报告题目：C20 estimates from GRACE: Recent Results

报 告 人：Orhan Akyilmaz, Ph.D，Istanbul Technical University

邀 请 人：方 兴 副教授

报告时间：2019年8月26日下午15:00

报告地点：测绘学院218会议室

报告人简历：

He was born in 1978 in Gaziantep, Turkey and had his BSc degree from Istanbul Technical University (ITU), Department of Geodesy and Photogrammetry Engineering in 1999. He had his MSc and PhD degrees on mathematical and physical geodesy at the Graduate School of Science Engineering and Technology at ITU, Istanbul in 2001 and 2005, respectively. Between 2002 and 2003, he has been in German Geodetic Research Institute (DGFI), Munich as a visiting scholar funded by a DAAD research grant. He also worked at the Ohio State University, Laboratory for Space Geodesy and Remote Sensing Research, between 2004-2006 as a visiting scholar and as a post doctoral fellow funded by the Scientific and Technological research Council of Turkey (TÜBİTAK) and NSF's CMG program of USA. He served at ITU Department of Geomatics Enginering as assistant and associate professors between 2006-2007, and 2007-2013, respectively. Since 2013, he is enrolled as a full professor at the same department. His research fields cover mathematical and physical geodesy and space geodetic techniques, including GNSS, satellite radar altimetry, SAR interferometry and LEO dedicated to static and time variable gravity field recovery such as GRACE/GRACE-FO. He has conducted and participated into various research projects either as a PI or a Co-PI, funded by the TUBITAK, European Union and NSF focusing on gravity field recovery using low earth orbiters (LEO), surface deformation monitoring with multi-mission geodetic satellite observations, monitoring Earth mass transport and hydrogeodesy. His current research is focused on processing of GRACE/GRACE-FO data at all levels (L1, L2, L3), including POD of LEO, regional and global gravity field recovery, downscaling Ground Water Storage changes using Artificial Intelligence Techniques as a novel methodology to combine various satellite geodesy and hydrogeologic data, and big (geospatial) data analysis to retrieve efficient and high resolution geophysical parameters. He is also the chair of the “Gravity Field” Working Group within Turkish National Geodetic Commission.

报告摘要：

C₂₀ (degree 2, order 0) harmonic coefficient of the global gravity field is related to the Earth’s oblateness and has particular importance in terms geophysics. Monthly C₂₀ solutions from official GRACE data analysis centers (GFZ, CSR, JPL) include periodic errors (which are usually assumed to be driven by the errors in S2 tide models). Therefore, it is usually recommended to replace the C₂₀ coefficients in monthly GRACE gravity field models (i.e. L2 data products) with those estimated from SLR (satellite laser ranging) observations.

In this study, we applied an alternative GRACE data processing method, namely improved energy balance approach (EBA) to achieve monthly temporal global gravity field solutions. In contrast to conventional dynamic approach used by official data analysis centers, our approach is consist of two main steps, (i) estimation of in situ geopotential difference (GPD) observables and (ii) least squares estimation of the spherical harmonic coefficients of monthly gravity field models using the GPD observations computed in step (i).

Our results show that our methodology/approach can resolve the low degree harmonics, and in particular C₂₀, with an accuracy comparable with the uncertainty of SLR estimates, and the estimated global gravity field models can directly be used without any need for replacing this coefficient. We conclude that the so-called error of C₂₀ in the solutions of official data analysis centers are not due to the S2 tide error, but it is likely to depend on the methodology they us efor GRACE data processing.