$V=a\sum _{k=0}^{K}\sum _{m=0}^{k}{\left(\frac{a}{r}\right)}^{{n}_{k}\left(m\right)+1}{P}_{{n}_{k}\left(m\right)}^{m}\left(\mathrm{cos}\theta \right)\left[{g}_{k}^{m}\mathrm{cos}\left(m\lambda \right)+{h}_{k}^{m}\mathrm{sin}\left(m\lambda \right)\right]$
The coefficients, ${g}_{k}^{m}$ and ${h}_{k}^{m}$ are functions of time so the CGRF is valid over the time interval 1960 to 2010.