What is passive radar and how is it used for civilian and military applications?

Radar is one of the most important and widely used sensing technologies in the modern world. Whether it’s air traffic control, weather monitoring, or national defense, radar plays a central role. However, in recent years, a lesser-known yet increasingly promising technology has gained attention: passive radar.

Unlike conventional (active) radar systems that emit their own radio signals, passive radar operates differently: it uses existing electromagnetic signals already present in the environment, such as television broadcasts, mobile networks, or GPS transmissions. This approach has numerous advantages—it emits no signal (thus remaining undetectable), is more resilient to jamming, and can be highly cost-effective.

In this article, we’ll explore in detail:

• what passive radar is and how it works,

• its military and civilian applications,

• the technological challenges and opportunities it presents,

• and how it might become a key component of future sensing systems—even in Hungary.

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Active vs. Passive Radar: A Quick Comparison

Active Radar – the classic model

An active radar includes both a transmitter and a receiver. It emits radio waves that bounce off a target, and the receiver detects the reflected signals to determine position, direction, and speed.

• Pros: accurate and under full control

• Cons: high energy use, easily detected and jammed

Passive Radar – the stealth sensor

A passive radar does not transmit any signals. Instead, it relies on existing emitters in the environment (e.g. TV towers, mobile base stations, radio transmitters). The system receives two signals:

1. Reference channel – the direct signal from the transmitter

2. Target channel – the signal reflected off the target

By analyzing the delay and spectral differences between the two, the radar computes the target’s position and velocity.

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How Does Passive Radar Work?

Key Steps

1. A signal source in the environment (e.g. DVB-T television transmitter) emits radio waves.

2. A target (e.g. aircraft, drone, vehicle) reflects these signals.

3. The passive radar system simultaneously receives:

   • the direct signal from the emitter (reference),

   • and the reflected signal from the target.

4. Signal processing algorithms (e.g. cross-correlation) compare both signals.

5. The system calculates the target’s location, speed, and potentially altitude.

✅ No transmission required → making the system:

• invisible to detection,

• lower in energy consumption,

• and suitable for use in populated areas.

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What Signal Sources Can Be Used?

DVB-T (digital terrestrial television) is often the preferred source due to its high power and stable signal structure.

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Application Areas

Military Use

1. Stealth aircraft detection

   • Passive radar can detect aircraft designed to be invisible to active radar.

2. Passive air defense systems

   • Enables silent monitoring of airspace without exposure to detection.

3. Battlefield movement detection

   • Low-flying drones, ground vehicles, and helicopters

4. Electronic warfare resilience

   • Harder to jam since it doesn’t emit its own signal

Civilian Use

1. Air traffic monitoring

   • Can track aircraft movement even without ADS-B transmissions

2. Urban traffic monitoring

   • Detects vehicles and pedestrians anonymously, without cameras

3. Weather observation

   • Detects precipitation or cloud movements using environmental signals

4. Infrastructure protection

   • Perimeter monitoring for airports, power plants, etc.

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Key Advantages

• ✅ Undetectable: no transmission, so it cannot be easily located or targeted

• ✅ Cost-effective: no need for a high-power transmitter

• ✅ Difficult to jam: since it uses third-party broadcast signals

• ✅ Integrates with existing systems: useful for civilian and military use

• ✅ Privacy-friendly: does not collect images or personal data

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Limitations and Challenges

• ❗ Depends on environmental signal sources

• ❗ May be less precise than active radar in some measurements

• ❗ Affected by signal interference and bandwidth limitations

• ❗ Requires complex, real-time signal processing

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The Role of Artificial Intelligence

Passive radar signals can be noisy and difficult to interpret. Here, AI and machine learning play a critical role:

• ML models to distinguish between noise and real targets

• Neural networks to predict motion patterns

• Anomaly detection – automatic alerting for unusual activity

• Adaptive learning – systems learn from their environment over time

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Passive Radar in Hungary and the EU

In Hungary

• Universities such as ELTE and BME are actively involved in passive radar R&D

• Defense firms like HM EI Zrt. are exploring military applications

• The Balaton region has hosted international air monitoring tests with passive systems

In Europe

• Horizon Europe projects fund research into passive sensing in urban environments

• NATO tests have included passive radar for stealth aircraft tracking scenarios

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A Technology for the Future?

Passive radar development in the next 5–10 years is expected to move toward:

• ? Multistatic systems – multiple receivers and signal sources working together

• ? AI-powered analysis engines – real-time interpretation of complex signals

• ? Networked radar platforms – distributed and decentralized sensing

• ? Vehicle-integrated passive radar – adding an invisible sensing layer to autonomous cars

• ?️ Space-based passive monitoring – ultra-low signature observation from orbit

Passive radar is not science fiction—it’s a real, evolving technology that could revolutionize how we monitor our surroundings. With no signal emission, it offers stealth, efficiency, and flexibility in environments where traditional radar might be too visible or vulnerable.

As demand grows for discreet, low-energy, and privacy-respecting sensing solutions, passive radar may become a cornerstone of future civil, military, and urban systems. Whether you work in radio engineering, defense, or smart infrastructure—this is a technology worth watching closely.