Formation Flying SAR: Analysis of Imaging Performance by Array Theory

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

Real-Time Hardware-in-the-Loop Laboratory Testing for Multisensor Sense and Avoid Systems

This paper focuses on a hardware-in-the-loop facility aimed at real-time testing of architectures and algorithms of multisensor sense and avoid systems. It was developed within a research project aimed at flight demonstration of autonomous non-cooperative collision avoidance for Unmanned Aircraft Systems. In this framework, an optionally piloted Very Light Aircraft was used as experimental platform. The flight system is based on multiple-sensor data integration and it includes a Ka-band radar, four electro-optical sensors, and two dedicated processing units. The laboratory test system was developed with the primary aim of prototype validation before multi-sensor tracking and collision avoidance flight tests. System concept, hardware/software components, and operating modes are described in the paper. The facility has been built with a modular approach including both flight hardware and simulated systems and can work on the basis of experimentally tested or synthetically generated scenarios. Indeed, hybrid operating modes are also foreseen which enable performance assessment also in the case of alternative sensing architectures and flight scenarios that are hardly reproducible during flight tests. Real-time multisensor tracking results based on flight data are reported, which demonstrate reliability of the laboratory simulation while also showing the effectiveness of radar/electro-optical fusion in a non-cooperative collision avoidance architecture

Real-Time Hardware-in-the-Loop Laboratory Testing for Multisensor Sense and Avoid Systems

This paper focuses on a hardware-in-the-loop facility aimed at real-time testing of architectures and algorithms of multisensor sense and avoid systems. It was developed within a research project aimed at flight demonstration of autonomous non-cooperative collision avoidance for Unmanned Aircraft Systems. In this framework, an optionally piloted Very Light Aircraft was used as experimental platform. The flight system is based on multiple-sensor data integration and it includes a Ka-band radar, four electro-optical sensors, and two dedicated processing units. The laboratory test system was developed with the primary aim of prototype validation before multi-sensor tracking and collision avoidance flight tests. System concept, hardware/software components, and operating modes are described in the paper. The facility has been built with a modular approach including both flight hardware and simulated systems and can work on the basis of experimentally tested or synthetically generated scenarios. Indeed, hybrid operating modes are also foreseen which enable performance assessment also in the case of alternative sensing architectures and flight scenarios that are hardly reproducible during flight tests. Real-time multisensor tracking results based on flight data are reported, which demonstrate reliability of the laboratory simulation while also showing the effectiveness of radar/electro-optical fusion in a non-cooperative collision avoidance architecture

Effects of Orbit and Pointing Geometry of a Spaceborne Formation for Monostatic-Bistatic Radargrammetry on Terrain Elevation Measurement Accuracy

During the last decade a methodology for the reconstruction of surface relief by Synthetic Aperture Radar (SAR) measurements – SAR interferometry – has become a standard. Different techniques developed before, such as stereo-radargrammetry, have been experienced from space only in very limiting geometries and time series, and, hence, branded as less accurate. However, novel formation flying configurations achievable by modern spacecraft allow fulfillment of SAR missions able to produce pairs of monostatic-bistatic images gathered simultaneously, with programmed looking angles. Hence it is possible to achieve large antenna separations, adequate for exploiting to the utmost the stereoscopic effect, and to make negligible time decorrelation, a strong liming factor for repeat-pass stereo-radargrammetric techniques. This paper reports on design of a monostatic-bistatic mission, in terms of orbit and pointing geometry, and taking into account present generation SAR and technology for accurate relative navigation. Performances of different methods for monostatic-bistatic stereo-radargrammetry are then evaluated, showing the possibility to determine the local surface relief with a metric accuracy over a wide range of Earth latitudes

Performance, Applications and Technological Issues of next generation Bistatic Synthetic Aperture Radar

Bistatic Synthetic Aperture Radar is defined when antennas for reception and transmission are physically separated. Such a spatial separation allows bistatic SAR to operate in different geometric and functional configurations, thus being able to achieve a vast amount of scientific and practical applications. Even if these applications are known from a long time, nowadays, Bistatic SARs can be only considered experimental systems. No operative missions based on Bistatic SAR have never been realized (the sole exception being TanDEM-X mission, scheduled for launch within 2010, and aiming at realizing a spaceborne bistatic SAR interferometer based on two LEO satellites flying in close formation). This situation has a twofold origin. First bistatic SAR observation involves a series of implementation challenges. Indeed it requires the coordinated use of two systems, with accurate time synchronization and antenna pointing between transmitter and receiver and with accurate antenna separation measurement and control. Then, the study of possible applications was very often conducted with a qualitative approach, in which, besides the basic principle of a given application, rarely quantitative methodologies for performance evaluation were proposed. This thesis, on the basis of ongoing national and international research programs and strategies, selects and analyzes three possible future realizations of bistatic SAR and, for each of them, it individuates some promising applications, develops and settles methods for performance estimation and feasibility analysis, and suggests technological solutions towards their implementation. The proposed tools, although defined for specific realization of bistatic SARs, can be considered of general validity

Performance of Stereoradargrammetric Methods Applied to Spaceborne Monostatic–Bistatic Synthetic Aperture Radar

This paper aims to investigate the performance of stereoradargrammetric methods applied to spaceborne monostatic–bistatic synthetic aperture radar (SAR) data for digital elevation model (DEM) generation. Stereoradargrammetric techniques for robust DEM generation were successfully experienced on monostatic repeat-pass SIR-A, SIR-B, SIR-C/X-SAR, ERS1/2, JERS-1, and Radarsat data. However, novel configurations achievable by modern spacecraft flying in formation will allow for the attainment of very large baselines between the antennas in a single-pass bistatic geometry so that the height determination accuracy can benefit from both stereo effect and simultaneous acquisition. Fivemodels for relief reconstruction by monostatic–bistatic SAR stereoradargrammetry are presented, and an error budget is assessed for each of them. Results of the sensitivity analysis exhibit metric accuracy, and therefore, the technique could be applied for height reconstruction as a methodology complementary to SAR interferometry

Synthetic Aperture Radar for Earth Observation from a Lunar Base: performance and potential applications

Starting from the reborn international interest in lunar exploration and the widely documented need for hyper-accurate measurements of Earth crustal dynamics, at a high revisit frequency and on a global scale, this paper presents the idea of a Moon-based interferometric synthetic aperture radar. After introducing models to describe synthetic antenna formation and discussing key issues of a Moon-based radar for Earth remote sensing, a preliminary system design and performance analysis are conducted. Quantitative results are presented in terms of achievable resolutions and needed radar parameters. Although the Moon-based observatory requires technical solutions and a budget that make likely its realization only in the far term, a comparison with currently planned spaceborne interferometric systems shows that it offers height measurement accuracies at a level and with a frequency not achievable otherwise

Moon-Based Synthetic Aperture Radar: Review and Challenges

Based on a recent renewed interest in the utilization of Moon as a platform for Earth remote sensing, this paper reviews the concept of a Moon-based Synthetic Aperture Radar (SAR). Such a system presents some features, which differentiate it from a conventional spaceborne SAR. Indeed process of antenna synthesis and the observation geometry are quite different. The platform, on which the observatory shall be build up, is not an orbiter or a space formation, but a whole celestial body. Earth-Moon relative motion generates the synthetic aperture and the properties of such a motion allow for the exploitation of very long synthetic antennas. Moreover two or more antennas can be located over lunar surface realizing a distributed SAR and enabling single-pass interferometric SAR applications, e.g. crosstrack interferometry and tomography. System parameters and technological challenges of such a Moon-based observatory are also discussed