designed for off-the-air TV reception, a Raspberry Pi 4 Model B single-board computer for processing, running the KrakenSDR-specific fork of the Raspberry Pi Operating System, which comes pre-loaded with the software required to drive the radio, and a "heavy-duty battery pack" to allow for portable use. The user interface, meanwhile, is provided as a web app accessible over the Raspberry Pi's built-in Wi-Fi — allowing the whole setup to be controlled from a smartphone or tablet.
"With one hand I pointed the surveillance antenna at the overcast skies and held my phone in the other," Cass writes. "Gratifyingly, I almost instantly started seeing a blip on the speed-versus-range radar plot, matched a few moments later by the rumble of an approaching jet. (The plot updates about once every 3 seconds.) Because of the strength of the echoes, I was able to raise the signal-cutoff threshold significantly, giving me radar returns uncluttered with noise, and often with multiple aircraft. A win for SDR!"
Cass' full article, which was published in the November 2022 issue of the print magazine, is available on the IEEE Spectrum website now.