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Problem on spacecraft

Assuming that ground stations are equally distributed on the Earth, how many ground stations are required to maintain constant contact with a spacecraft at 750 km altitude, and 72 degrees inclination?

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Given:

θ (theta) = 72°, Perpendicular distance of spacecraft=750 km.

We know that, 1° = π/180 radian, Radius of earth=6400 km. 
So, 72°= (72*π)/180 radian,
           = 2*π/5
Since,
θ = Arc/Radius,

Therefore, Arc = θ*Radius,
                      =2*π*750/5,
                      = 300π

Circumference of earth = 2*π*Radius
                                  = 2*π*6400

No. of space station = Circumference of earth/2*Arc
                             = (2*π*6400)/(2*300*π)  
                             = 21 space station (approx.)

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