In part I of this paper, we presented the basic architectures and theory for passive and active phased arrays. Here, we review array implementation, state-of-the-art applications, and identify future trends in phased-array technology.

Phased arrays-part II: implementations, applications, and future trends

With the geosynchronous synthetic aperture radar (SAR) satellite as the transmitter, the unmanned aerial vehicle (UAV) can passively receive the echo within the illuminated ground area and achieve 2-D imaging of the interested target. This SAR system, known as GEO-UAV bistatic SAR, is capable of autonomously accomplishing the bistatic SAR mission in rough terrain environments by prespecifying a path for the UAV receiver. In this paper, the GEO-UAV bistatic SAR system is first investigated. The practical advantages and spatial resolution are then analyzed in detail. The spatial resolution of GEO-UAV bistatic SAR is dependent on the observation geometry, which is determined by the UAV path. Therefore, the path planning for GEO-UAV bistatic SAR aims at identifying a set of optimal paths for the UAV receiver to travel through a 3-D terrain environment that simultaneously guarantees the safety of the UAV and achieves SAR imaging with optimized performance during the flight. The path planning is modeled as a constrained multiobjective optimization problem (MOP), which accurately represents the two main aspects for the path planning problem, i.e., UAV navigation and bistatic SAR imaging. Then, a path planning method based on a constrained-adaptive-multiobjective-differential-evolution algorithm is proposed to solve the MOP and generate multiple feasible paths for the UAV receiver with different tradeoffs between navigation for UAV and bistatic SAR imaging performance. The GEO-UAV bistatic SAR mission designer can choose a path from the solution set according to the application requirements, which makes the method more pragmatic.

Path Planning for GEO-UAV Bistatic SAR Using Constrained Adaptive Multiobjective Differential Evolution

Geosynchronous synthetic aperture radar (GEO-SAR) offers new opportunities for continuous Earth observation missions with large coverage and short revisit cycle. The unique features of GEO-SAR present huge potentials for bistatic observation applications. In this paper, the concept and advantages of GEO bistatic SAR (GEO-BiSAR) are first investigated. The system consists of a GEO illuminator and an airborne receiver, such as an airplane or a near-space vehicle. Compared with a monostatic GEO-SAR system, the bistatic configuration can provide finer spatial resolution and higher signal-to-noise ration (SNR) with less system complexity. The spatial resolution characteristics are then analyzed based on generalized ambiguity function, where the time-varying GEO velocity, Earth rotation, and ellipsoid Earth surface are taken into consideration. Meanwhile, the bistatic SNR is analyzed using the integration equation model. In this paper, the mission design for GEO-BiSAR aims at identifying a set of receiver flight parameters and bistatic configurations to obtain the desired spatial resolution and SNR. Based on the desired imaging performance of a specific application background, the mission design process is modeled as a nonlinear equation system (NES). Finally, a mission design method based on fast nondominated sorting genetic algorithm is proposed to solve the NES and obtain multiple optimal solutions to guide the receiver flight missions. Examples of the mission design process are given to validate the effectiveness of the proposed method. The results of the mission design can be conveniently used to guide the receiver flight mission for the desired imaging performance, which is highly desirable in practical applications.

Inclined Geosynchronous Spaceborne–Airborne Bistatic SAR: Performance Analysis and Mission Design

The detection performance of airborne bistatic radar depends on suitable clutter mitigation techniques. Space-time adaptive processing (STAP) is a candidate for suppressing bistatic clutter. Since STAP implementation relies on secondary data for covariance estimation, the nonstationary nature of typical bistatic angle-Doppler contours over range is a point of concern. In this paper we find that bistatic STAP suffers greater performance degradation than commonly observed in monostatic STAP analyses. Additionally we briefly examine approaches to enhance bistatic STAP capability.

Adaptive clutter cancellation in bistatic radar

We investigate bistatic STAP performance. We show that typical bistatic clutter environments appear nonstationary. Nonstationary behavior exacerbates STAP implementation. In the absence of corrective measures, SINR losses due to covariance estimation error approaches 30 dB for the numerical examples considered herein. Using localized STAP processing coupled with a time-varying weight procedure, we show that much of the performance loss can be restored.

Bistatic STAP: application to airborne radar

This paper describes a space-based radar surveillance concept employing geosynchronous illumination and bistatic reception on either unmanned airborne vehicles (UAVs) or low-orbit satellites. Two classes of surveillance are considered: that of aircraft targets (generically designated as "airborne moving target indication" or AMTI) and that of slow-moving ground targets (generically designated as "ground moving target indication" or GMTI). While the bistatic concepts are hardly new, the emergence of high-speed processing to support large-scale beamforming for AMTI, and MTI-SAR for GMTI, have made these approaches feasible for space-based theater surveillance resulting in substantial cost and performance advantages over more conventional monostatic approaches when only limited regions (i.e. on or two theater-sized areas) need to be covered at any one time.

W.E. Sievers

Papers

Wide area surveillance concepts based on geosynchronous illumination and bistatic unmanned airborne vehicles or satellite reception