The IRIDE constellation is an ambitious Italian space program that will support the national authorities in their analyses and monitoring activities, with a focus on Italian territory mapping. It will comprise a series of small satellite subconstellations exploiting a wide range of remote sensing technologies. This article analyses the NIMBUS X-Band synthetic aperture radar (SAR) IRIDE subconstellation, exploring potential orbital configurations beyond the more conventional and widespread dawn-dusk sun-synchronous orbit (SSO) one. In particular, starting from the mission target, we show that a 49° mid-inclination orbit (MIO) in a right-looking StripMap acquisition mode represents a highly effective choice for NIMBUS. We demonstrate that this configuration enhances the systematic coverage of the Italian territory with six nodal days of interferometric revisit time and high spatial resolution, thereby facilitating detailed observations of both natural phenomena and anthropic activities. In terms of differential SAR interferometry (DInSAR) performance, we prove that MIOs do not show significant limitations for what attains the critical baseline and geometric distortions. In addition, MIOs may lead to future advances in creating 3-D displacement maps because they allow for the recovery of the North-South deformation component that, conversely, cannot be precisely measured with DInSAR systems operating in SSO.

In this work, we investigate potential orbital configurations of NIMBUS, one of the Synthetic Aperture Radar (SAR) components of the Italian constellation IRIDE, which will be completed by 2026 under the management of the European Space Agency (ESA), with the support of the Italian Space Agency (ASI). IRIDE is conceptualized as a "constellation of constellations", comprising multiple sub-constellations of satellites positioned in Low Earth Orbit (LEO) and equipped with various sensing technologies, including SAR and optical sensors with medium- and high-resolution capability, operating at different frequency ranges. Given both institutional and commercial users the constellation is expected to fulfill, this work introduces a methodology to compare different orbital configurations and finally identify the optimal orbital arrangement that maximizes the Italian national coverage within the shortest revisit time possible, considering both the capabilities and imaging performance of a SAR system planned for deployment in orbit.

This paper focuses on the timing analysis for a Distributed Synthetic Aperture Radar (DSAR) system exploiting an opportunity illuminator. This is the case of RODiO, which is a new mission concept funded by the Italian Space Agency (ASI) for a Phase A study in the framework of ALCOR program. RODiO’s aim is to match the growing trend towards the miniaturization of satellites and new Synthetic Aperture Radar (SAR) applications. For this reason, RODiO consists in a cluster of four receiving-only CubeSats flying in a close formation and exploiting the independent PLATiNO-1 satellite as a transmitter. Because of the nature of RODiO mission, simultaneous observations with respect to the monostatic illuminator are needed. Through the comparison of monostatic and bistatic chronograms, a timing analysis is performed in the paper in order to assess the effects of DSAR geometry on simultaneous observation opportunities, aiding the design of the system.

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.

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.

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.

This paper presents a segmentation algorithm for marine synthetic aperture radar (SAR) images. The proposed approach is based on the analysis of image sublooks along azimuth direction and exploits a novel indicator, called incoherent entropy, to divide the input targets into three classes, namely, ship, sea, and ambiguity. A global threshold detector is then implemented to discriminate between stable targets (ships), providing low incoherent entropy amplitude, and ambiguity or sea, implying very high and high values of the indicator, respectively. An integration of the segmentation algorithm into maritime surveillance systems is investigated, proposing its use as a discrimination tool to limit the false alarm rate of traditional prescreening algorithms. The main steps of a standard SAR-based ship detection system are thus implemented to provide a first screening of the candidate targets, which are, then, processed by sublook analysis using incoherent entropy to identify false detections. The proposed technique is tested on stripmap SAR images collected by the TerraSAR-X mission, over the Gulf of Naples, Italy. Algorithm detections are validated using ground-truth data, provided by automatic identification system. The results confirm the capability of the proposed discrimination approach to reject the vast majority of the false detections resulting from the prescreening algorithm.

A targets detection algorithm is proposed for maritime surveillance by single-channel SAR images. It foresees a preliminary prescreening step, carried out using an adaptive threshold algorithm, followed by a discrimination phase, performed by sub-look analysis. The latter discriminates the pixels detected by the former step in three classes, i.e. targets, sea, and azimuth ambiguity. The algorithm is tested on single channel StripMap TerraSAR-X data. Results indicate that the selected algorithm shows promising detection performance and ambiguity rejection capabilities.

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. cross-track interferometry and tomography. System parameters and technological challenges of such a Moon-based observatory are also discussed.

This paper investigates the capability of a miniaturized radar sensor developed for Micro Aerial Vehicles (MAVs) to cope with complex mission scenarios that involve integrating and satisfying either Synthetic Aperture Radar (SAR) or Interferometric SAR and navigation and Sense-and-Avoid (SAA) requirements. A radar sensor based on Frequency Modulated Continuous Wave (FMCW) technology developed by IMST GmbH is exploited both for SAR and SAA acquisitions. The set of tests helps with understanding main issues and problems related with integration of different requirements and demonstrate that valuable data for each envisaged application can be saved with a single acquisition, thus paving the road for further and more detailed investigations.

This paper deals with the exploitation of Doppler centroid measurements for ship velocity estimation from focused single-look complex synthetic aperture radar (SAR) images. An algorithm is presented, which can be used as a discrimination tool to reduce the false alarm rate of standard adaptive threshold detectors and to complement the ship detection task with velocity estimation. The outputs are the indication of the presence of a moving target and the estimate of its slant range velocity. After a review of its theoretical background, algorithm features and performance are verified by application to TerraSAR-X data. The proposed method performs robust Doppler spectrum derivation for candidate ship targets and background pixels. The presented results show that the estimated radial velocity is in very good agreement (5% root-mean-square deviation) with that resulting from the azimuth offset method. Rejection of bright sea features and azimuth ambiguities is also demonstrated, taking advantage of the performed velocity analysis.

This paper discusses the performance of a novel millimeter wave radar sensor for UAS autonomous indoor operations. A sensor based on the interferometric Synthetic Aperture Radar (SAR) principle should be able to satisfy the stringent requirements set by indoor operations in terms of high-resolution three-dimensional mapping and autonomous navigation. A solution based on a Frequency Modulated Continuous Wave (FMCW) technology is proposed. The outlined nominal configuration serves as a basis to analyze the performance of the sensor in possible operational scenarios by means of a software simulator. Four different focusing algorithms are implemented and tested and capabilities of the novel sensor are finally discussed, demonstrating that high-resolution, high-quality observation of an assigned control volume can be achieved.

The present paper reports on some insights in the use of Synthetic Aperture Radars (SAR) for bathymetric data retrieval by exploiting variation in swell wave parameters approaching the shoreline. Since SAR signals are unable to penetrate sea surface and to reach seabed, echoed signals from sea surface are used to investigate underwater bathymetry in coastal area. A suitable data processing methodology is proposed to properly detect swell shoaling and refraction phenomena, and to accurately measure swell parameters. The described methodology is tested by using ALOS L-band SAR images over the Gulf of Naples, Italy. A Digital Elevation Model (DEM) of the investigated area is thus obtained. Results are consistent with the values reported in the Official Nautical Chart provided by the Italian Navy Hydrographic Institute.

The paper presents a method to discriminate ship targets from sea background in focused Single-Look Complex (SLC) Synthetic Aperture Radar (SAR) images. The method is based on Doppler Centroid analysis and it is able to generate an estimate ship velocity. Experimental results are presented showing the application of the method to TerraSAR-X data of the Gulf of Naples, Italy.

This paper analyzes the spatial resolution of bistatic synthetic aperture radar (SAR) in general hybrid configurations, such as air- and spaceborne systems moving along independent trajectories. The gradient method is utilized to point out the effects of the acquisition geometry, namely, position and velocity of both the transmitter and the receiver, on image resolution. This general approach is applied to different realizations of bistatic SAR, such as low-Earth-orbit monostatic-bistatic SAR, spaceborne-airborne bistatic SAR, and a bistatic system consisting of a high-altitude long-endurance illuminator and lower altitude airborne receivers. The main features of the method are then put in evidence, including the derivation of analytical tools to individuate adequate relative geometries for achieving satisfactory resolutions. A comparison to the other proposed techniques for computing the spatial resolution of bistatic SAR is also reported in order to highlight some peculiarities of all presented methodologies. Finally, the good agreement between the image resolution results achieved by recently carried out bistatic SAR experiments and the ones derived by the gradient method strengthens the potentialities of the proposed approach.

The study of a novel navigation system for Unmanned Airborne System (UAS) based on bistatic Synthetic Aperture Radar (SAR) is presented. The innovative bistatic configuration builds on spaceborne radar transmitters and airborne receivers, the latter mounted in a forward-looking geometry. Such approach, impossible or extremely demanding with a monostatic approach, allows one to achieve dual information with two different radar working modes: imaging capability can be in fact coupled with the possibility of moving target indication. The study is particularly suited on one of the most common UAS platforms: a close range, medium takeoff weight, with an endurance of roughly 7 hours and cruise speed of about 50m/s, whose requirements have been identified. The finalization of the study is achieved by the definition of a strawman system concept with different approaches. Four options are identified, with different performance and system complications/challenges. The study herein reported was carried out under ESA contract 22449/09/F/MOS.

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. Five models 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.

This paper presents the tracking algorithms developed for a multisensor anti-collision system for unmanned aerial vehicles. This system will be developed by the Italian Aerospace Research Center (CIRA) within a research project named TECVOL, funded in the frame of the National Aerospace Research Program (PRO.R.A.) on UAV. The hardware setup is composed by a pulsed radar, two infrared cameras, and two visible cameras used as aiding sensors, thus the adoption of a fusion algorithm was mandatory to obtain the most accurate and reliable tracking estimate of obstacles. The paper describes the different modes and the relevant attainable performances of the developed tracking algorithm. The adopted data fusion technique for tracking is the Kalman filter. In particular, three different algorithms are compared in a typical collision scenario, namely conventional filter in rectangular coordinates, conventional filter in spherical coordinates, and extended filter in rectangular coordinates. Though all the three algorithms exhibited satisfying performances, the extended filter in rectangular coordinates resulted the most adequate for this airborne application

This work presents an analysis of the attainable performance of a bistatic SAR for oceanographic applications, such as the determination of sea state parameters and the reconstruction of the sea wave spectrum. The analysis has been developed with regard to the spaceborne bistatic SAR of the BISSAT mission. It is based on a receiving-only antenna, flying on board a small satellite, that gathers echoes from the area illuminated by an active SAR operating on an independent large platform. It is shown that in the case of bistatic acquisitions the region of the sea wave spectrum that can be retrieved through linear inversion is wider than the one relevant to a monostatic survey. Specifically, the improvement consists of higher sea states (up to 15-20%) and a wider range of azimuth angles of propagation of the sea waves, increasing for larger antenna baselines. Also, the image degradation due to acceleration smearing is reduced. Improvements can achieve 20% for larger azimuth angles of propagation and baselines.

This paper summarizes scientific rationale and technical approach for a bistatic synthetic aperture radar (SAR) mission (BISSAT). The study has been funded by the Italian Space Agency for a competitive Phase-A study along with other five missions. Its concept consists in flying a passive SAR on board a small satellite, which observes the area illuminated by an active SAR, operating on an already existing large platform.

This paper presents an application of the European Remote Sensing (ERS) satellites' radar data to digital elevation model (DEM) generation. The selected test site is the Sannio-Matese area in southern Italy, where several corner reflectors (CRs) were deployed to be used as ground control points (GCPs) for height measurement accuracy validation. First of all, an analysis of the CR response in radar images is presented. Then, the procedure for image pair geometric registration and interferogram formation is described in detail. A quantitative analysis is also performed by comparing these interferograms to the corresponding products obtained by using the ISAR software, officially distributed by the European Space Agency (ESA). Reported correlation values show that only tandem pairs allow an efficient interferometric processing to be performed, thanks to their short-time baseline (one day), whereas correlation adequate for differential interferometry could not be achieved. The method adopted for the computation of the interferometric baseline components on the basis of satellite orbital data is described, including the GCP-based corrections. The procedure was applied to obtain DEMs of a 10/spl times/10 km/sup 2/ subarea characterized by very high correlation coefficients (0.6). The best attained values of the GCP height measurement accuracy were about 4 m. Finally, the DEMs were compared, giving root mean square (rms) differences less than 20 m in the best case.

Presents a spaceborne SAR system simulator based on a coherent approach. After a description of the original aspects of the computer simulator (i.e., digital terrain model, Earth ellipsoid rotation, space platform orbital and attitude dynamics, antenna pattern, chirp sampling, and two-dimensional compression), the characterization of passive and active calibrators embedded in different backgrounds is discussed in terms of the SAR impulse response function. An assessment of the SAR processor performance is derived by computing the one-dimensional ground range and azimuth resolutions of the simulated calibrators. Finally, various plots and numerical results are shown in order to illustrate impulse responses under clutter and no clutter assumptions.>

Analyzes a spaceborne interferometric synthetic aperture radar (SAR) for high-resolution, topographic applications. Two physical antennas are vertically spaced and are carried along parallel paths by two different platforms connected by a tether. Tethered space systems have been proposed by several authors for different applications and a joint US-Italian program exists to deploy in 1992 a small satellite (Tethered Satellite System) from a Space Shuttle. A system performance analysis is carried out, considering a theoretical study and a numerical simulation. By evaluating the tethered interferometric SAR impulse response, the height of various point scatterers is computed and an error budget is obtained. Results show that this system is capable of achieving a root-mean-square (RMS) error in height measurement adequate for several applications.>