Contribution Analysis of GOCE SGG Observations at Different Orbital Altitudes to Tongji-GMMG2021S Gravity Field Model

Gravity field and steady-state ocean circulation explorer (GOCE) satellite gravity gradiometry (SGG) data play an important role in modeling static gravity fields, particularly considering the decrease in orbital altitude from 259.5 to 229.0 km during the final 15 months. However, there is limited discussion regarding the contribution of decreases in the GOCE satellite's orbital altitude to the static gravity field solutions. Therefore, by utilizing reprocessed Level-1B SGG data and gravity recovery and climate experiment (GRACE) satellite data, we have solved a satellite-only static gravity field model entitled Tongji-GMMG2021S up to 300 d/o. Subsequently, a quantitative analysis was conducted to assess the contribution of SGG data during the period of low-orbital altitude (LOA) from August 2012 to October 2013 towards the Tongji-GMMG2021S solution. The validation results indicated that the accuracy of the Tongji-GMMG2021S model was comparable to that of the latest combined GOCE/GRACE models. During the LOA period, the analysis conducted at the normal matrix level demonstrated that the SGG data obtained during the LOA period significantly contributed to the solutions at medium and high degrees. Notably, the final 5-month SGG data contributed more than 25% to the Tongji-GMMG2021S solution within the range of 150 to 260 d/o. Furthermore, beyond 180 degrees, the contribution of the solution for the LOA period was even more significant than that for the high-orbital altitude (HOA) period from November 2009 to July 2012. The LOA solution exhibited a standard deviation of 7.24 cm in geoid height error with respect to Tongji-GMMG2021S, resulting in a reduction of 51.15% in standard deviation compared to the HOA solution (14.82 cm).