SST (Satellite to Satellite Tracking) observations between GPS and LEO (Low Earth Orbiter) play an important role to determine precise 3D orbits of LEO satellites. The Zero difference technique has been proven to be very efficient for this task. Zero difference means that the observations between the GPS satellites and the LEO satellite must be used without any differencing (in time or in position). In this work, as a first step to determine precise LEO orbits, initial LEO positions have been estimated based on the Bancroft method with an accuracy of a few meters. These positions can be used subsequently as initial values for LEO positions based on pseudo-range ionosphere free observations. Then the position differences between two sequential epochs can be estimated with an accuracy of approximately 1 cm based on ionosphere free carrier phase observations. The absolutely estimated positions and position differences can be used afterwards to estimate the final absolute positions of a LEO satellite at every epoch, if a sufficient number of GPS satellites i.e. equal or more than four are available.