The objective of this work is to investigate the application of radio link between the mothercraft and a CubeSat to the geodesy of Phobos. The origin of Martian moons (i.e. Phobos and Deimos) is still unknown. There are possibilities that they are ejected from Mars after an impact, or they are captured C-type asteroid. The knowledge of Phobos interior structure (e.g. homogeneous, porous, or fractured) will lead to better understanding towards its formation as well as the early solar system. The mothercraft and the CubeSat are flown in a safe high quasisatellite orbit (QSO) and a sustaining low QSO, respectively. Besides the Doppler measurement noise (0.1 mm/s), the ephemeris error of Phobos at least 200 m is found to be very disturbing for Phobos proximity operations. Nevertheless, the relative motion between the high QSO and the low three-dimensional QSO are resistant enough to reveal C20, C22, and to an accuracy of 0.3%, 1%, and 0.6% after one week. Such an accuracy can effectively constrain the moment of inertia as well as the interior structure of the target. The inter-sat radiometric measurement can also support the orbit determination of spacecraft with an accuracy of 240 m and 5 cm/s with respect to Phobos.