How to calculate GPS incidence, heading angle?

Dear all,
I have only descending orbit data for my study area. This is the reason I couldn’t calculate the vertical as well as East-west velocities using SAR data. But I have high-density GPS data, So I’m planning to convert all GPS velocities to LOS (at least I can compare GPS LOS to descending orbit PS-InSAR LOS ). For that I got a formula(below attachment), In this formula, I required GPS heading, Incidence angle How can get these values. Please help me.