Kinematic precise orbit determination for gravity field determination

In this paper we first present approaches and results in precise orbit determination (POD) for satellites in Low Earth Orbit (LEO) based on one or two frequency GPS measurements and, secondly, we focus on the relations between kinematic POD and gravity field determination. Using GPS measurements of the CHAMP satellite we show that it is possible to estimate kinematic positions of a LEO satellite with the same level of accuracy (approximate to 1-3 cm w.r.t. SLR) as with the widely applied reduced-dynamic or dynamic approaches. Kinematic precise orbit determination (POD) as presented here is based on GPS phase measurements and is independent of satellite dynamics (e.g. gravity field, air-drag, etc.) and orbit characteristics (e.g. orbit height, eccentricity, etc.). We also looked at the LEO POD based on GPS measurements from only one frequency, where we make use of what we call the LP linear combination. We show that with this linear combination LEO POD can be performed with one frequency at the 10 cm level. In the second part of the paper the use of kinematic POD is discussed in the framework of the CHAMP, GRACE and GOCE gravity missions. With simulated GPS measurements we studied the impact of ambiguity resolution for the kinematic baseline between the two GRACE satellites. At the end of the paper we present an alternative approach in gravity field determination by measuring the gravitational frequency shift between (optical) atomic clocks in space. In this approach we require very accurate clocks positions, which may be obtained from kinematic POD.