The choice of a satellite orbit mostly depends on its main application. From this point of view it represents a crucial part of a satellite mission design. The most important parameters to describe a satellite orbit are the inclination angle i (of the orbit plane respect to the equatorial plane) its eccentricity e and its height H from the Earth's surface. In principle whatever eigth H can be used, provided that the speed of the satellite on its orbit allows the centrigugal force to exactely compensate the gravitational one at that heigth. Polar (i close to 90°) and Geostationary (i=0, H=35.800 km) orbits are the most common choices for EO satellites. In principle one single polar satellite can be sufficient to guarantee the global coverage of the Earth with equal quality of the images at all latitudes. All Geostationary satellites share the same circular orbit with H around 36000 km where the required speed exactely correspond to the one required to travel an entire orbit in 1 sideral day (orbital period P = 1 sideral day). This means that the satellite footprint is permanently in place over a specific Earth's location (e.g. for Meteosat 0°N, 0°E) allowing a quasi-continuous monitoring of a whole Earth's emisphere (with poor visibility of Earth's edges including Poles). Polar satellites' heigths are usually in between 700-800 km, with orbital periods around 100min (i.e. about 14,5 orbits/day) even if, lower orbits are also chosen particularly for very high spatial resolution payloads. Lower inclinations are also used (quasi-polar orbits) for specific applications. Due to the asphericity (and mass inhomogeneity) of the Earth, satellite orbit plane rotates around the Earth's polar axis with a period Pp producing (for elliptical orbits) the rotation of the orbit itself in its plane. A common choice for most EO polar satellites is to choose the orbital parameters in a way that Pp=1 year (Sun-Synchronous orbits). Due to the synchronism between Earth's revolution around the Sun and the orbit plane precession around Earth' axis, satellite passages happens at the same local solar time (similar illumination conditions) each time it flies over a specific region. This ensure repeatable sun illumination conditions facilitating image interpretation particularly for change detection or land monitoring applications. Other choices are possible when it is required to monitor with continuity high latitude regions. This is the case of Molniya orbits which combine the continuity of observations typical of geostationary satellites with the possibility, offered by polar orbits, to overfly the highest latitudes regions. Its characteristics are: high eccentricity (e.g. e=0,74, axes 500 and 23.000 km), P=1/2 sideral day (Geo-Synchronous), inclination (i=63,4° or i=116,6°) which guarantees the satellite footprint at the apogee remaining positioned on a fixed ground point (non-rotating orbit). This way the satellite will spend more than 93% of its orbital period looking to the same emisphere even from a high latitude point of view. So called altimetric orbits respond to the specific needs of altimetry. In this case the orbital parameters are chosen in order to guarantee, for example: a) that the ascending and descending sub-satellite tracks intersect at roughly 90 degrees on the Earth’s surface (so that orthogonal components of the surface slope can be determined with equal accuracy; b) the possibility to monitor all phases of tidal effects on ocean surface. Particularly important for several applications (multi-temporal analyses, change detection, etc.) are the Exactly repeating orbits. They are conceived in order that the sub-satellite track will repeat itself exactly after a certain interval of time. This allows images having the same viewing geometry during the satellite’s lifetime making moreover available a particularly simple method of referring to the location of images (navigation or geo-referenciation) for example by referring to a ‘path and row’ system used for instance by the Landsat World Reference System (WRS). It is possible to arrange satellite orbits parameters in order to contemporary guarantee the sun-syncronism so that, not only satellite images collected on the same region can be easily super-imposed each-other but the same illumination and viewing geometry can be achieved. This is, for instance, the choice adopted for LANDSAT satellites whose images are typically available as a collection of scene of fixed dimension always similar each other when covering the same terrestrial area.
Illustrate the importance of the choice of the satellite orbit for the design of a satellite mission devoted to specific applications
Discuss different types of satellite orbits
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