In this chapter we discuss GPS geodesy at medium distances which in some ways is the most challenging. As indicated in Chapter 1, GPS can be considered a global geodetic positioning system providing nearly instantaneous position with 1–2 cm precision with respect to a consistent terrestrial reference frame. Nevertheless, as we shall see in this chapter, the relation between intersite distance and geodetic precision is still important, particularly for integer-cycle phase ambiguity resolution. Currently, the main sources of error at medium distances are due to tropospheric and ionospheric refraction of the GPS radio signals.