Satellite communications systems based on non-GEO constellations of satellites can be categorised in two groups : the 'satellite fixed cell' systems, and the 'earth fixed cell' ones. 'Earth-fixed cell' satellite systems overcome the negative effects of handover that are unavoidable with 'satellite-fixed cell' systems. However this requires overlapping between adjacent satellite footprints, thus increasing the constellation size. A method is proposed for the determination of the minimum size of an 'earth-fixed cell' constellation with circular orbits, given the orbit altitude, the user's minimum elevation angle and the cell size. The method takes into account the effects of imperfect station keeping. Comparisons are given for 'satellitefixed cell' systems and 'earth-fixed cell' ones. keywords: 'earth-fixed cell', 'satellite-fixed cell, nongeo satellites, constellation sizing, station keeping.