Bistatic Radar Technique

The Bistatic Radar Technique refers to a radar system configuration in which the transmitter and receiver are located at separate positions, as opposed to the conventional monostatic radar system where both are co-located. This spatial separation between the transmitting and receiving antennas introduces unique operational advantages and analytical challenges, enabling applications in remote sensing, stealth detection, atmospheric studies, and military surveillance.
Bistatic radar exploits the physical principles of electromagnetic wave propagation, reflection, and scattering to measure the characteristics of a target or medium. Its distinctive geometry allows for enhanced observation of target signatures, reduced vulnerability to jamming, and improved detection of low-observable objects.

Principles and Basic Configuration

In any radar system, electromagnetic waves are transmitted toward a target, reflected (or scattered) back, and then received and analysed. In a bistatic radar, the transmitter emits a signal that illuminates the target, while the receiver—placed at a different location—collects the reflected energy. The bistatic baseline is the straight line joining the transmitter (T) and receiver (R). The target’s location defines a third point, forming a bistatic triangle that governs the system’s geometry.
Key parameters include:

• Bistatic angle (β): The angle between the transmitter–target and target–receiver lines.
• Bistatic range: The sum of the distances from the transmitter to the target and from the target to the receiver.
• Bistatic Doppler shift: The change in frequency observed due to the relative motion of the target with respect to both the transmitter and receiver.

By analysing these quantities, the system can determine the target’s position, velocity, and reflective characteristics.

Comparison with Monostatic Radar

The bistatic configuration allows for greater operational flexibility. For example, the receiver can operate passively, detecting only reflected signals without emitting any energy—making it effectively invisible to adversaries.

Types of Bistatic Radar Systems

Bistatic radars can be categorised according to the level of separation between the transmitter and receiver:

1. Quasi-Bistatic Radar: The transmitter and receiver are separated by a short distance relative to the target range, creating a geometry nearly identical to a monostatic system. This arrangement is often used in ground-based imaging radars.
2. Wide-Baseline Bistatic Radar: The transmitter and receiver are separated by large distances, often hundreds of kilometres. This setup provides unique perspectives for remote sensing and atmospheric measurements.
3. Passive Bistatic Radar (PBR): Utilises existing non-cooperative transmitters—such as commercial broadcast signals, television towers, or satellite links—as sources of illumination. The radar receiver analyses the reflections of these ambient signals to detect and track objects without transmitting its own energy.
4. Multistatic Radar: An extension of the bistatic concept involving multiple transmitters and/or receivers. Multistatic configurations enhance spatial coverage, target recognition, and resistance to countermeasures.

Working Principle

The basic operation of a bistatic radar involves the following steps:

1. Transmission: The transmitter emits electromagnetic waves that propagate toward a target.
2. Scattering: The target reflects and scatters the incident wave in multiple directions.
3. Reception: The receiver collects the portion of scattered energy directed toward it.
4. Processing: The system processes the received signal to determine target parameters such as range, speed, cross-section, and location.

The bistatic range equation governs the power of the received signal:
Pr=PtGtGrλ2σb(4π)3Rt2Rr2P_r = \frac{P_t G_t G_r \lambda^2 \sigma_b}{(4\pi)^3 R_t^2 R_r^2}Pr​=(4π)3Rt2​Rr2​Pt​Gt​Gr​λ2σb​​
Where:

• Pₜ is the transmitted power,
• Gₜ and Gᵣ are the gains of the transmitter and receiver antennas,
• λ is the wavelength,
• σ_b is the bistatic radar cross-section (RCS) of the target,
• Rₜ and Rᵣ are the distances from the transmitter and receiver to the target.

The bistatic radar cross-section differs from the monostatic RCS, as it depends on the bistatic angle and the target’s reflective properties from different viewing directions.

Applications of the Bistatic Radar Technique

1. Military Surveillance and Stealth Detection: Bistatic radar can detect stealth aircraft designed to evade monostatic radar because it receives reflections from different angles. Its spatial diversity makes it less predictable and harder for stealth designs to absorb or deflect.
2. Remote Sensing and Earth Observation: Bistatic radar techniques are widely used in synthetic aperture radar (SAR) missions to study surface topography, vegetation, and oceanography. The bistatic configuration provides complementary scattering information that enhances imaging accuracy.
3. Passive Radar Systems: Using existing commercial transmitters (e.g., FM radio or television signals), passive bistatic radar enables surveillance and aircraft detection without emitting any radiation—providing low-cost, covert monitoring capabilities.
4. Atmospheric and Ionospheric Research: The technique is used to study ionospheric irregularities, meteors, and atmospheric density variations. By positioning the receiver at a distant location, scientists can analyse scattered signals that reveal upper-atmospheric properties.
5. Space Missions: In planetary exploration, bistatic radar observations have been used to analyse planetary surfaces and atmospheres. For example, radar reflections from planetary surfaces recorded by distant receivers provide insights into surface roughness and composition.
6. Maritime and Airborne Surveillance: Airborne bistatic radar systems enhance detection of sea-surface targets, submarines, and small vessels by exploiting scattered wavefronts from multiple angles.

Advantages of Bistatic Radar

• Stealth Detection: Greater capability to detect low-observable targets.
• Reduced Vulnerability: Passive or separated receivers make the system less susceptible to electronic countermeasures and anti-radiation attacks.
• Extended Coverage: The transmitter and receiver can cover a larger area or observe from multiple perspectives.
• Flexible Geometry: Allows optimisation for specific detection conditions, such as ground clutter reduction.
• Energy Efficiency: Passive receivers do not require transmission power, lowering operational costs.

Limitations and Challenges

• Complex Geometry: Target localisation and tracking require more complex calculations than in monostatic radar.
• Synchronisation: Accurate timing and frequency synchronisation between transmitter and receiver are essential for precise measurements.
• Data Fusion: Combining data from multiple receivers or transmitters demands sophisticated algorithms and communication links.
• Calibration and Alignment: System accuracy depends heavily on precise knowledge of transmitter and receiver positions.
• Environmental Sensitivity: Atmospheric variations can distort the signal path and affect measurement accuracy.

Recent Developments and Future Trends

Modern advances in digital signal processing, software-defined radar systems, and global navigation satellite systems (GNSS) have revitalised interest in bistatic and multistatic radar. Innovations include:

• GNSS-Based Bistatic Radar: Using navigation satellites as transmitters to monitor the Earth’s surface and oceans.
• Spaceborne Bistatic SAR: Dual-satellite radar imaging systems that produce three-dimensional Earth surface data.
• Networked Radar Systems: Integration of multiple radar nodes operating cooperatively, providing enhanced detection and tracking.
• Artificial Intelligence and Data Fusion: Machine learning algorithms for improved signal interpretation and clutter discrimination.

These developments promise greater sensitivity, wider coverage, and more efficient use of non-cooperative signal sources in the future of radar technology.