Space Instruments

Bistatic Radar (Space)

Q: What is the bistatic range equation?

The bistatic radar range equation replaces the monostatic R^4 term with the product R_T^2 * R_R^2, where R_T is the transmitter-to-target range and R_R is the target-to-receiver range. The bistatic RCS sigma_B replaces the monostatic RCS. SNR = (P_t * G_t * G_r * lambda^2 * sigma_B) / ((4*pi)^3 * R_T^2 * R_R^2 * kT_s * B * L). This means bistatic geometry can be more favorable than monostatic when the target's bistatic RCS is larger, as occurs near forward scatter.

Q: What is GNSS Reflectometry?

GNSS-R uses GPS, Galileo, or BeiDou navigation satellites as bistatic radar transmitters of opportunity. A dedicated receiver satellite collects the reflected GNSS signals from the Earth's surface. NASA's CYGNSS constellation (8 microsatellites) uses L-band GNSS-R to measure ocean surface wind speed by analyzing the delay-Doppler map of reflected GPS signals. The technique provides global coverage with no dedicated transmitter, dramatically reducing mission cost.

Q: Why does forward scatter enhance detection?

At bistatic angles near 180 degrees (forward scatter), Babinet's principle predicts that the bistatic RCS equals the physical cross-section of the target projected onto the scattering plane, regardless of the target's shape or stealth coatings. A stealth aircraft designed to minimize monostatic RCS may have a forward-scatter bistatic RCS hundreds of times larger. This makes forward-scatter bistatic radar a potential counter-stealth technique.

Bistatic Radar (Space) uses physically separated transmitter and receiver platforms in orbit to illuminate and detect targets or surface features. Unlike monostatic radar where the same antenna transmits and receives, bistatic geometry places the transmitter and receiver at different locations, creating a bistatic triangle. In space applications, this includes dedicated bistatic SAR pairs (like TanDEM-X), GNSS Reflectometry (GNSS-R) where navigation satellites serve as illuminators of opportunity, and forward-scatter surveillance concepts for detecting stealth targets whose monostatic RCS is minimized but whose forward-scatter cross-section remains large.

Category: Space Instruments

Key Missions: TanDEM-X, CYGNSS

Understanding Bistatic Radar (Space)

In a monostatic radar, the signal travels to the target and back along the same path, experiencing R⁴ path loss. In bistatic radar, the signal travels from transmitter to target (R_T) and from target to receiver (R_R), experiencing R_T²×R_R² loss. When one leg is short (e.g., the transmitter is a GNSS satellite at 20,200 km but the receiver is in LEO at 500 km), the geometry can be highly favorable.

The bistatic RCS (σ_B) differs from monostatic RCS and depends on the bistatic angle β between the transmitter and receiver directions as seen from the target. Near forward scatter (β ≈ 180°), Babinet's principle causes σ_B to equal the target's geometric silhouette area, independent of stealth shaping or RAM coatings.

Bistatic Radar Range Equation

Bistatic SNR:
SNR = P_t·G_t·G_r·λ²·σ_B / ((4π)³·R_T²·R_R²·kT_sBL)

Forward-scatter RCS (Babinet):
σ_B(β→180°) = 4πA²/λ²
A = geometric silhouette area of target

Space Bistatic Radar Missions

Mission	TX Source	Band	Geometry	Application
TanDEM-X	TerraSAR-X	X-Band	Dedicated pair	DEM generation
CYGNSS	GPS sats	L-Band	GNSS-R	Ocean wind speed
SAOCOM-CS	SAOCOM	L-Band	Companion sat	Soil moisture
Passive Bistatic	TV/FM broadcast	VHF/UHF	Ground RX	Air surveillance

Common Questions

Frequently Asked Questions

What is the bistatic range equation?

It replaces monostatic R⁴ with R_T²×R_R² and uses bistatic RCS σ_B. Geometry can be more favorable when the target's bistatic RCS exceeds its monostatic RCS, as in forward scatter.

What is GNSS Reflectometry?

GNSS-R uses GPS/Galileo/BeiDou as bistatic transmitters of opportunity. NASA's CYGNSS (8 microsats) measures ocean wind by analyzing delay-Doppler maps of reflected L-band GPS signals, requiring no dedicated transmitter.

Why does forward scatter enhance detection?

At β≈180°, Babinet's principle makes σ_B equal to the target's physical silhouette, regardless of stealth coatings. A stealth aircraft's forward-scatter RCS can be hundreds of times larger than its monostatic RCS.

Related Terms

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