**2. GRACE mission orbit and its on-board instruments**

The GRACE mission consists of two 0.5 ton satellites that followed each other at a distance of ~220 km, which were placed at a relatively low average altitude of around 450 km with a quasi-polar orbit inclination of 89.5 degrees to ensure a quasi-global coverage (**Figure 2**). The relative distance between the two satellites was measured with a accuracy of 1 μm.s−1 by a radar telemeter operating at K-Band microwave Ranging (KBR) [4]. The inter-satellite distance depends of the gravitational acceleration changes that affect each GRACE satellite [6]. The A three-axis accelerometer that senses the dynamical effects as non-dissipative forces (the mean solar and Earth's radiation pressure, the atmospheric drag) is also placed on-board. Afterwards the non-gravitational effects are removed from the raw accelerations, the geopotential change along the track of the GRACE satellites is estimated from the residual perturbations in distance and inter-satellite distance changing rate. Changes of the observed inter-satellite distances reflect the variations of the Earth's gravity field related to topography and density heterogeneities.

The payload was composed of five instruments on-board, the satellite components of the GRACE mission are listed below [3]:

