Numerical study on the mixed model in the GOCE polar gap problem

Gravity gradients acquired by the Gravity field and steady-state Ocean Circulation Explorer (GOCE) do not cover the entire earth because of its sun-synchronous orbit leaving data gaps with a radius of about 6.5° in the polar regions. Previous studies showed that the loss of data in the polar regions deteriorates the accuracy of the low order (or near zonal) coefficients of the earth gravity model, which is the so-called polar gap problem in geodesy. In order to find a stable solution for the earth gravity model from the GOCE gravity gradients, three models, i.e. the Gauss-Markov model, light constraint model and the mixed model, are compared and evaluated numerically with the gravity gradient simulated with the EGM2008. The comparison shows that the Best Linear Uniformly Unbiased Estimation (BLUUE) estimator of the mixed model can solve the polar gap problem as effectively as the light constraint model; furthermore, the mixed model is more rigorous in dealing with the supplementary information and leads to a better accuracy in determining the global geoid.