Defense and Military RF Military RF Systems Informational

How does a passive radar system detect targets without transmitting its own signal?

A passive radar system detects targets without transmitting its own signal by using existing radio frequency transmissions (called illuminators of opportunity) as the radar illumination source. The system uses two or more receive channels: a reference channel that receives the direct signal from the transmitter, and one or more surveillance channels that receive the same signal after it has been reflected from targets. By correlating the reference and surveillance signals, the system measures the time difference (bistatic range) and frequency difference (bistatic Doppler) between the direct and reflected paths, determining target position and velocity. Common illuminators include FM radio broadcasts (88-108 MHz), digital television (DVB-T, 470-790 MHz), cellular base stations (700 MHz-2.6 GHz), WiFi, and DAB radio. The primary advantages of passive radar are that it emits no RF energy (making it inherently covert and immune to anti-radiation missiles), it requires no allocated spectrum, and it can be deployed with relatively simple, low-cost receive-only hardware. The primary limitations are that detection performance depends on the power and location of available illuminators, spatial resolution is typically poorer than active radar, and the signal processing is computationally intensive.

Category: Defense and Military RF

Updated: April 2026

Product Tie-In: Military Components, GaN Devices, Antennas

Passive Radar: Detection Using Illuminators of Opportunity

Passive radar (also called passive coherent location, or PCL) has evolved from a laboratory curiosity to an operationally deployed military and civilian sensor technology. Its covert nature makes it particularly attractive for air defense in environments where active radar transmissions would be detected and targeted.

System Architecture

A passive radar system consists of a reference antenna (typically directive, pointed at the illuminator transmitter), a surveillance antenna (typically with wide angular coverage to detect targets), multichannel digital receivers that simultaneously capture both signals, and a signal processing chain that performs direct-path interference cancellation and cross-ambiguity function computation.

Illuminator Characteristics

FM radio (88-108 MHz): High power (10-250 kW ERP), wide coverage, but narrowband (75 kHz per channel) limiting range resolution to approximately 2 km. VHF wavelengths provide detection of stealth targets
DVB-T digital TV (470-790 MHz): Moderate power (10-100 kW ERP), 8 MHz bandwidth providing 19-meter range resolution. The most commonly used illuminator for operational passive radar
Cellular (LTE/5G, 700 MHz-3.5 GHz): Dense network provides good geometric diversity. Bandwidth of 10-100 MHz gives range resolution of 1.5-15 meters. Lower power per base station limits range against small targets
DAB radio (174-240 MHz): 1.5 MHz bandwidth, moderate power. Used as supplementary illuminator

Signal Processing

The core processing step is computing the cross-ambiguity function (CAF) between the reference and surveillance signals, which produces a 2D map of bistatic range and Doppler velocity. Direct-path interference from the illuminator must be canceled from the surveillance channel (typically 60-100 dB suppression required) using adaptive filtering. CFAR detection applied to the CAF output identifies targets. Multiple illuminators provide triangulation for target localization.

Passive Radar Detection Parameters

Bistatic range: R_b = c x tau_d (time delay between paths)
Bistatic Doppler: f_d = (v/c) x (cos(beta/2)) x f_c
Direct path cancellation required: > 60-100 dB
Cross-ambiguity function: CAF(tau,f) = integral[s_surv(t) x s_ref*(t-tau) x e^(-j2pi f t) dt]

Common Questions

Frequently Asked Questions

Can passive radar detect stealth aircraft?

Yes, passive radar has potential advantages against stealth targets. Stealth shaping is optimized to redirect energy away from the monostatic radar direction, but the bistatic geometry of passive radar means the target reflects energy in directions other than back toward the transmitter. Additionally, FM radio at VHF frequencies has wavelengths comparable to aircraft dimensions, reducing the effectiveness of RCS shaping.

What detection range does passive radar achieve?

Detection range depends heavily on the illuminator power and the target RCS. Using 100 kW FM transmitters, typical detection ranges are 100-250 km for large aircraft (RCS 10-100 m^2) and 30-80 km for small targets (RCS 1 m^2). DVB-T-based systems achieve 50-150 km for large aircraft.

Which countries have deployed military passive radar?

Czech Republic (VERA/VERA-NG by ERA), Poland (PET/PCL), Germany (Hensoldt TwInvis), China, and several others have deployed or developed passive radar systems. NATO has recognized passive radar as a valuable complement to conventional active radar for air surveillance.

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