a. What is a repeat ground-track orbit? b. Explain why repeat ground-track and Sun-synchronous orbits are typically used for Earth observation missions. m k c. The constraint for a Sun-synchronous and repeat ground-track orbit is given by T = 2 86, 400, where T is the orbital period in seconds, m the number of days and k the number of revolutions. Explain why this is, in fact, a constraint on the semi-major axis of the orbit.