# A New Orbiting Deployable System for Small Satellite Observations for Ecology and Earth Observation
## 3 Study Cases
### Application 7: Sea State from SAR and Instrumentation Located Onboard the Vessel  
As shown in the previous chapter Section 3.6, remote sensing observations represent a powerful tool for providing the kinematic properties of vessels. In turn, the ship motion analysis allows for the real-time assessment of the sea state parameters, which can contribute to minimizing the navigation risks by supporting the onboard decision to handle potentially dangerous phenomena in following and quartering seas, such as surf-riding, broaching, and parametric rolling, and to reduce the fuel costs (e.g., [270; 271]). Sea state condition monitoring improves the statistics of long-term wave data, providing additional information, especially in open ocean waters, where the weather buoys are very scattered [272]. In addition, it allows to optimize the ship route by balancing variations of the weather conditions and navigational constraints [273]. Additional benefits arise from the assessment of the onboard comfort level, mainly related to the motion sickness incidence (MSI) parameter, which needs to be continuously monitored to increase the seakeeping performances of passenger ships [270; 274]. The characteristic parameters of the sea waves can be efficiently determined from SAR imaging systems, because they are operative in all the light and weather conditions and have a wide swath. The radar system needs an X- or C-band, with a resolution better than a few tens of meters. So far, several large (e.g., the pioneer USA SeaSat [275], ESA Sentinel-1 [276], and DLR TerraSAR-X [259]) and medium-large (e.g., Italian COSMO-SkyMed [233; 234]) satellites have been employed to accurately estimate the wave parameters, such as height, average length, and direction, using SAR system in various bands, e.g., C-, X-, and L-bands [277; 278]. However, despite their lower swath and space resolution, small platforms are taking hold (e.g., UK NovaSAR-1 and Israeli TecSAR [57]), thanks to the opportunity of multiple launches and the use of non-sun-synchronous orbits [57]. The characteristic parameters of the sea waves provided by the SAR system mounted on small platforms can be used as references to verify the performance of a parametric wave spectrum resembling procedure that can be applied to detect the sea state parameters based on the measurement and analysis of the heave and pitch motions of a vessel in a seaway, recorded by positioning sensors located onboard the ship [279; 280]. In detail, the heave and pitch motions in a seaway can be estimated starting from the survey of the amplitudes of the ship vertical motions in a point of known coordinates, located on the centerline plane, using the GNSS (Global Navigation Satellite System), and pitch acceleration, using a gyroscope [281; 282]. The pitch acceleration allows to determine the pitch motion amplitude of the ship (by double integration in the time domain), and, from the vertical translational motion (composed by heave and pitch motions), it is possible to compute the heave motion amplitude of the center of mass. These data allow to determine the response spectra related to the heave and pitch motions in the encounter wave frequency domain [280; 281; 283]. The response spectra as a function of the absolute frequency can be evaluated on the basis of two additional input data, namely the vessel speed, which is measured by the GNSS, and the encounter angle between the true course of the ship (determined by GNSS system) and the prevailing direction of the sea, which is derived from images obtained by the SAR systems. Given both the response spectra and the ship complex transfer functions, it is possible to determine the sea spectrum by generating the motions [282]. Finally, from the spectral distribution of the sea in the absolute frequency domain, the characteristic parameters are obtained, namely, significant wave height, wave peak period, and spectrum peak enhancement factor.