Astrometric observations of Phobos with the SRC on Mars Express

Abstract

Aims. From April 2008 to August 2011 Mars Express carried out 74Phobos flybys at distances between 669 and 5579 km. Images taken with the Super Resolution Channel (SRC) were used to determine the spacecraft-centered right ascension and declination of this Martian moon. Methods. Image positions of Phobos were measured using the limb-fit and control-point measurement techniques. Camera pointing and pointing drift were controlled by means of background star observations that were compared to corresponding positions from reference catalogs. Blurred and noisy images were restored by applying an image-based point spread function in a Richardson-Lucy deconvolution.Results. Here, we report on a set of 158 Phobos astrometric observations with estimated accuracies between 0.224 and 3.405 km circular w.r.t. the line of sight to the satellite. Control point measurements yield slightly more accurate results than the limb fit ones. Our observations are in good agreement with the current Phobos ephemerides by the Jet Propulsion Laboratory (JPL) and the Royal Observatory of Belgium (ROB) with mean offsets of up to 335 m. Our data can be used for the maintenance and update of these models.

Mutual event observations of solar system objects by SRC on Mars Express

“…In contrast to previous astrometric observations (Oberst et al 2006;Willner et al 2008;Pasewaldt et al 2012Pasewaldt et al , 2015, where the absolute positions of Phobos or Deimos in the stellar sky as seen from the MEX spacecraft were provided, the mutual event observations are free of any influences from spacecraft pointing. This is due to the fact that primarily the angular difference between two observed bodies, that is, the relative position, is observed and provided.…”

“…Hence the far distant object can be used as a fiducial point that can help to correct SPICE kernel information on the attitude of the camera with respect to the stellar sky about two axes (x and y or yaw and pitch). The twist or roll angle, with the rotation axis along the boresight vector of the camera, was reported to be consistent (Pasewaldt et al 2015;Willner et al 2008) with the information in the SPICE kernel data and is not separately corrected. Likewise, the scaling factor was reported to be 1.0, with sufficient accuracy throughout the SRC observations (Willner et al 2008), and is not separately analyzed here.…”

“…In the past Phobos' and Deimos' positions in space were observed separately by several spacecraft during their approach to Mars and at times also during flyby or far distance observations, for example, by Mariner 9 (Duxbury & Callahan 1989), Viking (Duxbury & Callahan 1988), Phobos 2 (Kolyuka et al 1991), MGS (Bills et al 2005;Jacobson 2010, and references therein), MRO (Jacobson 2010;Jacobson & Lainey 2014) and Mars Express (Oberst et al 2006;Willner et al 2008;Pasewaldt et al 2012Pasewaldt et al , 2015.…”

“…Several different techniques have been applied to gather astrometric information from SRC images (see Oberst et al 2006;Willner et al 2008;Pasewaldt et al 2012Pasewaldt et al , 2015. In contrast to the method that is described below, Willner et al (2008) and Pasewaldt et al (2012Pasewaldt et al ( , 2015 used stars in the background to control the camera pointing for astrometric observations. According to Willner et al (2008) pointing errors of MEX mainly relate to attitude uncertainties about the X and Y axes of the camera leading to translations of the observed bodies' location in the image.…”

Mars moon ephemerides after 14 years of Mars Express data

“…The uncertainty estimation is still incomplete so far because of occasional spacecraft jitter during image acquisition (see Pasewaldt et al 2015 for details). For sequences with continuous observations of reference stars or other celestial objects, this causes no problem.…”