Low altitude flying for high resolution imaging satellite: Comparison of low circular and elliptical orbits

Low Flying enables higher resolution imaging capabilities within a given instrument and satellite format, but at price of higher atmospheric drag and propulsion constraints if we keep with circular orbit. The intuitive interest of elliptic orbit is to enable low altitude flight with much less atmospheric drag since it occurs during the limited duration of perigee. The working restriction to perigee leads to coverage limitation within a latitude window of 30 degrees. This is not a fundamental obstacle since there are (will be) in the world several regional concentrations of needs for high resolution civilian or military surveillance. For the only goal of high resolution near the vertical of the satellite, the elliptic has no equivalent, but under given coverage and revisit constraints the circular orbit reduces the need for off-nadir steering and the resulting degradation of (optical) resolution with respect to vertical. The purpose of this preliminary study is to compare the two orbit approaches. This is based on existing formats of satellite designed to fly above the usual altitude floor (500 km) and therefore according to limited mass extension (similar to 5%) to accommodate the extra AV and propulsion needs (electrical or chemical). Emphasis is given to the microsatellite format (< 200 Kg). It comes up that the elliptic orbits call he very efficient solutions when the imaging needs are geographically concentrated (1000 km circle). We call have constellations of satellites of circular orbits giving the global coverage, plus one or few extra satellites placed on elliptical orbits to drastically upgrade both the resolution and the revisit over one or several theatres with tuneable locations.