Marco D’Errico

Wake Component Detection in X-Band SAR Images for Ship Heading and Velocity Estimation

A new algorithm for ship wake detection is developed with the aim of ship heading and velocity estimation. It exploits the Radon transform and utilizes merit indexes in the intensity domain to validate the detected linear features as real components of the ship wake. Finally, ship velocity is estimated by state-of-the-art techniques of azimuth shift and Kelvin arm wavelength. The algorithm is applied to 13 X-band SAR images from the TerraSAR-X and COSMO/SkyMed missions with different polarization and incidence angles. Results show that the vast majority of wake features are correctly detected and validated also in critical situations, i.e., when multiple wake appearances or dark areas not related to wake features are imaged. The ship route estimations are validated with truth-at-sea in seven cases. Finally, it is also verified that the algorithm does not detect wakes in the surroundings of 10 ships without wake appearances.

Relative Trajectory Design

An analysis of orbital relative motion models is presented with emphasis on their application to formation design. Relative motion model evolution from the first Hill’s schematization (circular orbit, close satellites) is described, considering the inclusion of chief’s orbit eccentricity and orbital perturbations. In particular, the inclusion of J2-secular effects is treated in depth considering various approaches in literature. Literature is also reviewed for both small and large eccentricities. Further details are presented to model formations with small chief’s eccentricity (order of 10−3), which are typical of Earth observation missions, for both the case of close formations, i.e. with satellite distance of the order of tens of kilometers, and large formations, i.e. satellite distance up to hundreds of kilometers. Finally, design applications are presented, with derivation of relative trajectories from application requirements. As an example, relative orbits for SAR interferometry are derived from the requested altitude measurement uncertainty and considering different candidate geometries (pendulum, cartwheel, etc.). Relative orbits for SAR tomography and large baseline bistatic SAR applications are also analyzed.

Relative Trajectory Design for Bistatic SAR Missions

Trajectory design for formation-based bistatic SAR missions is presented for both close (cross-track interferometry) and large (radargrammetry) formations. Trajectory design process is simplified by using analytical relative motion models that include Earth oblateness perturbations and optimize mission performance. In addition, proposed design reduces trajectory establishment and maintenance efforts. examples are briefly summarized where best relative trajectory is derived starting from application models.

Performance Analysis of Ship Wake Detection on Sentinel-1 SAR Images

A novel technique for ship wake detection has been recently proposed and applied on X-band Synthetic Aperture Radar images provided by COSMO/SkyMed and TerraSAR-X. The approach shows that the vast majority of wake features are correctly detected and validated in critical situations. In this paper, the algorithm was applied to 28 wakes imaged by Sentinel-1 mission with different polarizations and incidence angles with the aim of testing the method’s robustness with reference to radar frequency and resolution. The detection process is properly modified. The results show that the features were correctly classified in 78.5% of cases, whereas false confirmations occur mainly on Kelvin cusps. Finally, the results were compared with the algorithm performance on X-band images, showing that no significant difference arises. In fact, the total false confirmations rate was 15.8% on X-band images and 18.5% on C-band images. Moreover, since the main criticality concerns again the false confirmation of Kelvin cusps, the same empirical criterion suggested for the X-band SAR images yielded a negligible 1.5% of false detection rate.

P-Band Distributed SAR

This chapter discusses a spaceborne P-band synthetic aperture radar concept based on a distributed architecture and formation flying technologies. This approach can in principle allow overcoming physical constraints that limit the performance of monolithic SARs, leading in the P-band case to huge antennas and hard swath/resolution trade-offs. The proposed SAR is based on a larger transmitting satellite and a set of lightweight receiving-only platforms. This architecture also enables multi-mission capabilities. In particular, forests observation and biomass estimation based on side-looking SAR data can be in theory combined with near nadir interferometric ice sounding. Payload concept is clarified, and a preliminary performance analysis in terms of ambiguity and coverage is proposed. Then, mission analysis, preliminary spacecraft design, and formation control architecture are briefly described.

Future Trend, Potential, Risks

During the last decades, the concept of distributed space systems has significantly progressed in terms of space applications, including Earth remote sensing. This chapter is devoted to a critical analysis of the achieved improvements and of the areas of major key issues in order to analyze potential and risks of distributed space systems. A discussion of future activities needed to prepare more advanced distributed space missions is also provided. In particular, payloads and applications are first discussed. Then, guidance, navigation, and control as well as other technological challenges, including modularity and architecture, follow.

Preliminary Concepts and Analysis of Future Earth Observation Missions Based on Distributed Radars

Spaceborne synthetic aperture radars can gain great advantage from the concepts of formations and distributed space systems. In fact, combination of signals from multiple coherent receivers allows to overcome intrinsic performance limitations of monolithic SAR systems. This paper deals with an overview of recent advances and ideas in the field of distributed sparse filled aperture radar concepts and required signal processing. Then, preliminary system considerations are given and a preliminary conceptual analysis of a distributed P-band SAR is presented. At low frequency distributed SARs offer greater advantages in overcoming the minimum area constraint (order of several tens of meters) and reduce the impact on formation control feasibility (requirement depends on λ). A formation of 6 cooperating satellites carrying a 2 xm x 2 m antenna operating at an undersampling pulse repetition frequency of 1000 Hz would allow to achieve 1 m azimuth resolution (ionospheric effect not included) over the whole range swaths.