Abstract: An earth observation system capable of observing the earth globally and uniformly in a steady observation operation mode while observing a given district on the earth in a high-frequency observation operation mode, as occasion requires, includes a plurality of observation satellites each equipped with an orbit transfer facility, an observation data receiving system, an observation satellite operating ground system, an observation command facility such as anti-disaster task forces, institutes or the like. In the steady observation operation mode for observing globally the earth, M observation satellites are constellated on each of L sun-synchronized quasi-recurrent orbits each having a same altitude and a same inclination (where L and M represents natural numbers). In the high-frequency observation operation mode for observing a particular district on the earth, M satellites are constellated on each of L sun-synchronized recurrent orbits each having a nominal altitude of 561 km and a nominal inclination 97.