High Precision Positional Measurement For The Five Major Uranian Satellites

The dynamical models of planets and satellites are of great significance for studying the formation and evolution of solar system,supporting space exploration tasks and further understanding the physical characteristics of a specific planetary system.In order to improve and study the dynamic models of planets and satellites and construct more accurate orbital models,a large amount of observations are needed.710 CCD frames,observed by 1 m telescope at Yunnan Observatory over 15 nights during 2014-2016,have been reduced to derive the precise positions of the five Uranian major satellites(Ariel,Umbriel,Titania,Oberon and Miranda).For the position measurement of the faint satellite(Miranda),it is quite close to Uranus and is greatly affected by the halo light of the planet;we have carried out a halo removal treatment to attenuate the influence of the halo light of its host planet.At the same time,we also discussed the case of Miranda's suitability to use a function with a polynomial of several orders after halo removal.A total of 248 CCD images on the four nights of November 2016 in the above observations were tested by using a Gaussian function with a 1-3 order polynomial background.The results show: after the halo removal,positional precisions when using Gaussian function with a 3rd order polynomial background are better than the results when using 1st and 2nd order methods.In addition,the positions of satellites were measured with reference stars taken from Gaia DR1,a newly published star catalogue.The theoretical positions of Uranian satellites were retrieved from the Jet Propulsion Laboratory(JPL)Horizons System that includes the satellite ephemeris ura111,while the position of Uranus was obtained from the JPL ephemeris DE431 mx.In order to discuss the influence of different satellite theories on the accuracy of position measurement,a comparison of the other two satellite ephemerides from the Institut de Mécanique Céleste et de Calcul des éphémérides(IMCCE)was also performed.The results show that the observations of the five satellites are in good agreement with the three satellite ephemerides.When Gaia DR1 is used for the reference star catalogue and meanwhile satellites ephemeris is obtained from JPL,our results show that the absolute values of the mean(O-C)(observed minus computed)are not greater than 0".020 in both right ascension and declination.The positional precision is better than 0.030 arcsec for the four greatest Uranian satellites and 0.050 arcsec for Miranda in each direction.