Passive Multistatic ISAR Imaging of Ship Targets with GNSS Signals of Opportunity

This work develops multistatic Inverse Synthetic Aperture Radar (ISAR) imaging techniques of ship targets using global navigation satellites signals (GNSS) as illuminators of opportunity and a single, ground-based receiver. This comprises an image scaling procedure to focus the data pertaining to different bistatic angles on a reference domain independent of the specific emitter, thus allowing a direct combination of images obtained under multiple and wide spatial separations. The procedure exploits the target state information made available by the very same spatial diversity offered by the system, also including the possibility of ship maneuvers during the image time. Moreover, to maximize the quality of the resulting image, an adaptive combination rule has been derived to preserve the highest information content from the individual bistatic data despite the variation of the target signature under the different perspectives. To explore the effectiveness of the technique under non-ideal conditions, theoretical and numerical performance analyses have been also provided considering errors in the target kinematic data. Moreover, the whole processing chain has been tested against experimental data comprising four Galileo satellites, showing the effectiveness of the proposed approach to achieve meaningful multistatic radar images of vessels at sea. The approach derived here is verified using GNSS-based radar, however it is generic and directly applicable to the broader class of passive radar imaging systems with satellite illuminators.