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Formation Flying Synthetic Aperture Radar (FF-SAR) is defined as a SAR in which the signal emitted by the transmitter and scattered from the area of interest is not collected by a single receiver but by many, conveniently distributed, formation flying, receivers. The concept of distributed aperture can enable new SAR working modes, but more important, thanks to passive operations, can achieve very high performance through a series of very compact, low weight, agile, satellite platforms. Such a distributed space system can be regarded as a system in which the payload functionality is broken apart and distributed among the different elements of the system. While fractionation and formation flying may lead to many advantages, distributed space systems pose a number of technological and operational issues at system and subsystem level. Signal modeling, radar processing, system operations and formation flying aspects are analysed in this paper and an end-to-end space system demonstrator concept is also proposed including 3 satellites working in X-band, flying in a LEO close formation. Mission operations and system budgets are performed at a preliminary level showing the possibility to achieve mission objective by platforms of micro-satellite class (

Formation Flying Synthetic Aperture Radar (FF-SAR) is a specific case of distributed multistatic SAR system, in which many co-flying platforms cooperate with each other to implement new and complex SAR modes, otherwise impossible with the current monolithic systems. No space mission has been realized yet, demonstrating and exploiting FF-SAR properties and peculiarities. Examples of mission concepts for a SAR distributed among several formation flying satellites have been recently proposed based on sub-500 kg class satellites. However, their realization is slowed down by major challenges that still affect the feasibility and the reliability of FF-SAR concept. These include: i) satellite formation-flying, intended as the problem of relative motion design (including formation acquisition and reconfiguration) and relative navigation, as well as autonomous formation maintenance, reconfiguration and control; ii) multistatic SAR synchronization, both in time and in phase to enable the suitable coherent combination of signals collected by different platforms; iii) multistatic SAR processing, that is digital beamforming to generate higher performance images/products from the low performance signals collect by each receiver separately. The paper investigates a precursor FF-SAR mission based on a formation of CubeSats (each less than 15 kg) for testing and validating solutions to the above-mentioned challenges. Each satellite shall use actuators for formation control, GNSS sensors for accurate orbit reconstruction, and an inter-satellite link to support autonomous formation flying. The paper is focused on definition and preliminary design of the main mission elements.

This paper presents the idea of a hybrid bistatic synthetic aperture radar (SAR) formed by a constellation of spaceborne illuminators and receiving-only airborne forward-looking SARs. The study aims at identifying and developing new imaging techniques for avoidance, landing, and observation, able to guarantee forward-looking real-time, all-weather, day and night, high geometric and radiometric resolution images for Unmanned Airborne Systems (UAS). Nowadays, UAS Navigation is mainly performed with satellite navigation systems (e.g. GPS, Galileo) which offer accurate and reliable navigation data, but no information about the surrounding environment. A vision-based navigation system can greatly improve UAS autonomy with an additional beneficial impact on obstacle avoidance capability. SAR systems have been already experienced on-board aircrafts of different classes. However, such sensors have been only used as remote sensing payloads in side-looking geometry. For navigation purposes a forward looking geometry is preferable, but it has been rarely experienced due to major limitations (left/right ambiguity and poor Doppler resolution) which can be only partially mitigated at the cost of strong complexities. These limitations can be overcome by the proposed space airborne bistatic system provided that specific requirements on the acquisition geometry are satisfied. In addition, the airborne receiver can be much more compact and lightweight, with a reduced power request thus complying with typical UAS requirements. Preliminary feasibility assessment and definition of strawman system concept have been conducted under ESA contract (22499/09/F/MOS) considering UAS vision-based navigation as main application even if further uses, e.g. remote sensing or surveillance, have been envisaged.

This paper presents a mission concept for an end-to-end space demonstration of Formation Flying Synthetic Aperture Radar (FF-SAR) system. This is realized by means of 3 satellites working in X-band, flying in Low Earth Orbit (LEO) formation. One satellite embarks a Transmitting-Receiving (Tx/Rx) radar, i.e. it is a monostatic SAR. The other two satellites are Receiving-only platforms. From the FF-SAR perspective, starting from the design of a general purpose monostatic SAR, targeted to achieve 8m x 8m resolution on ground from a 550-km orbit, FF-SAR properties are exploited to demonstrate performance improvement, namely the achievement of 4m x 4m resolution with the same Noise Equivalent Sigma Zero (NESZ) requirement, testing suitable combinations of FF-SAR techniques like Signal-to-Noise Ratio (SNR) enhancement, Pulse Repetition Frequency (PRF) reduction, Coherence Resolution Enhancement (CRE). Moreover, at the nominal 8m x 8m resolution, 3D imaging and Ground Moving Target Indication (GMTI) techniques can be also tested. The paper focuses on space system design aspects related to the end-to-end demonstration mission, aiming at showing the feasibility of a FF-SAR with micro-satellite class platforms (~100 kg) .

The paper focuses on space system design aspects related to an end-to-end demonstration mission, aiming at showing the feasibility of a Formation Flying Synthetic Aperture Radar (FF-SAR) with microsatellite class platforms (~100 kg). Trajectory design approaches that can fulfil payload requirements are addressed to enable selected FF-SAR applications. The exploitation of these applications relies on suitable combinations of FF-SAR techniques like Signal-to-Noise Ratio (SNR) enhancement, High-Resolution Wide Swath (HRWS) SAR imaging, and Coherence Resolution Enhancement (CRE). In this framework, a cluster of 3 micro-satellites, working in X-band, flying in a Low Earth Orbit (LEO) close-formation, has been designed as a candidate end-to-end system demonstration mission. One satellite embarks a Transmitting-Receiving (Tx/Rx) radar, i.e. it is a monostatic SAR. The other two satellites are Receiving-only platforms. Critical design aspects related to spacecraft subsystems and formation-flying analysis are addressed to confirm the technical feasibility of the spaceborne distributed system implementing the FF-SAR principle.

This paper presents the design of the receiving antenna of an upcoming formation-flying synthetic aperture radar (FF-SAR) based on the CubeSat standard and on a preexisting spaceborne SAR of opportunity. The receiving antenna operates at X-band and is compliant with the imaging modes of the FF-SAR, namely a stripmap mode for signal-To-noise ratio improvement (IM1) and a High-Resolution Wide-Swath mode for the monitoring of large areas (IM2). A large reflector has been designed to meet the high-gain requirement in IM1, while the wide coverage needed in IM2 is ensured by a reconfigurable patch array feed. Full-wave analysis shows that the designed receiving antenna achieves 37.42 dBi gain in IM1 and 12.1 degree half-power beamwidth in IM2.

Monocular Visual Odometry (VO) is widely used for small aerial platforms navigation. However, environment conditions can limit the use of cameras. Radar Odometry (RO) can be envisaged when illumination conditions hinder visual navigation. Indeed, radar can help estimation of platform speed and interframe platform displacement, thus solving also the inherent monocular VO scale estimation problem. Performance of radar-aided monocular VO navigation solution, to be used then within the framework of all-illumination multi-sensor navigation, is assessed with test in real operational scenario. Results show acceptable standalone performance, that can be further improved with more complex architectures.