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Active Filters

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Active filters are electronic circuits that use operational amplifiers (op-amps), resistors, and capacitors to filter out unwanted frequency components from a signal. A first-order low-pass active filter is designed to pass signals with a frequency lower than a certain cutoff frequency and attenuate frequencies higher than that cutoff frequency. The transfer function for a first-order low-pass active filter is:

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Updated: May 13, 2026

Tracking Infiltration Front Depth Using Time-lapse Multi-offset Gathers Collected with Array Antenna Ground Penetrating Radar

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Metasurface-Enabled Active-Like Passive Radar.

Mingyi Li¹, Jiawen Xu¹, Hanting Zhao²

¹State Key Laboratory of Photonics and Communications, Peking University, Beijing, China.

Advanced Science (Weinheim, Baden-Wurttemberg, Germany)

|May 12, 2026

Summary

This summary is machine-generated.

Metasurface-enabled passive radars (MEPRs) overcome limitations of traditional passive radar systems. This novel approach enhances object detection and tracking in cluttered environments without source control.

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Last Updated: May 13, 2026

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Area of Science:

Electromagnetics and Metamaterials
Radar Systems Engineering
Wireless Communication

Background:

Passive radars (PRs) offer cost-effective object detection by utilizing existing wireless signals.
Conventional PRs face challenges with unknown source waveforms, interference, and detecting slow targets.

Purpose of the Study:

To introduce a novel metasurface-enabled passive radar (MEPR) concept.
To enable interference suppression, signal enhancement, and accurate target localization without source control.

Main Methods:

Integration of a space-time-coding programmable metasurface into a passive radar system.
Imprinting distinct spatiotemporal tags onto ambient wireless wavefields.
Proof-of-concept implementation and real-time testing at 5.48 GHz.

Main Results:

Demonstrated real-time imaging and tracking of unmanned aerial vehicles.
Achieved performance comparable to active radar systems.
Successfully operated under interference-rich conditions.

Conclusions:

MEPR technology transforms passive radar into an active-like sensing platform.
MEPR offers a scalable, adaptive, and energy-efficient solution for next-generation sensing.
This technology is foundational for integrated sensing and communication systems.