Marta Bucciarelli

Multistatic and MIMO Distributed ISAR for Enhanced Cross-Range Resolution of Rotating Targets

In this paper, we present a new technique to exploit the data acquired simultaneously by multiple radar sensors carried by multiple air platforms to increase the cross-range resolution of inverse synthetic aperture radar (ISAR) images of rotating targets. This distributed ISAR technique is devised for two different cases: 1) multiple-input-multiple-output (MIMO) case with each platform carrying an active radar that transmits and receives RF waveforms and 2) multistatic case with a single platform carrying an active radar (transmitting and receiving) and the remaining platforms equipped with passive sensors (namely, receiving only). The processing chain proposed for the distributed ISAR is shown, together with the results obtained against simulated ISAR data for both the MIMO and the multistatic cases. The performance analysis shows that the proposed technique is able to provide an increase of the cross-range resolution up to the number of platforms in the multistatic case and even higher in the MIMO case, if the platforms are properly located. This is of great benefit in applications where the target rotation angle is insufficient to guarantee the desired resolution. A typical case is the imaging of ship targets with rotation induced by the sea swell structure under low sea state conditions. To make the results appealing for practical application, the performance degradation is also analyzed arising from errors in the knowledge of both the target rotation motion and the acquisition geometry. Experimental data collected by a ground-based radar operating together with a rotating platform are processed by following the presented distributed ISAR technique to validate the proposed approach.

Multi-platform ISAR for flying formation

In this paper we exploit a formation of two radar sensors carried by two air platforms to increase the cross-range resolution of ISAR imaging. This is particularly challenging when the motion of the target is limited, as for ISAR images of ship targets from helicopters when the sea state is low. A distributed ISAR technique is devised to process coherently the echoes received by the two radar systems appropriately separated in angle; if both of them are active sensors, one is assumed able to receive and separate the echoes both from its own transmission and from the transmission of the other sensor (MIMO distributed ISAR), otherwise a configuration with one active radar system and one receiving only device is considered (bistatic distributed ISAR). The distributed ISAR technique is tested on simulated data to evaluate its effectiveness in achieving an improvement of the cross-range resolution; for a given number of flying platforms better values of cross-range resolution are obtained when more than one receiving channel per sensor is considered. Specifically, with two platforms, an increase in resolution up to three times the single sensor ISAR case is demonstrated in the MIMO configuration, as opposed to the enhancement of a factor up to two achieved in the bistatic case. Moreover, results obtained by processing the experimental data collected by a ground based radar operating together with a rotating platform are shown to prove the effectiveness of the proposed technique.

Array optimization and adaptive processing for sub-array based thinned arrays

In this work a new approach is proposed for the optimized design of a thinned array, based on a small set of different sub-array types (with assigned properties), properly disposed on a planar surface. The small number of different sub-array shapes is relevant for industrial production, to reduce design and manufacturing costs, as well as to allow scalable antenna designs for different applications. The optimization of the design is based on the maximization of an objective function, involving the side lobe level and/or the main lobe width. The possibility to split the obtained thinned array antenna in a set of sub-apertures is investigated, corresponding to an integer number of sub-arrays connected to independent receiving channels. This is required to apply adaptive processing techniques to cancel external e.m. interferences and/or clutter echoes. The performance obtained by using the adaptive techniques with sub-apertures taken from the optimized thinned array are evaluated with reference to assigned clutter and jamming scenarios, and compared to the sub-array based filled array.

Spatial Resolution Improvement in GNSS-Based SAR Using Multistatic Acquisitions and Feature Extraction

This paper considers the exploitation of navigation satellite systems as opportunity transmitters for bistatic and multistatic synthetic aperture radar (SAR). The simultaneous availability of multiple satellites over a scene of interest at different viewing angles allows multistatic SAR acquisitions using a single receiver on or near the ground. The resulting spatial diversity could be used to drastically improve image resolution or to enhance image information space. To exploit the availability of multiple satellites, two data fusion approaches are here considered. In the former, point features of the single images obtained from different perspectives are extracted and then combined, whereas in the latter, a multistatic image is first obtained by combining the single channel data at the image level and then the point features are extracted. This is achieved by considering ad hoc CLEAN-like techniques. These techniques have been developed on both the analytical and simulation levels and experimentally verified with real GNSS-based SAR imagery. The techniques described here are not limited to GNSS-based SAR but may be applied to any multistatic SAR system.

MIMO distributed imaging of rotating targets for improved 2-D resolution

This letter deals with the multiple-input-multiple-output distributed radar imaging of rotating targets. The distributed system is based on formation-flying platforms, which can be configured with proper cross-track and along-track displacements. The platforms carry active radar systems transmitting almost orthogonal waveforms; exploiting both monostatic and bistatic acquisitions, range and cross-range resolution improvement can be achieved, and a maximum theoretical 2-D resolution cell improvement factor can be obtained significantly greater than the number of flying platforms. The required distributed focusing technique is developed, and its effectiveness and robustness is tested against simulated data. The proposed distributed system could be suitable to the application of ship target imaging, using a reconfigurable formation of platforms, for maritime surveillance in wide sea areas.

Multi-sensor ISAR techniques for motion estimation of pitching, rolling and yawing targets

This paper addresses the problem of the estimation of the rotation motion of a target interested by roll, pitch and yaw. In a previous work it was proved that the exploitation of ISAR data acquired by multiple sensors located with angular separations among them lying on the same plane greatly improves the accuracy of the estimation of the rotation motion component normal to the sensors baseline with respect to the single sensor case. In this work we consider a joint multi-aspect multi-grazing formation of sensors and exploit the multi-sensor data to estimate the three components of the rotation motion. Both model based and model free estimation techniques are proposed and the Cramer Rao Bound is derived. The provided performance analysis shows the effectiveness of the proposed approach for both constant and sinusoidal motions.

Multi-grazing ISAR for side-view imaging with improved cross-range resolution

The use of multiple radar systems, carried by a set of air platforms flying in formation, is demonstrated to be able to provide side-view ISAR images of pitching and rolling targets with increased cross-range resolution. The multi-grazing ISAR technique is devised for two different cases: (i) multistatic case with a single platform carrying an active radar and the remaining equipped with receiving only devices, (ii) MIMO case with each platform carrying an active radar. The needed focusing technique is presented with the results obtained against simulated ISAR data; the performance analysis shows that the proposed technique is able to provide an increase of the cross-range resolution up to the number of radar systems in the multistatic case or even higher in the MIMO case. The multi-grazing ISAR is especially suitable for the application to ship target imaging in the presence of low sea state giving rise to side-views of poor quality when imaged with conventional ISAR.

Maritime Moving Target Indication Using Passive GNSS-Based Bistatic Radar

This paper is a first introduction to the concept of using global navigation satellite systems (GNSS) as illuminators of opportunity in a passive bistatic real-time radar system for maritime target indication applications. An overview of the system concept and the signal processing algorithms for moving target indication is provided. To verify the feasibility of the system implementation as well as test the developed signal processing algorithms, an experimental test bed was developed and the appropriate experimental campaign with the new Galileo satellites and a ferry as the target was carried out. The results confirm the system concept and its potential for multistatic operation, with the ferry being detected simultaneously by two satellites.

Maritime target detection using GNSS-based radar: Experimental proof of concept

This paper investigates the feasibility of using the Global Navigation Satellite Systems (GNSS) as transmitters of opportunity in a passive bistatic radar system for maritime target detection applications. To this purpose, the experimental test bed was developed, and the practical campaign was conducted for maritime data acquisition with the new Galileo satellites and a ferry as the target. By developing and applying the appropriate signal processing algorithm, the obtained experimental results have not only verified the system performance, but also showed the potentialities of a multi-static configuration utilising multiple satellites simultaneously.

Multi-sensor ISAR technique for feature-based motion estimation of ship targets

The topic of estimating the rotation motion of a target is investigated in this paper. Specifically the focus is on ship targets observed by multiple sensors and a new technique is proposed based on the exploitation of the multi-sensor data. The technique works entirely in image domain and makes use of linear features that can be extracted directly from the different radar images acquired by the different sensors for the target under consideration. The performance of the proposed technique is investigated in order to highlight the effectiveness and robustness of the proposed approach.