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The generation of a Distributed SAR (DSAR) image can be achieved by combining, both in amplitude and in phase, a stack of low performance bistatic raw matrixes. The paper addresses the complexity of multi-platform image formation developing a high-fidelity simulation tool for the generation of DSAR products, which is useful for the validation of theoretical performance models and thus for both system/payload design and mission analysis.

The ARGO project brings forward an innovative concept in the field of Maritime Situational Awareness (MSA). Currently, the maritime traffic monitoring is mainly supported by the Automatic Identification System (AIS), a system in which medium and large ships (>300 tons) are equipped by a transponder transmitting vessels identification data and information about their route, enabling ship-to-ship and ship-to-shore data exchange within about 60 nautical miles from the coast. The AIS system is conceived on a collaborative approach: only the ships whose AIS transponder is turned on can be detected and monitored. Hence, non-collaborative vessels do not leave a trace in the AIS system. The ARGO project aims to improve the security of ships providing an effective understanding of maritime traffic, including also non-collaborative vessels. Currently, several space-based monitoring systems have been proposed and tested. All of them carry out the ship detection based on echoes in optical or radar images. This have strongly improved the MSA in the last decade, but such systems are useless to fight against illegal activities performed by go-fast boats. In fact, this kind of vessels, thanks to their design, materials and high speed, cannot be imaged in radar images and the illegal activities are typically performed during the night-time when optical sensors cannot provide data. However, thanks to their high velocity and narrow beam, these boats are likely to produce long visible wakes. Such wakes are very persistent, lasting for distances of several kilometers, and they can be observed in Synthetic Aperture Radar (SAR) imagery. Hence, ARGO project will enable the space-based surveillance of seas under all weather and day-and-night conditions, overcoming the idea of the ship detection based on echoes in optical or radar images and exploiting the wake presence as an indirect signature of the ship presence. The unique capabilities of the proposed system make it suited for the detection of very fast ships, typically used for illegal affairs, strongly contributing to border security and passengersgoods safety.

The exploitation of modular CubeSat as main platforms in high-performance Distributed Synthetic Aperture Radar (DSAR) systems can be a breakthrough for the future of small Earth Observation satellites. If on one side, the distribution of SAR payload among several co-flying platforms, as in Formation-Flying SAR system (FF-SAR), enables to obtain high SAR imaging capability, from the other side, the modularity, applied to CubeSat platforms, allows to significantly reduce the whole timeframes for the development and launch, that means low-cost and easier access to space. Therefore, the number of elements in the formation can be increased to have more acquisitions and improved performance. The goal of this work is twofold: to propose feasible solutions for the design of each modular subsystem, and to demonstrate that modular PnP CubeSat can operate as main platform in a FF-SAR mission. Indeed, the design is driven by both the need to make hardware and software components as more reusable as possible, in such a way to facilitate the readiness of using the same modules in different platform configurations for different mission scenarios, and to ensure typical spacecraft operations as well as FF-SAR applications. For example, the Attitude Determination Control Subsystem (ADCS) software has been abstracted from specific sensors and actuators to allow the user to change either the type of ADCS hard-ware and its mounting location with no (or few) required further modifications. By standardizing hardware interface and by decomposing the single integrated ADCS software block in different single software components, only the appropriate software modules can be easily selected, when reused, based on available sensors and actuators. At the same time, FF-SAR mission-related multiple pointing modes required for SAR operations, as well as attitude determination and control capabilities for the acquisition, reconfiguration and maintenance of the formation, must be ensured. To this end, a simulator developed in MATLAB/Simulink environment will be discussed in this paper to show the feasibility of this approach.

The paper investigates the capabilities of a formation flying SAR (FF-SAR) for Earth observation from space. FF-SAR is a versatile remote sensing concept which is aimed at replacing large and complex monolithic systems and satellites by multiple, very compact, units (e.g. CubeSats), flying in formation, that are able to achieve the same or even better performance. Moreover, FF-SAR enables new observation techniques that are not possible or not reliable by single-satellite monolithic missions. The paper presents some tools that can be used to support the selection of design parameters. The tools are applied to a specific realization of an FF-SAR in which a cluster of passive CubeSats is considered using a pre-existing SAR mission as an illuminator of opportunity.

Distributed Synthetic Aperture Radar (DSAR) 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. This paper focuses on basic system requirements and formation flying aspects relevant to DSAR. In particular, trajectory design approaches that can fulfil payload requirements while relaxing formation control issues are addressed. Long term stability is also investigated.

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.

Distributed Synthetic Aperture Radar (DSAR) is one of the most promising approaches to enable the utilization of small satellite platforms for gathering radar images as well as reducing the time-to-market and costs. The paper aims at estimating the main imaging properties achievable by a DSAR under realistic satellite trajectories and working conditions. To this end, a simulation environment has been developed in which DSAR performance has been investigated including the most significant error sources, i.e. radar synchronization errors, position and pointing errors, co-registration errors, antenna pattern errors and signal noise. The estimation has been carried out considering satellite formations with both a relevant along-track distance among the receivers and a dominant cross-track/vertical separation, to investigate different application scenarios. The conducted sensitivity analysis provides some valuable insights into the uncertainty in the error sources which can be tolerated. From an application perspective, the results show that the impact of the error sources can be made negligible thus confirming that DSAR enables azimuth ambiguity suppression, SNR improvement and coherent resolution enhancement. Moreover, DSAR can achieve an accuracy in pointing, antenna pattern, coregistration, and the clock error, in line with well-assessed techniques.

The recent launches of Earth Observation (EO) satellites have made numerous SAR images available to dynamically monitor the ocean with improved spatial resolution at shorter revisit time. Focusing on automatic target detection of ships with the specific aim of improving our Maritime Domain Awareness (MDA), this work assesses the capabilities of multi-frequency/multi-mission spaceborne Synthetic Aperture Radar (SAR) data. Specifically, Sentinel-1 (C-band), COSMO-SkyMed (X-band), and SAOCOM (L-band) missions have been considered in this analysis. The aim of the paper is to present an efficient approach for interpreting images acquired within small time gaps, ensuring fisheries and pollution control, anti-piracy actions, and surveillance over coastal/protected regions.

Maritime trade and trasport occupy a pivotal position in the current era of globalization. Thus, monitoring ships at sea represents the starting point of this paper in which a novel approach to detect ships by wake has been proposed, based on Instance Segmentation deep learning architecture Mask R-CNN. In order to train and test this network, 766 wake chips cropped from 50 multispectral images acquired from Sentinel-2 satellites were observed. In particular, B2 (blue), B3 (green), B4 (red) and B8 (Infrared) bands were considered since they are all characterized by same resolution. The results proved that Mask R-CNN is capable to detect the vast majority of ship wakes with high confidence percentage in different configurations, i.e. slanted wakes, multiple wake scenarios or wakes in dark areas not related to their features.

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.

The paper shows a preliminary assessment of a novel hybrid technique for wake-based ship detection in SAR images. The combined approach leverages the more mature Radon-based wake detection in synergy with more recent deep learning object detection. The proposed methodology aims to handle the intrinsic limitations of both approaches. Results confirm that the diversity of the techniques can be deeply exploited to achieve a near-real time and reliable tool to improve the maritime situational awareness.

This article analyzes the process of image synthesis for a formation flying synthetic aperture radar (FF-SAR), which is a multistatic synthetic aperture radar (SAR) based on a cluster of receiving-only satellites flying in a close formation, in the framework of the array theory. Indeed, the imaging properties of different close receivers, when analyzed as isolated items, are very similar and form the so-called common array. Moreover, the relative positions among the receivers implicitly define a physical array, referred to as spatial diversity array. FF-SAR imaging can be verified as a result of the spatial diversity array weighting the common array. Hence, different approaches to beamforming can be applied to the spatial diversity array to provide the FF-SAR with distinctive capabilities, such as coherent resolution enhancement and high-resolution wide-swath imaging. Simulation examples are discussed which confirm that array theory is a powerful tool to quickly and easily characterize FF-SAR imaging performance.

The paper shows the strong potentialities of multimission/multifrequency SAR data for improving the maritime situational awareness in coastal areas. Two main issues are analyzed: the detection of ships that are visible in SAR images and the identification of non-collaborative vessels, which are not visible in SAR images. In the first case, the multimission/multifrequency data guarantees: (a) smaller revisit time with respect to a single mission, enabling cross-check of the detection in several images and, thus, improving the detection rate, and (b) the availability of images covering large areas at low resolution as well as smaller swath observed with higher resolution. This is crucial in particular for the coastal areas where local phenomena can strongly affect the detection performance. In the second case, the multimission/multifrequency data enables innovative approaches exploiting the different appearance of ship and its wake at different frequencies.

This paper discusses a new receiving antenna for remote sensing applications to be mounted onboard a formation-flying synthetic aperture radar (FF-SAR) bistatic system based on the CubeSat standard. The formation works as a bistatic SAR collecting microwave signals coming from a transmitting SAR unit. The receiving antenna has been designed according to the acquisition modes of the formation, namely a stripmap mode for signal-to-noise ratio improvement and a High-Resolution Wide-Swath mode for the monitoring of large regions. In order to meet the very different requirements for both operating modes with the physical constraints imposed by the nanosat geometry, a large reflector with reconfigurable feed has been conceived and simulated. The results show that the proposed antenna accomplishes the design specifications.

The paper focuses on the concept of a formation-flying synthetic aperture radar (FF-SAR) bistatic system composed of a set of compact, low-weight satellite receivers in close formation (within 1 km) placed in the same low-Earth orbit at large distance (about 100 km) from a transmitter. Each receiver is conceived to fit a 12-unit CubeSat. A signal model adapted to the proposed formation geometry is also presented, and a corresponding processing scheme to achieve range swath widening and signal-to-noise ratio (SNR) improvement is illustrated and discussed.

With the specific aim of improving our Maritime Domain Awareness, satellite data enable a wide range of applications, including fisheries and pollution control, anti-piracy actions, and surveillance over coastal/protected regions. Among all the available data, the ones gathered by space-borne synthetic aperture radar (SAR) are attracting large interest thanks to their coverage and all-weather and all-time observation capabilities. Currently, Artificial Intelligence (AI) has been widely recognized as the only way to take fully advantages of increasing amount of Earth Observation (EO) data, and Deep Learning-based detectors have been successfully applied for the detection of ships from cluttered sea surface. However, nonetheless their exploitation for ship route estimation purposes, the problem of wake detection by deep learning has been barely touched. With this concern, the paper investigates one of the latest deep learning architecture for object detection, i.e. RetinaNet, as an effective means to achieve a robust wake detector.

The paper investigates timing aspects of the design and the operations of a formation flying distributed Synthetic Aperture Radar (SAR). The analyzed system considers a satellite cluster including one transmitter and $N$ receivers flying in formation with a dominant along-track baseline. This realizes a sensor receiving $N$ samples at azimuth-displaced positions for each transmitted pulse, enabling several applications. In fact, besides the possibility to implement single-pass multi-baseline interferometric SAR techniques, the bistatic raw data collected by each receiver can be combined to generate a single higher performance image, e.g. High-Resolution Wide-Swath (HRWS) imaging. The paper investigates the latter topic, proposing dedicated solutions to preserve the imaging performance against the effects of relative orbit dynamics and residual orbit control errors. In details, the factor $N$ can be interpreted as the redundancy, or the maximum number of degrees of freedom of the distributed system when compared to a single-channel monostatic SAR. Such a redundancy can be exploited in different ways pending on the application of suitable signal reconstruction and it represents an important parameter affecting the system timing. In this contest, the manuscript proposes an approach to system timing, satisfying not only ambiguity-related issues but also requirements concerning signal parameters, like SNR and\or Peak-to-Side-Lobe Ratio (PSLR). Finally, the timing considerations are recast in the framework of the overall design of a formation flying SAR.

New design concepts for sensors, systems and satellites are crucial to enable innovative and high-performance working modes. In this contest, the concept of distributed aperture is becoming of a strong interest for the future Synthetic Aperture Radar (SAR) systems. It is based on the idea that 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, and, thanks to passive operations, can achieve very high performance through a series of very compact, low weight, agile, satellite platforms. The advantages of such system are innumerable and the potentialities of reliability, flexibility and modularity make it a very interesting approach for the future missions. However, a number of technological and operational issues shall be solved. The formation acquisition after launch, control and maintenance are big challenge. Also, a small satellite-based distributed SAR system may pose challenging requirements in terms of electrical power, communication link, and payload synchronization. In this ambit, the recent technology upgrades have led to the development of small platforms with increasing capabilities, thus opening the way for using distributed space systems also for cost-effective operational, public and commercial services. Finally, signal processing related issues shall also be solved to enable high-performance applications, not reachable by existing systems. The paper is focused on the distributed SAR concepts, showing: (a) main design drivers, (b) main issues to be solved and related approaches and (c) main potentialities.

Recently, international agencies for border security ask for an improvement of the actual Maritime Situational Awareness. This manuscript presents preliminary results of a detection technique of go-fast boats, whose utilization in illegal affairs is strongly increasing. Their detection is very challenging since: (i) their echo is not visible in SAR images, and (ii) the illegal activities are carried out in the nighttime making useless the optical sensors. However, their wakes are very persistent and extent in SAR images for some kilometers. Hence, the manuscript shows an innovative deterministic methodology for the ship detection based on the wake signature. It firstly identifies pixels crossed by the wakes, whose presence is, then, validated in two steps. The first level of validation estimated how prominent the wake components are with respect to their background. The second level of validation exploits the presence of the wakes among neighbor pixels. The approach has been applied on ships imaged by TerraSAR-X mission showing the same peculiarities of go-fast boats. Results highlight the potentialities of the proposed approach, which can be also conceived as a subsequent step in a hybrid system, whose preliminary wake detection screening is carried out by different techniques.

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.

A novel technique for ship wake detection has been recently proposed and applied on X-band Synthetic Aperture Radar images provided by COSMO/SkyMed and TerraSAR-X. The approach shows that the vast majority of wake features are correctly detected and validated in critical situations. In this paper, the algorithm was applied to 28 wakes imaged by Sentinel-1 mission with different polarizations and incidence angles with the aim of testing the method’s robustness with reference to radar frequency and resolution. The detection process is properly modified. The results show that the features were correctly classified in 78.5% of cases, whereas false confirmations occur mainly on Kelvin cusps. Finally, the results were compared with the algorithm performance on X-band images, showing that no significant difference arises. In fact, the total false confirmations rate was 15.8% on X-band images and 18.5% on C-band images. Moreover, since the main criticality concerns again the false confirmation of Kelvin cusps, the same empirical criterion suggested for the X-band SAR images yielded a negligible 1.5% of false detection rate.