Measurements of the Doppler frequency change in a microwave beam sent to and transponded back from an interplanetary spacecraft are very sensitive to the quadrupole gravitational field of a nearby natural satellite. This method will be used in Cassini's mission to the saturnian system in relation to Titan during some of the numerous flybys. The use of very stable coherent signals at X and Ka bands will allow an excellent determination of the quadrupole dimensionless coefficients, with absolute accuracy between 10 -8and 10 -9. Titan's quadrupole deformation is due to its rotation and the saturnian tidal forces; the determination of the corresponding two parameters will greatly constrain the interior models. The measurement accuracy should be sufficient to determine also the variation of the tidal parameter with the orbital period, as a consequence of Titan's eccentricity; this will provide information on its rigidity and indicate whether it has an internal ocean of water and ammonia. The analysis of the complicated dependence on the geometry of Doppler measurements of the quadrupole can be done by means of an analytical expression of the covariance matrix.