Bistatic Radar

Bistatic radar is a radar system in which the transmitter and receiver are located at separate sites, unlike a traditional monostatic radar, where both are co-located. This configuration allows the system to detect, measure, and analyse electromagnetic reflections from a target using spatially separated elements. Bistatic radar plays a crucial role in modern surveillance, remote sensing, and scientific research, offering distinct advantages in terms of stealth detection, coverage, and flexibility.

Concept and Basic Principles

The fundamental principle of bistatic radar is based on the transmission and reception of electromagnetic waves between two different points. The transmitter emits a radio signal that illuminates a target. The signal reflects from the target and is received by a receiver located at another position. The geometry formed by the transmitter (T), receiver (R), and target (P) defines the bistatic configuration.
A key parameter in bistatic radar operation is the bistatic angle (β) — the angle subtended at the target by lines drawn to the transmitter and receiver. When the transmitter and receiver are close together, β is small, and the system behaves similarly to a monostatic radar. As β increases, the differences between the two systems become more pronounced.
Bistatic radars can measure various target characteristics such as range, velocity, radar cross-section (RCS), and Doppler shift. The system’s geometry provides additional data that can be exploited for advanced applications such as passive radar detection and stealth technology analysis.

Classification and Configurations

Bistatic radar systems can be classified based on their geometry, signal source, and level of coordination between transmitter and receiver.

1. By Geometry:
   • Forward Scatter Radar: The receiver is located directly opposite the transmitter, with the target passing between them. This configuration is sensitive to small targets and useful for detecting low-observable objects.
   • Side-Looking Bistatic Radar: The receiver is placed at an angle to the transmitter, allowing for wide-area surveillance.
   • Backward Scatter Radar: The receiver is positioned behind the transmitter, similar to monostatic radar, but with a spatial offset.
2. By Signal Source:
   • Active Bistatic Radar: Uses a dedicated transmitter and receiver, both under the control of the radar operator.
   • Passive Bistatic Radar (PBR): Utilises existing transmitters of opportunity (e.g., FM radio, television broadcasts, or cellular networks) and a dedicated receiver, reducing system cost and detectability.
3. By Synchronisation:
   • Synchronous Bistatic Radar: Transmitter and receiver share precise timing information, ensuring accurate range and Doppler measurements.
   • Asynchronous Bistatic Radar: Operates without perfect synchronisation, relying on signal correlation techniques to extract information.

Working Mechanism

The operational cycle of a bistatic radar involves three key stages: transmission, reflection, and reception.

1. Transmission: The transmitter emits a pulse or continuous wave of electromagnetic energy toward a target area.
2. Reflection: The target reflects part of this energy, depending on its shape, size, material, and orientation.
3. Reception: The receiver, located elsewhere, detects the reflected signal. By measuring the time delay between transmission and reception, and the frequency shift due to the Doppler effect, the radar determines the target’s distance, speed, and motion characteristics.

The bistatic range — the sum of the distances from the transmitter to the target and from the target to the receiver — forms the basis for calculating target position. Unlike monostatic radar, where the range is simply the two-way distance from one location, bistatic radar uses elliptical loci: all possible target positions lie on an ellipse with the transmitter and receiver as foci.

Advantages of Bistatic Radar

Bistatic radar systems offer several operational and strategic advantages:

• Stealth Detection: The spatial separation of transmitter and receiver makes it difficult for stealth aircraft to avoid detection, as their design is optimised to deflect radar signals back toward the transmitter, not toward a remote receiver.
• Low Vulnerability: Since the receiver is passive (in some cases), it does not emit detectable signals, making it less susceptible to enemy jamming or anti-radiation missile attacks.
• Flexible Deployment: The transmitter and receiver can be independently located to optimise coverage, terrain adaptation, or line-of-sight limitations.
• Cost Efficiency: In passive configurations, existing broadcast or communication transmitters can be used, reducing system costs.
• Improved Coverage: Bistatic networks can cover larger or multiple regions by deploying multiple receivers around a single transmitter.
• Enhanced Target Characterisation: The diversity of angles provides better information on target shape, material, and scattering properties.

Limitations and Challenges

Despite its advantages, bistatic radar faces several technical and operational challenges:

• Complex Geometry: The non-coincident transmitter and receiver introduce complex geometric relationships that complicate signal processing and tracking.
• Synchronization Requirements: Accurate timing between the transmitter and receiver is crucial for determining range and velocity. Any error affects measurement precision.
• Clutter and Multipath Interference: Terrain reflections and environmental noise can distort received signals, requiring advanced filtering techniques.
• Limited Range Resolution: In passive systems, the radar’s resolution depends on the characteristics of the external transmitter, which cannot be controlled.
• Data Fusion Complexity: Integrating data from multiple bistatic pairs or networks demands sophisticated algorithms and computational resources.

Applications

Bistatic radar technology finds application in various military, civilian, and scientific domains:

1. Defence and Surveillance:
   • Detection of stealth aircraft, cruise missiles, and unmanned aerial vehicles (UAVs).
   • Over-the-horizon radar systems for long-range surveillance.
   • Passive radar systems for covert operations and electronic warfare.
2. Remote Sensing and Earth Observation:
   • Bistatic Synthetic Aperture Radar (BiSAR) for detailed terrain mapping, topography, and soil moisture analysis.
   • Spaceborne bistatic missions, such as those using satellite constellations, for high-resolution imaging.
3. Meteorology and Atmospheric Studies:
   • Measurement of precipitation, wind patterns, and ionospheric irregularities.
   • Monitoring of atmospheric disturbances and weather forecasting.
4. Civilian and Commercial Uses:
   • Air traffic control, maritime surveillance, and environmental monitoring.
   • Passive radar systems using existing communication signals to track aircraft or vehicles without dedicated transmitters.

Comparison with Monostatic Radar

Future Developments

Modern research in bistatic radar focuses on networked and multistatic radar configurations, where multiple transmitters and receivers operate simultaneously. These networks provide comprehensive coverage and redundancy, improving detection reliability. Integration with Artificial Intelligence (AI) and Machine Learning (ML) is enhancing target identification, tracking accuracy, and clutter suppression.
Emerging applications include spaceborne bistatic radar systems, unmanned aerial platforms, and distributed radar networks for real-time surveillance and environmental monitoring. With advances in digital signal processing, sensor miniaturisation, and communication technologies, bistatic radar continues to evolve into a key component of next-generation sensing systems.