In the frame of the NETLANDER project we are presently developing a ground penetrating radar, the GPR experiment, aimed at the exploration of the Martian subsurface. Initial observations of the geological features in the subsurface and the detection of water reservoirs either in the form of ground ice or of liquid water are among the main objectives of all missions that will be flown to Mars in the coming years since they should largely improve our understanding of the processes that have been active during the planet evolution and, in particular, provide crucial measurements on the evolution of the climate and atmosphere of the planet. The GPR is designed for deep sounding and operates at a low frequency of 2 MHz. Contrary to the normal mode of operation of subsurface radars which are moved over the soil to be explored, the GPR will operate from fixed landers. We have thus proposed a new concept which aims at a 3D imaging of the reflectors in the subsurface. This is achieved by measuring not only the distance of the reflectors but also the direction of the reflected waves. The direction of the reflected waves is deduced from the analysis of 2 electric and 3 magnetic components of the wave field that are measured using 2 electric antennas, also used for transmission, and one magnetic antenna which can be rotated along 3 successive mutually orthogonal directions. The perfect stability of the underground allows to perform a very large number of coherent integrations which, in addition to an improved pulse transmission scheme (bi-phasing, BPSK coding, ...), provide a high sensitivity as well as a good resolution. Numerical simulations with a model of the Martian subsurface have shown that it is thus possible to detect a liquid water layer at a depth of more than 2 kilometers. The instrument can also measure the electric permittivity of the first layers of the subsurface, down to ∼30m. In special modes of operation it will be used as a ionospheric sounder to probe the lower ionosphere and as a riometer to measure the absorption of radiowaves propagating through the ionosphere. We shall provide a general description of the electronics and antennas illustrated by a number of numerical simulations run to determine the radar capabilities in the anticipated Martian conditions. Initial ground tests were recently performed on the Dune du Pyla in Southern France, a sand dune where the operation of the radar can be validated in an environment which is a reasonable analog of the Martian subsurface. Results of this test will be reported.