Earth to Satelite Signal Time Delay Formula

Equation for calculate earth to satelite signal time delay is,

${\Delta }_t=\frac{2}{c}·\sqrt{R^2+r^2+2·R·r·\cos \varphi }$

where,
c = Speed of light (299792458 m/s)
R = Earth' s radius (6.378 m)
r = The orbital radius (the distance from the centre of the Earth)
ϕ = Latitude
Δ_t = Time taken for a signal to send from earth to the satellite and back again