Station-keeping For Earth Observation Spacecraft
For Earth observation spacecraft typically operated in an altitude above the Earth surface of about 700 - 800 km the air-drag is very faint and a re-entry due to air-drag is not a concern. But if the orbital period should be synchronous with the Earth rotation to maintain a fixed ground track also the faint air-drag at this high altitude must be counter-acted by orbit raising manoeuvres in the form of thruster burns tangential to the orbit. These manoeuvres will be very small, typically in the order of a few mm/s of delta-v. If a frozen orbit design is used these very small orbit raising manoeuvres are sufficient to also control the eccentricity vector.
To maintain a fixed ground track it is also necessary to make out-of-plane manoeuvre to compensate for the inclination change caused by Sun/Moon gravitation. These are executed as thruster burns orthogonal to the orbital plane. For Sun-synchronous spacecraft having a constant geometry relative to the Sun the inclination change due to the solar gravitation is particularly large, a delta-v in the order of 1-2 m/s per year can be needed to keep the inclination constant.
Read more about this topic: Orbital Station-keeping
Famous quotes containing the words earth and/or observation:
“The earth is not a mere fragment of dead history, stratum upon stratum like the leaves of a book, to be studied by geologists and antiquaries chiefly, but living poetry like the leaves of a tree, which precede flowers and fruit,—not a fossil earth, but a living earth; compared with whose great central life all animal and vegetable life is merely parasitic.”
—Henry David Thoreau (1817–1862)
“That observation which is called knowledge of the world will be found much more frequently to make men cunning than good.”
—Samuel Johnson (1709–1784)