N-hour repeating equatorial orbit: a new method for remote sensing satellites

Sun-synchronous orbit (SSO) has a characteristic that passes the given target area at the same local mean solar time, and is commonly used by remote sensing satellites. The ground track of these satellites could repeat precisely, by applying specific design methods. However, the revisit time of a single SSO remote sensing satellite could be greater than one day, and hard to achieve continuous Earth observation, especially for low latitudes. The N-hour repeating equatorial orbit introduced in this paper is a new concept medium Earth orbit (MEO) for remote sensing satellites, which can realize several revisits in one day, and same local time revisits on different days for a designated target at low latitudes. In this paper, the inertial period definition of equatorial satellites is clearly defined and calculated, the design of prograde and retrograde repeating equatorial orbits is given and analyzed theoretically and numerically. The longitude coverage features of equatorial satellites are also evaluated.