Geocenter motion - Degree 1长时间序列变化
The spherical harmonic coefficients of degree 1 represent the distance between the center of mass of the Earth and its 'center of figure'【球谐一阶项代表了地球形状中心和地球质量中心之间的距离】, which in practice is approximated by the center of a set of tracking stations on the surface of the Earth【确定地球形状中心通常是在地表设置一系列跟踪站，这些站点所确定的中心为地球的形状中心】. Because of their physical meaning, time changes in degree 1 coefficients can be expressed in several equivalent forms
As distances in mm along the Z (along the axis of rotation), X and Y axes;
As (fully normalized) coefficients of the geopotential;
As the changes in mass (per unit area) that would give rise to the geopotential coefficients, expressed either in kg/m^2 or cm of equivalent water thickness.
此处我们可视化第一类表达方式。我们打开GRACE数据，我们可以发现一阶项是0，这是因为在解算数据时，默认为地球的质量中心，而我们在研究中通常采用地球的形状中心，因而两者存在差异，需要补充一阶项。
我们在这个网站可以下载以X,Y,Z形式表达的一阶项的长时间变化https://filedrop.csr.utexas.edu/pub/slr/geocenter/
显示的结果如下图：
