Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRHigh‑resolution imaging despite the simultaneous motion of both the sensor platform and the target scene, combined with rapid changes in viewing direction—this is the performance profile envisioned for a novel airborne radar system. An eagle possesses an exceptionally high visual acuity and can precisely detect and interpret ground structures even while in flight. Researchers at Fraunhofer FHR are working to replicate this capability technologically: a radar system mounted on an aircraft that provides an electronically steerable, highly agile field of view. The viewing angle is intended to be redirected toward relevant scenarios within extremely short time intervals. Which scenarios are considered relevant will in the future be determined by a specially developed processing and decision‑making algorithm.
High resolution despite rapid changes in viewing direction
The objective is to develop a multifunctional radar system capable of capturing ground scenes at high resolution, detecting moving targets, tracking them with precision, and steering its transmit and receive beams within microseconds and with extremely high accuracy. Whereas conventional ground‑surveillance radars predominantly operate in the X‑band, the new concept is based on the Ka‑band, which offers approximately four times higher frequencies.
To achieve this, a SAR system is combined with techniques for Ground Moving Target Indication and tracking (GMTI). High system bandwidths are required, as they form the basis for high‑resolution imaging modes. Current systems already achieve ground resolutions below 2 cm × 2 cm.
A key challenge lies in maintaining image quality during rapid changes in viewing direction. At high signal bandwidths, deviations of the radar beam from its intended direction and frequency‑dependent phase distortions (squint effects) occur. New hardware approaches aim to minimize these effects by enabling the antenna front‑end to perform inertial‑free, fully electronic 2D beam steering with highest precision in both azimuth and elevation. The goal is distortion‑free signal propagation in all desired spatial directions.
Compact, lightweight, and suitable for small platforms
A major advantage of the Ka‑band is the potential for miniaturization. The antenna aperture can be reduced by up to 90 %. This makes the system particularly suitable for small airborne platforms such as light aircraft, drones, or future swarms of cooperating flying sensor platforms.
In parallel, the remaining system electronics are adapted to the reduced aperture. Compact, integrated electronic architectures are required, with heat generation and dissipation posing particular challenges in the confined installation space.
The reduction in volume is enabled by integrated high‑frequency chips for the Ka‑band and compact packaging and interconnection technologies—while simultaneously increasing system performance. A technical challenge arises from the higher free‑space attenuation in the Ka‑band compared to the X‑band, which tends to reduce range at equal transmit power. Here, the reduced sensor size again offers advantages: the smaller area allows for a significantly higher density of antenna elements, increasing the radiated power per unit area. This helps compensate for range losses. Additionally, the higher reflectivity of many ground structures when illuminated with Ka‑band signals has a positive effect on achievable distances.
Technically, prototype array modules have already been developed and corresponding test environments established. A demonstrator consisting of a transmit‑array module group has been built and characterized. The next step is the complete construction of a phased‑array demonstrator comprising four receive arrays, a high‑power transmit array, and a matched digital signal‑generation and signal‑processing chain.
Perspectives for radar and communication
The primary application of the new system is high‑resolution airborne observation of ground scenes. Beyond this, electronic beam steering is also highly relevant for modern communication systems. Electronically steerable broadband antennas for high‑data‑rate communication between ground stations and satellites benefit from this technology, as do air‑to‑ground communication systems that must successively track different satellites and perform rapid changes in viewing direction.
The insights gained thus lay the foundation for novel radar systems that combine traditional reconnaissance capabilities with modern communication functions in a single integrated platform.
