Satellite Orbits and Measurement Characteristics

Orbits for ocean satellites are geostationary, polar or inclined orbits. A geostationary orbit is one in which the satellite is always in the same position with respect to the rotating Earth. The satellite orbits at an elevation of approximately 36,000 km because that produces an orbital period equal to the period of rotation of the Earth. By orbiting at the same rate, in the same direction as Earth, the satellite appears stationary. Geostationary satellites provide a large field of view (up to 120°) at very high frequency enabling coverage of weather events. Because of the high altitude, spatial resolution is of a few km while it is of 1 km or less for polar orbiting satellites. Because a geostationary orbit must be in the same plane as the Earth's rotation, that is the equatorial plane, it provides distorted images of the polar regions. Five or six geostationary meteorological satellites can provide a global coverage of the earth (for latitudes below 60°).

Polar-orbiting satellites provide a more global view of Earth by passing from pole to pole, observing a different portion of the Earth with each orbit due to the Earth's own rotation. Orbiting at an altitude of 700-800 km these satellites have an orbital period of approximately 90 min. These satellites usually operate in a sun-synchronous orbit. The satellite passes the equator and any given latitude at the same local solar time each day. Inclined orbits have an inclination between 0° (equatorial orbit) and 90° (polar orbit). They are used, in particular, to observe tropical regions (e.g. TMI on TRMM mission). High accuracy altimeter satellites such as TOPEX/Poseidon and Jason use higher altitude and non synchronous orbits to reduce atmospheric drag and (mainly) to avoid aliasing of the main tidal signals.

Depending on instrument types (along-track, imaging or swath), frequencies and antennas (see above), the sampling pattern of a given satellite will be different. In addition, in the visible and infrared frequencies, cloud cover can strongly reduce the effective sampling.

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