Direction Finding & Passive Radar

The hobby end of radio direction finding (RDF). With 4–5 phase-coherent receive channels you can compute angle-of-arrival in real time, mount the array on a vehicle, and drive to a transmitter. Passive radar uses the same arrays to detect aircraft / vehicles by reflections off ambient broadcast signals (FM, DAB, DVB-T) without transmitting anything yourself. Cross-link with SDR Receivers (KrakenSDR), SDR Drivers & Frameworks (GNU Radio).

Coherent multi-channel SDR hardware

• ★ KrakenSDR — 5-channel coherent RTL-SDR in a single enclosure. The only mainstream <$500 coherent SDR; CrazyCap'n / Tamas Pető's project, productized. ~$385. Open-host-software (GPL) on top of librtlsdr; FPGA-side coherence is hardware. Powered over USB-C / barrel; needs a 5V/2A supply. Comes with a 5-port SMA phase-matched cable kit.
  • https://www.krakenrf.com
• KerberosSDR (legacy) — 4-channel predecessor to KrakenSDR; sunset; not recommended for new builds.
• USRP X310 / X410 + multiple daughterboards — research-grade coherent rigs; thousands of dollars. Used for academic passive radar / SIGINT research.
• HackRF cluster + GPSDO — DIY coherent setup using multiple HackRFs locked to one GPSDO; technically possible but software is glue-by-yourself.

DOA / direction-finding software

• ★ krakensdr_doa — official direction-of-arrival app for KrakenSDR. Vehicle-mount mode with map display, real-time bearing, automated triangulation as you drive past a transmitter. Web UI; runs on a Pi 4 in the car or any Linux box. Killer use case: foxhunting, locating illegal pirate transmitters, finding stolen vehicle GPS tracker, ADS-B / AIS site triangulation. GPLv3.
  • https://github.com/krakenrf/krakensdr_doa
• gr-doa / gr-doaplotters — GNU Radio out-of-tree DOA modules; research-oriented. GPL.
• DF-Aggregator — central server that aggregates bearings from multiple KrakenSDR nodes for fixed-station triangulation. Python. GPL.
• kraken_to_aprs — bridges DF output into APRS for cross-pollination with APRS & Packet.

Passive radar

• ★ krakensdr_pr (Passive Radar) — companion to krakensdr_doa; uses two channels (reference + surveillance) of the KrakenSDR to detect aircraft / vehicles via reflections off ambient FM broadcast / DAB / DVB-T signals. No transmit license needed; no transmit hardware needed. Web UI shows range-Doppler plots. GPL.
  • https://github.com/krakenrf/krakensdr_passive_radar
• gr-pr / academic passive radar code — research repos; varies in quality.
• Pasiv — older passive radar attempt; sunset.

Antenna arrays

• 5-element circular UHF array (KrakenSDR's recommended configuration) — five λ/4 monopoles on a magnetic ground plane on top of a car; ~$60 of magnet bases + whips. The standard mobile-DF array.
• Linear arrays — for fixed installations; better for narrow azimuth fans.
• Yagi + rotator — old-school manual DF; works but slow. Pair with Ham Satellite Tracking rotator gear.

Practical guidance

• GPS lock matters. Mobile DF needs the host (Pi or laptop) to know its own position so the bearing math gets cumulative. Stick a USB GPS dongle on it; krakensdr_doa speaks NMEA and displays a track.
• Calibration. The KrakenSDR ships with a built-in noise source for in-band coherent calibration; recalibrate when you change frequency. 30 seconds.
• Frequency window. KrakenSDR is RTL-SDR-based, so 24 MHz – 1.766 GHz. Direct-sampling adds 0–24 MHz at reduced performance. Most fox-hunts are at 144 / 440 MHz; FCC-illegal jammers tend to be in the 100–500 MHz range too — squarely in band.
• Don't move while bearing. Each bearing is computed from a few hundred ms of samples; if you're driving at 60 mph the array's pointing-direction needs to match.
• Multi-station fixed triangulation is more accurate than mobile DF for stationary targets — use DF-Aggregator across 2–3 KrakenSDR nodes spaced kilometers apart.
• FM passive radar limits. Range ~50 km in flat terrain with a strong nearby FM station as illuminator; resolution ~5 km. Useful as a curiosity / research demo, not for serious surveillance.

License / pricing notes

• All KrakenSDR host software is GPLv3.
• KrakenSDR hardware — schematics and host code public; FPGA bitstream precompiled. ~$385.
• gr-doa / passive radar academic code — varies, mostly GPL.
• Patent landscape. Many DOA algorithms (MUSIC, ESPRIT, root-MUSIC, MVDR) are out-of-patent and freely implementable. Modern beamforming with deep-learning calibration is patent-jungly; the krakensdr stack uses classical methods that are legally clear.

Pick this if…

• Default coherent SDR, hobbyist: KrakenSDR.
• Mobile fox-hunting: KrakenSDR + magnetic 5-element array on the car roof + krakensdr_doa.
• Fixed multi-station triangulation: 2–3 KrakenSDRs + DF-Aggregator.
• Passive radar curiosity: KrakenSDR + krakensdr_passive_radar with a strong FM broadcaster nearby.
• Research / publishable DOA: USRP X310 + custom GNU Radio DOA flowgraphs.
• Old-school manual DF: a Yagi, a rotator, and a stopwatch. Still works.