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.
For medium and low latitudes, the N-hour repeating equatorial orbit is a new option for remote sensing satellites, which can realize multiple revisits per day, and revisit at the same local time revisits on different days. The remote sensing constellation formed by equatorial satellites could easily achieve rapid revisit, or even continuous observation. Therefore, the ground surveillance performance is critical for the system design of equatorial remote sensing constellations. In this paper, the latitude coverage characteristics of equatorial satellites are analyzed. This paper also assesses the constellation size required to achieve continuous observation of targets in different latitudes. The staring imaging time durations of satellites in the prograde and retrograde constellation are analyzed respectively.