Build a Passive Radar Receiver
~180 minAdvancedUse FM broadcast towers as illuminators and two coherent RTL-SDRs to detect and track aircraft and vehicles via passive bistatic radar — no transmitter required.
Hardware
- •Two coherent RTL-SDR dongles (KrakenSDR or KerberosSDR recommended)
- •Reference antenna (directional Yagi toward FM tower)
- •Surveillance antenna (dipole or discone, broad coverage)
- •SMA cables, powered USB hub, outdoor mounts
Software / Skills
- •GNU Radio with gr-osmosdr
- •Python 3 with NumPy, SciPy, Matplotlib
- •Basic DSP knowledge (FFT, correlation)
- •dump1090 for ADS-B validation (recommended)
- •Linux recommended for real-time performance
Passive bistatic radar (PBR) uses existing radio transmissions — FM broadcast, DAB, DVB-T, or GSM — as the illuminator of opportunity. The receiver does not transmit anything; it simply listens for echoes of the illuminator's signal reflected off targets of interest. This makes it completely covert and legally straightforward.
Bistatic geometry
In a bistatic system, the transmitter (Tx) and receiver (Rx) are separated by a baseline distance (L). A target at position T creates a bistatic range:
R_bistatic = R_Tx + R_Rx - L
R_Tx = range from FM tower to target
R_Rx = range from receiver to target
L = baseline (Tx-Rx distance)
Targets on the same ellipse with
foci at Tx and Rx have equal R_bistatic
Doppler velocity measurement
A moving target shifts the frequency of the reflected signal by the bistatic Doppler:
f_d = (v/λ)(cos α + cos β)
v = target speed
λ = wavelength (FM ≈ 3 m)
α = angle from Tx to target velocity
β = angle from Rx to target velocity
A 250 m/s aircraft at FM: ~±150 Hz
Illuminators of opportunity
FM broadcast (87.5–108 MHz)
BW: ~200 kHz | Range: 100–200 km
Best for beginners. High power, wide coverage, simple signal.
DAB digital radio (174–240 MHz)
BW: 1.5 MHz | Range: 150–250 km
Better range resolution than FM. Requires DAB-capable receiver.
DVB-T (470–790 MHz)
BW: 8 MHz | Range: 200+ km
Highest range resolution. Used in academic passive radar research.
GSM/LTE base stations
BW: 200 kHz–20 MHz | Range: 20–80 km
Short baseline only. Urban clutter is a challenge.
Passive radar using FM broadcast towers typically achieves detection ranges of 100–200 km for aircraft. Commercial FM stations transmit at 87.5–108 MHz with up to 100 kW EIRP — far more power than any active radar an amateur could build.