The dc current collected by a rocket-borne cylindrical probe with various bias potentials has been measured, along with the plasma parameters (by in situ independent measurements on the same payload). Periodic variations synchronous with the rocket spin cycle appear; they are related to wake effects for the ion current and to both wake and V¿B effects for the electron current. The ion current modulations are quantitatively explained by a theory on the wake which takes account of the angle between the velocity and the rocket axis; the electron current modulations due to the V¿B field are separated from the others and quantitatively explained by simple calculations. As a by-product the ion current outside the wake gives an unperturbed density measurement in agreement to within 15% with an independent measurement.