Transfer of synthetic aperture radar approaches to the infrared range

• Applicant: Professor Dr.-Ing. Christian Waldschmidt
• Subject Area: Communication Technology and Networks, High-Frequency Technology and Photonic Systems, Signal Processing and Machine Learning for Information Technology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541802722

The joint evaluation of measurements of a radar sensor with a wide field of view along a known trajectory of the sensor allows to generate a synthetic aperture radar (SAR). This principle is used in radar engineering to achieve significantly improved image resolution along the trajectory. The basic requirement for this evaluation technique is a coherent addition of all received target signals, which requires an extraction of the phase of the received signal relative to the transmitted signal. In addition, this achieves an improvement in the signal-to-noise ratio, since non-target signals do not complement or even cancel each other out during processing. Real targets, on the other hand, ideally experience a constructive integration gain. Infrared lidar systems do not use SAR techniques yet. In contrast to coherent processing, where received signals are mixed with a local oscillator, commercial lidar systems operate mainly on time-of-flight measurements of pulsed signals. With advances in optical components for telecommunication applications, coherent processing does now present a promising perspective for further improvements of optical sensor performance. Coherent sensors with intricate modulation schemes resembling those of radar sensors could potentially yield decisive improvements in processing versatility and detection sensitivity. However, optical systems are often constructed from within the optical domain without establishing and exploiting crosslinks to established radar concepts. The challenges of sensor system design and signal processing are already well-known and thoroughly researched in radar engineering. In this project, the aim is to transfer existing knowledge on coherent sensing and SAR imaging from radar systems to an optical system concept, prove its functionality and research the underlying cause-effect relationships. A prime focus is placed on stripmap SAR evaluation via a robust backprojection approach for sceneries up to few centimeters in front of the sensor. Building on the first goal of a fundamental proof-of-concept, the new measurement technique are analyzed to understand its influential parameters. The relationship of sensor construction and components, of system parameters such as power loss, noise, and phase noise, and of target properties shall be investigated. For SAR functionality, the optical properties of targets and scenery as seen from different angles are of critical importance, as well as interference from surface imperfections known as speckle. Finally, the aim is to understand the impact and origin of limiting constraints on resolution and signal dynamic range in coherent optical imaging.

• DFG Programme: Research Grants