In order to use GPS or Galileo receivers on board a geostationary satellite so as to compute its position, we need to determine the specific characteristics of the signals which reach a satellite in a geostationary environment. There are a lot of differences between the signal that an earth user can receive and the one that a geostationary satellite receives. The localisation of geostationary satellites thanks to the GPS or Galileo systems is harder than for an earth user because of the spatial configuration of the problem. Indeed, the geostationary orbit is 'above' the GPS/Galileo orbit. Consequently, the received signals have a low C/No. With this constraint, specific acquisition and tracking techniques are required for the geostationary satellites' positioning process. The aim of this article is to describe the specific constraints of the geostationary environment. In a first part, we interest in the link budget between the geostationary satellite and the GPS/Galileo satellite and in the number of GNSS satellites that we can use for the acquisition of the signal depending on the C/No. The main factors which drive the C/No are the gain of the GPS/Galileo transmitting antenna, the gain of the receiver antenna and the free space losses. So we have studied different shape of the receiver antenna pattern in order to make it fit with the geometry of the problem: the spatial area which interests us is the area on each side of the earth. Then, we interest in the impact on the link budget when we consider signals transmitted through the main lobe only or also through the side lobes of the transmitting antenna. In the first case, the link budget is >35dBHz. In that case, the number of visible GPS/Galileo satellites for a given position of the geostationary satellite is very low. In order to get more satellites, we process the signals transmitted trough the side lobes of the GPS/Galileo antenna. The number of visible satellites will be increased. But, as the signals are transmitted through the side lobes, the global C/No is lower too. The results obtained in this part will show us that it will not be possible to use a classic acquisition technique but the performances obtained with typical high sensibility processing techniques will match our requirements. Then, once the environment has been characterised, we interest in the reliability of the positioning for the geostationary satellites through the computation of the Dilution Of Precision (DOP). The area where the GPS/Galileo satellites are located is not wide from the GEO point of view and thus, their spatial configuration is not good for the DOP. In order to improve the DOP, we must take into account the signals transmitted through the main lobe and the side lobes, and so we will have to deal with low C/No signals. Again, high-sensibility processing techniques will show some major interests. Finally, we study the range of the Doppler affecting the signal and investigate its effect on the acquisition time and processing techniques. The velocity of the geostationary satellites is such that the signal can undergo a Doppler as big as +/-15kHz.