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Satellites’ Rotation and Shape


A typical task in planetary geodesy is the computation of Control Point Networks (CPNs) based on photogrammetric measurements within stereo image data. From the 3D-coordinates of the control points, maps as well as terrain models and shape models are gernerated. Hereby, it is important to know the body's orientation at image time. As reference frame acts the International Celestial Reference Frame  (Ma et al., 1998; Ma et al. 2009). The rotational elements of a body model the self-rotation as well as the orientation of the spin axis as functions of time, including periodic perturbations induced by gravitational forces (e.g. precession movement of the spin axis). Such rotational models are available for multiple object in the solar system  (Archinal et al., 2011).

The Martian moon Phobos is in synchronous spin-orbit resonance around its primary body. Hence, Phobos' rotational period of  7h 39 min is in average equal to its orbital period. Because of the acting tidal forces, Phobos does not rotate uniform. The spin axis performs a precession movement of 826 days. Furthermore, Phobos gradually approaches Mars such that its orbit shrinks by about 5 cm per year. As a consequence the self-rotation of Phobos is accelerated. Duxbury und Callahan (1989)  showed that the moon undergoes a forced libration, they used astromometric observations of the Viking orbiters and found a longitudinal librational amplitude of 0.8 +/- 0.2 degree.

In the project rotational models are studies and imroved. Because of the large number of publications to Phobos, it is a suitable target to compare the results of a newly investigated computation method for rotational elements. The method of an inertial frame bundle block adjustment allows to directly determine rotational parameters, which formerly could only obtained indirecty. Also weakly determined parameters such as the secular acceleration term are evaluated. The improved CPN yields also an improved shape model, which is a.o. the foundation to study the thermal regimes on Phobos.



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