## 1. Introduction

[2] The launch of dedicated gravity satellite missions has revolutionized our knowledge of the Earth's gravity field. In 2002 the along-track satellite formation mission GRACE (Gravity Recovery and Climate Experiment) [*Tapley et al.*, 2007] was launched into a near polar low Earth orbit. Apart from high-low satellite-to-satellite tracking by the GPS constellation and a 3-axis accelerometer measuring the non-conservative accelerations (e.g., air drag, solar radiation pressure) acting on each of the two satellites, the key element is a microwave ranging system observing the distance variation between the two satellites with micrometer accuracy. The most recent static models derived purely from GRACE data resolve the global gravity field up to degree 180, e.g., GGM03S [*Tapley et al.*, 2007], EIGEN-5S [*Förste et al.*, 2008], AIUB-GRACE02s [*Jäggi et al.*, 2009], and ITG-Grace2010s [*Mayer-Gürr et al.*, 2010].

[3] The GOCE satellite (Gravity Field and Steady-state Ocean Circulation Explorer) [*Drinkwater et al.*, 2003] was successfully launched on 17 March 2009. GOCE is based on a sensor fusion concept: orbit analysis with a spaceborne GPS-receiver enabling continuous 3D tracking of the satellite with an accuracy 2–3 centimeters, plus on-board satellite gravity gradiometry (SGG). This completely new measurement concept is based on the observation of gravity gradients in space with accelerometers over short baselines within a platform flying in drag-free mode, i.e., by in-situ compensating the non-gravitational forces. Correspondingly, GOCE is the first mission that observes direct functionals of the Earth gravity field from space. First GOCE gravity field models based on 71 days of GOCE data from November 2009 to January 2010 have been computed in the frame of the ESA project “High-Level Processing Facility” by applying 3 independent and complementary processing strategies [*Bruinsma et al.*, 2010; *Pail et al.*, 2010; *Migliaccio et al.*, 2010].

[4] One of these three solutions, the so-called time-wise gravity field model [*Pail et al.*, 2010] (product ID: EGM_GOC_2__20091101T000000_20100111T000000_0002), will be used as a reference solution, and will be denoted as ESA GOCE-only model hereafter.

[5] Here, the first combined satellite-only gravity field model GOCO01S is presented, which is a combination of GRACE and GOCE measurements, to make optimal benefit of the individual strengths of these two satellite missions. (GOCO stands for “Combination of GOCE data with complementary gravity field information”, a project initiative with the objective to compute high-accuracy and high-resolution static global gravity field models based on satellite and terrestrial data. GOCO01S is the first step comprising a combined satellite-only model).

[6] The model is parameterized in terms of coefficients of a spherical harmonic series expansion of the gravitational potential *V* in spherical coordinates (with radius *r*, co-latitude ϑ, longitude *λ*):

with *G* the gravitational constant, *M* the mass of the Earth*, R* the mean Earth radius, _{lm} the fully normalized Legendre polynomials of degree *l* and order *m*, and {_{lm}, _{lm}} the corresponding coefficients to be estimated.