$750 and a Weekend Passive Direction Finding with KrakenSDR

The speaker set out to explore ground-based radio navigation as a cheap, GPS-like alternative, drawing on Cold War systems and research on Mars rover navigation. Using a KrakenSDR receiver with five antennas on a Raspberry Pi, they took bearings on transmitters around Portland, Oregon, including a hospital ham beacon that identifies itself every ten minutes. Most of the work was physical: finding sites where they were allowed to set up and trying to get clear sight lines.

Results varied widely. A professor's test from a tall building was accurate to about four degrees, while the speaker's early attempts were off by around eleven. Setup mattered more than software. Ground planes, antenna height (a music stand made a large difference), and metal and concrete reflections all changed the readings. Confusing true north with magnetic north also cost them data. Accuracy degraded past roughly half a mile, and multipath produced twin lobes in the plots.

The main takeaway is that a roughly 750 dollar kit can produce useful bearings, but urban environments make site selection and line of sight the dominant factors. The Q&A raised doubts about whether a strong peak came from the speaker's own transmitter or from the headphones being tested, and the speaker agreed localization might have been on the transmitter.