We analyze the spin modulation of the current of a boom-mounted positive biased spherical Langmuir probe observed with the Viking experiment. The satellite perturbs the probe current by two distinct effects: (1) photoelectrons emitted by the low potential part of the satellite, (2) capture of ambient electrons. These two effects are shown to be strongly dependent on the magnetic field orientation with respect to the probe-satellite axis. We quantify experimentally and theoretically the range of such effects. We then deduce some basic rules for electron energy and density measurements and for the design of satellite-borne Langmuir probe experiments. ¿ American Geophysical Union 1993