phase gradient

[kjz1997]

hello，[(EarthScope2023/1.3_InSAR_theory)]，I find it a bit difficult to understand the formula for the phase gradient in EarthScope2023/1.3-InSAR_theory. The phase difference is Delta_phi = -2pi p<b,l>/lambda . When calculating the gradient, my understanding is to take the derivative along the orbital direction and the slant direction separately. However, why does L take the derivative of s occur.