  LaRa (Lander Radioscience) is an experiment on the ExoMars 2020 mission that
uses the Doppler shift on the radio link due to the motion of the ExoMars
platform tied to the surface of Mars with respect to the Earth ground stations
(e.g. the deep space network stations of NASA), in order to precisely measure
the relative velocity of the lander on Mars with respect to the Earth. The LaRa
measurements shall improve the understanding of the structure and processes in
the deep interior of Mars by obtaining the rotation and orientation of Mars
with a better precision compared to the previous missions. In this paper, we
provide the analysis done until now for the best realization of these
objectives. We explain the geophysical observation that will be reached with
LaRa (Length-of-day variations, precession, nutation, and possibly polar
motion). We develop the experiment set up, which includes the ground stations
on Earth (so-called ground segment). We describe the instrument, i.e. the
transponder and its three antennas. We further detail the link budget and the
expected noise level that will be reached. Finally, we detail the expected
results, which encompasses the explanation of how we shall determine Mars'
orientation parameters, and the way we shall deduce Mars' interior structure
and Mars' atmosphere from them. Lastly, we explain briefly how we will be able
to determine the Surface platform position.
