Today GPS has evolved into a widely accepted tracking system for Low Earth Orbit Satellites. In particular, carrier-phase differential GPS technology is applied in the area of precise relative navigation for LEO satellites in formation. This paper introduces the errors both on carrier-phase and code measurements in space and compared the differences between the error sources and the counterparts on the ground surface. The ionospheric delays, which have new figures in space, are focused on, as they are regared as the main influence factor for the precise relative navigation for LEO satellites in formation.