What Is Passive Radar?

Passive radar is a class of radar system that detects, locates, and tracks targets without transmitting any electromagnetic signal of its own. Instead of generating a dedicated illumination waveform, a passive radar receiver exploits existing radio frequency transmissions in the environment, known as illuminators of opportunity, to detect the reflections scattered off moving targets such as aircraft or vehicles. Because no dedicated transmitter is required, a passive radar installation is covert, imposes no spectrum licensing burden, and avoids the cost and logistics of a high-power transmitter. The discipline draws from statistical signal processing, array signal processing, bistatic geometry, and radar theory. Its development accelerated significantly with the digitization of broadcast signals, which provided wideband, well-characterized waveforms suitable for high-resolution radar processing.

The operating geometry of passive radar is fundamentally bistatic: the transmitter and receiver are at separate locations, unlike monostatic radar where a single antenna both transmits and receives. Target range is not directly measured; instead, the system measures the difference in path length between the direct signal from the transmitter and the delayed reflected signal from the target, yielding a range-sum, or bistatic range, ellipse on which the target lies. Combining measurements from multiple transmitters or receiver positions resolves the target's position.

Illuminators of Opportunity

The choice of illuminator of opportunity determines the system's detection range, range resolution, and clutter characteristics. FM radio broadcasts, operating between 87.5 and 108 MHz, were among the first exploited illuminators: their high transmit power and continental coverage provide detection ranges of hundreds of kilometers, though the narrow bandwidth (each station occupies roughly 200 kHz) limits range resolution to several hundred meters. Digital Video Broadcasting - Terrestrial (DVB-T) signals, used for digital television in Europe and elsewhere, offer substantially wider bandwidths of 6 to 8 MHz and are described in detail in a paper published in IEEE Transactions on Signal Processing on DVB-T passive radar signal processing. This wider bandwidth improves range resolution to tens of meters, making DVB-T a preferred illuminator for short-range air traffic surveillance. Other exploited sources include Digital Audio Broadcasting (DAB), cellular base stations (GSM, LTE), and satellite downlinks.

Cross-Ambiguity Function and Target Detection

The core signal processing operation in passive radar is the cross-ambiguity function (CAF). As documented in passive radar signal processing research, the CAF correlates the reflected surveillance channel signal against a reference copy of the direct signal from the transmitter, computing correlations across a grid of bistatic range delays and Doppler frequency shifts. A peak in the two-dimensional CAF surface indicates a target at the corresponding bistatic range and velocity. The reference signal is typically extracted by pointing one antenna directly at the transmitter, while a second antenna or array is directed toward the surveillance volume.

A persistent challenge is clutter cancellation: the direct-path signal from the transmitter arrives at the receiver with far greater power than the target reflections, and multipath reflections from fixed terrain and structures create stationary clutter peaks. Adaptive cancellation algorithms, such as the extensive cancellation algorithm (ECA), suppress these masking sources before target detection is performed.

Radar Detection and Imaging

Once the CAF is computed and clutter suppressed, detection algorithms apply a threshold to the range-Doppler map, often using constant false alarm rate (CFAR) processing that adapts to local clutter statistics. Detections across successive coherent processing intervals are fed into tracking algorithms that associate range-Doppler measurements with target trajectories. DVB-T based passive radar systems have been demonstrated for simultaneous counter-drone operations and civil air traffic surveillance, exploiting DVB-T transmitters' continuous illumination to generate persistent range-Doppler coverage. Passive radar systems can also form images of stationary scenes using inverse synthetic aperture techniques, reconstructing spatial reflectivity maps from the Doppler history of targets or terrain features.

Applications

Passive radar has found applications across a range of surveillance and sensing domains, including:

• Air traffic surveillance and airspace monitoring without dedicated radar infrastructure
• Counter-drone detection and tracking in urban environments
• Covert military surveillance and border monitoring
• Space object tracking using high-power broadcast illuminators
• Ground vehicle tracking for traffic and security applications