Bistatic Radar
Understanding Bistatic Radar
In a bistatic configuration, the transmitter and receiver occupy different locations, creating a triangular geometry between the transmitter, target, and receiver. This separation fundamentally changes the radar phenomenology: the target scatters energy in all directions, and the bistatic receiver captures energy scattered at an angle different from the backscatter direction measured by monostatic systems. The bistatic angle (the angle at the target between the transmitter and receiver directions) is the key geometric parameter that determines the bistatic RCS and the system's detection capability.
Passive Coherent Location (PCL) systems represent the most operationally significant application of bistatic radar. By using existing broadcast infrastructure as the illuminating source, PCL eliminates the need for a dedicated high-power transmitter entirely. Modern PCL systems digitize both a direct-path reference channel (aimed at the illuminator) and a surveillance channel (aimed at the target area), then perform cross-ambiguity function processing to extract target range and Doppler. Systems like Hensoldt TwInvis and Thales Silent Sentry have demonstrated reliable aircraft tracking to 250 km using commercial FM transmitters as illuminators.
The Bistatic Radar Equation
Pr = (Pt × Gt × Gr × λ2 × σb) / ((4π)3 × Rt2 × Rr2)
Where:
Pt = Transmitter power, Gt = Transmit antenna gain
Gr = Receive antenna gain, λ = Wavelength
σb = Bistatic RCS (depends on bistatic angle β)
Rt = Transmitter-to-target range
Rr = Target-to-receiver range
Forward Scatter Enhancement:
At bistatic angle β → 180°, σb → 4πA2/λ2
Where A = physical cross-sectional area of the target. A fighter aircraft with A = 10 m² at λ = 3 m (FM band): σb = 1,400 m² (31.5 dBsm), regardless of stealth shaping.
Bistatic Radar Configurations
| Configuration | Illuminator | Bistatic Angle | Range | Key Advantage |
|---|---|---|---|---|
| Dedicated Bistatic | Purpose-built TX | Variable (0-180°) | 500+ km | Full waveform control, coherent processing |
| Passive (PCL/FM) | FM radio (88-108 MHz) | Variable | 150-250 km | Covert, no transmitter cost, wide area |
| Passive (DVB-T) | Digital TV (470-862 MHz) | Variable | 50-100 km | Higher bandwidth, better range resolution |
| Forward Scatter | Any (dedicated or opportunity) | ~180° | Fence-line only | Massive RCS enhancement, stealth-immune |
| Multistatic | Multiple TX and/or RX | Multiple | Network-dependent | Track continuity, geometric diversity |
Frequently Asked Questions
Why is bistatic radar effective against stealth aircraft?
Stealth aircraft are shaped to redirect monostatic radar reflections away from the transmitter. The faceted surfaces and curved edges deflect incident energy to predictable angles, typically 30 to 60 degrees away from the source direction. A bistatic receiver positioned at one of these deflection angles intercepts the redirected energy, effectively turning the stealth shaping into a reflector aimed at the receiver. The bistatic RCS of a stealth target can be 10 to 30 dB higher than its monostatic RCS. Near the forward scatter region (bistatic angle approaching 180 degrees), the RCS depends only on the target's physical cross-section, not its shaping, making all stealth treatments irrelevant.
What is passive bistatic radar and how does it work?
Passive bistatic radar (Passive Coherent Location, or PCL) uses existing broadcast transmissions as the illuminating source instead of a dedicated radar transmitter. FM radio stations (88-108 MHz) are the most common illuminator because they provide high transmit power (10-100 kW ERP), wide geographic coverage, and stable CW signals ideal for Doppler processing. The receiver correlates the direct-path reference signal with target echoes to extract range, Doppler, and bearing. Operational systems like Hensoldt TwInvis and Thales Silent Sentry achieve detection ranges of 150 to 250 km. Because the receiver emits no RF energy, it is inherently covert and immune to anti-radiation missiles.
How does the bistatic radar equation differ from the monostatic equation?
The monostatic radar equation uses a single range R raised to the fourth power because the signal travels to the target and back along the same path. The bistatic equation separates this into R_T squared (transmitter to target) and R_R squared (target to receiver), giving R_T squared times R_R squared in the denominator. This geometric separation can be advantageous: a receiver 10 km from the target with a transmitter 100 km away produces a range product of 10^6 km^4, compared to 10^8 km^4 for a monostatic radar at 100 km. The equation also uses bistatic RCS instead of monostatic RCS, which depends on the bistatic angle between transmitter, target, and receiver.