Relativistic transformation between τ and TCG for Mars missions under IAU Resolutions

Considering the fact that the general theory of relativity has become an inextricable part of deep space missions, we investigate the relativistic transformation between the proper time of an onboard clock tau and the Geocentric Coordinate Time (TCG) for Mars missions. By connecting tau with this local timescale associated with the Earth, we extend previous works which focus on the transformation between tau and the Barycentric Coordinate Time (TCB). (TCB is the global coordinate time for the whole solar system.) For practical convenience, the relation between tau and TCG is recast to directly depend on quantities which can be read from ephemerides. We find that the difference between tau and TCG can reach the level of about 0.2 seconds in a year. To distinguish various sources in the transformation, we numerically calculate the contributions caused by the Sun, eight planets, three large asteroids and the spacecraft. It is found that if the threshold of 1 microsecond is adopted, this transformation must include effects due to the Sun, Venus, the Moon, Mars, Jupiter, Saturn and the velocities of the spacecraft and Earth.