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.

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, the idea of a Moon-based interferometric synthetic aperture radar (SAR) is presented. 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 likely make its realization distant, 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.

The underwater environment poses numerous challenges and risks, making Unmanned Underwater Vehicles (UUVs) an indispensable alternative to human operators. Numerous Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) have been developed as a valuable resource in a broad spectrum of underwater operations. However, the deployment and operation of UUVs face significant challenges due to the unique underwater environment that critically affects Positioning, Navigation, and Timing (PNT) performance, making it incomparable to above-water applications. This discrepancy significantly impacts the decision-making process of industrial operators, particularly those in the sector of Critical Undersea Infrastructures (CUIs). Despite advancements, they persist in the use of heavy ROVs deployed from an expensive and environmentally impactful mothership for Inspection and Monitoring (I&M) tasks. To explore the potential revolutionary impact on underwater operations, we analyze the resilience of CUIs, and we review the most promising robotics developments that are currently or soon to be available. The forthcoming solutions not only promise to enhance the efficiency of I&M operations, thereby bolstering the security of CUIs, but they also have the potential to transform the broader field of underwater operations as a whole.

The synergic utilization of data from different sources, either ground-based or spaceborne, can lead to effective monitoring of maritime activities. To this end, the integration of synthetic aperture radar (SAR) images with data reported by the automatic identification system (AIS) is of high interest. Accurate matching of ships detected in SAR images with AIS data requires compensation of the azimuth offset, which depends on the ship’s velocity. The existing procedures interpolate the route information gathered by AIS to estimate the ship’s velocity at the epoch of the SAR data, to remove the offset. Matching accuracy is limited by interpolation errors and AIS route information unavailability or uncertainties. This paper proposes the use of SAR-based ship velocity estimations to improve the integration of AIS and SAR data. A case study has been analyzed, in which the method has been tested on TerraSAR-X images collected over the Gulf of Naples, Italy. Presented results show that the matching is improved with respect to standard procedures. The proposed method limits the distance between the AIS report and the SAR-based detection to less than 150 m, which is in line with maritime surveillance needs.

Spaceborne synthetic aperture radar (SAR) can be considered an operational asset for maritime monitoring applications. Well-assessed approaches exist for ship detection, validated in several maritime surveillance systems. However, measuring vessel velocity from detected single-channel SAR images of ships is in general difficult. This letter contributes to this problem by investigating the possibility of retrieving vessel velocity by wake analysis. An original method for velocity estimation is developed for calm sea (Beaufort scale 1-2) and applied over seven X-band SAR images, gathered by COSMO-SkyMed mission over the Gulf of Naples, Italy. The algorithm exploits the well-known relation between the wavelength of the waves composing the Kelvin pattern and the ship velocity. But the proposed approach extends the applicability of the existing wake-based techniques since it foresees evaluation of the wavelength along a generic direction in the Kelvin angle. Promising results have been achieved, which are in good agreement with those of more assessed techniques for ship velocity estimation in SAR images.