Leah-Nani Alconcel, University of Birmingham, UK; Lily Beesley, University of Birmingham, UK; Gruffudd Jones, University of Birmingham, UK; Morgan Coe, University of Birmingham, UK; Marina Gashinova, University of Birmingham, UK
Keywords: inverse synthetic aperture radar, ISAR, satellite characterisation, encounter modelling
Abstract:
To complement ground-based radar and optical sensor observations, an in-orbit sensing technology that achieves detailed multi-aspect imagery would enable on-orbit inspection, activity monitoring, and fault or anomaly detection, where resident space object (RSO) characterisation with fine resolution (at least 10 cm) is needed to enable feature identification. A radar technique particularly suitable for the provision of on-orbit SDA is inverse synthetic aperture radar (ISAR). The ISAR imaging principle has a number of advantages for imaging highly dynamic objects. It is formed by utilising differential Doppler shifts of different parts of moving objects within the surveillance volume during an observation. The use of auto-focussing algorithms relaxes the requirement on the knowledge of motion parameters, which enables imaging of e.g. non-cooperative objects and space debris travelling at high relative velocities to the monitoring platform.
Previous research by the authors defined the concept of sub-THz ISAR imagery from space-borne platforms [2]. Generalised scenarios of co- and counter-rotating LEO satellites were considered and initial estimates of ISAR resolution with fixed and tracking antennas were obtained. Entropy minimisation and phase gradient autofocus were used for motion compensation. A more recent study focused on the development of feasible deployment scenarios of a monitoring satellite (MS), an end-to-end metaheuristic simulator for persistent monitoring of GEO assets (observed satellite, OS), and an attitude-agnostic alignment method for feature tracking within the imagery [3]. A dual-band antenna system using two frequencies, 75 GHz and 300 GHz, was investigated for ISAR image formation, which is also used in this study.
In this work, the concept of cooperative optical and ISAR systems to detect targets and autonomously optimise the conditions for ISAR image formation during an encounter between two satellites is introduced and explored. The system analysis includes the geometry of the observed satellite (OS), reflectivity of the materials, solar array orientation, solar angle, and the orbital geometries of both OS and MS. A wide-angle camera has been identified as a suitable option, and the detection capability of a specific star tracker has been explored. The results show that detection of the OS is possible using the MS optical system at ranges of up to 2,000 km. When coupled with the pre-existing ephemeris knowledge of MS and OS positions, this should enable precise MS re-orientation and dual-frequency sub-THz antenna pointing to maximise the number of high-resolution ISAR frames produced at every encounter.
[1] Emidio Marchetti et al. “Space-Based Sub-THz ISAR for Space Situational Awareness-Concept and Design”. In: IEEE Transactions on Aerospace and Electronic Systems 58.3 (2022), pp. 1558–1573. ISSN: 15579603. DOI: 10.1109/TAES.2021.3126375.
[2] Gruffudd Jones et al. “Strategies for monitoring of assets in geosynchronous orbit (GEO) using Inverse Synthetic Aperture Radar (ISAR)”. Submitted to IEEE Transactions in Radar Systems. Available at TechRXiv. DOI: 10.36227/techrxiv.173750120.09426378/v1
Date of Conference: September 16-19, 2025
Track: Space-Based Assets