Panel a: sum of the Euclidian distance between the experimental covariance coefficients and the analytical values calculated from Eq. (13) with different equivalent height values. The equivalent height values are distributed from 2000 m to 4000 m with 80 steps and the best fitted equivalent height is 3164.6 m. Panels b–d: experimental values and best-fit analytical values when the separated field angle are 16″, 22.6″, 32″, respectively.