Abstract: Based on the demand of relative positioning in formation flying missions,the dynamic observation geometry between the global navigation satellite systems and the formation flying satellites is analyzed.The description of relative dilution of precision(RDOP) and its properties are illustrated.The geometric dilution of precision(GDOP) analysis is carried out firstly for a single satellite in the formation.The common visible satellites and the corresponding visible time intervals were then discussed with different formation relative distances.The RDOP for formation flying satellites in certain scenarios is calculated and compared with the values of PDOP.Simulations show that the masking elevation angle of the receivers is a vital factor for GDOP.The formation relative distance has an impact on the selection of common visible satellites.The integrated GPS-Galileo system has advantages over the single GPS navigation system for the relative positioning of formation flying satellites.The former possesses a smaller GDOP value and thus can benefit the high precision inter-satellite position determination.