Towards a new release of the combined global gravity field model GOCO: Preliminary results

Global models of the Earth's static gravity field are crucial for geophysical and geodetic applications such as oceanography, tectonics, and global reference systems. The combined global gravity field model GOCO2025 is the latest release of the GOCO* series and is derived solely from satellite observations. Data from the dedicated satellite gravimetry missions GRACE, GRACE-FO, and GOCE as well as satellite laser ranging observations and kinematic orbits of geodetic satellites are used to determine a static gravity field together with a regularized trend, an annual and a semi-annual oscillation. Starting with GRACE in 2002 the time series spans more than 20 years of data from satellite missions with complementary observation principles, which allows for the determination of a high-accuracy gravity field model with the best possible spatial resolution.

The unique instrumentation on GRACE-FO with two independent inter-satellite ranging systems (laser ranging interferometer (LRI) and K/Ka band ranging instrument (KBR)) that operate simultaneously allows for the determination of a stochastic model taking the cross-correlation between the instruments into account by using variance component estimation. This modeling approach results in formal errors of the spherical harmonic coefficients that align well with empirical estimates which is crucial for a combination with other data types and uncertainty propagation. The significantly higher measurement precision of the LRI compared to the KBR is especially beneficial for determining high-degree spherical harmonics. Proper stochastic modeling of all the input data results in realistic accuracy information for the derived combined gravity field solution (represented by a full variance-covariance matrix) that is crucial for further combination with, for example, terrestrial gravity data.

Consistent and up-to-date background models are used in the combination process of the different satellite data sets. A combined ocean tide model (GOT5.6 + FES2022 + TIME22) and additional corrections to this model, estimated over the entire GRACE/GRACE-FO time series, are employed and uncertainty information of the Atmosphere and Ocean De-Aliasing Level-1B product (AOD1BRL07) is incorporated in the estimation of short time gravity variations within the least-squares adjustment.