The spherical harmonic coefficients of degree 1 represent the distance between the center of mass of the Earth and its 'center of figure'【球諧一階項代表了地球形狀中心和地球質(zhì)量中心之間的距離】, which in practice is approximated by the center of a set of tracking stations on the surface of the Earth【確定地球形狀中心通常是在地表設(shè)置一系列跟蹤站，這些站點(diǎn)所確定的中心為地球的形狀中心】. Because of their physical meaning, time changes in degree 1 coefficients can be expressed in several equivalent forms

1. As distances in mm along the Z (along the axis of rotation), X and Y axes;

2. As (fully normalized) coefficients of the geopotential;

3. 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.

此處我們可視化第一類表達(dá)方式。我們打開GRACE數(shù)據(jù)，我們可以發(fā)現(xiàn)一階項是0，這是因為在解算數(shù)據(jù)時，默認(rèn)為地球的質(zhì)量中心，而我們在研究中通常采用地球的形狀中心，因而兩者存在差異，需要補(bǔ)充一階項。

我們在這個網(wǎng)站可以下載以X,Y,Z形式表達(dá)的一階項的長時間變化https://filedrop.csr.utexas.edu/pub/slr/geocenter/

顯示的結(jié)果如下圖：