Answer :
The altitude of these satellites above the earth's surface is 3,588 10⁷ m, and the radio signals from these satellites cannot directly reach receivers on earth that are north of 81.3 N latitude because by the curve of the planet; consequently, the signals cannot reach these places
to find the altitude of these satellites above the earth's surface requires the use of the law of universal gravitation and Newton's second law.
F = ma
G m M / r² = m a (1)
centripetal acceleration
a = v²/r
Why is the velocity factor (velocity) constant?
v = d/t
A complete orbit with a time (T) called the period circumnavigates the distance.
d = 2πr
v = 2πr/T
a = (4π²r²/T²)/r
insert in expression 1
GM/r² = 4π²r/T²
GM/r³ = 4π²/T²
r = ∛ GM T² / 4π²
R is the distance from the center of the earth which is the distance from the surface of the earth
R = Re + h
Re + h = ∛ GM T²/4π²
h = ∛(G M T²/4π²) - Re
Convert to SI units
T = 1 day (24 hours / 1 day) (3600 seconds / 1 hour)
T = 86400 seconds
let's do the math
h = ∛ (6.67 10⁻¹¹ 5.98 10²⁴ (8.6400 10⁴)² / 4π²) - 6.37 10⁶
h = ∛ (75.42 10²¹) - 6.37 10⁶
h = 4.225 x 10⁷ - 0.637 x 10⁷
h = 3,588 10⁷m
b) In the appendix we find that for very large latitudes the straight path from the satellite to the earth is interrupted by the curvature of the planet. As a result, the signal cannot reach these locations