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An ephemeris for a satellite is a precise set of data that describes the satellite's position and velocity at a specific point in time. It is typically provided in the form of a set of Keplerian elements, that are the set of parameters that defines the orbit of the satellite. It includes the following information:
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- The orbital elements: semi-major axis, eccentricity, inclination, right ascension of the ascending node, argument of perigee and mean anomaly.
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- The time of the ephemeris: Epoch or the time at which the orbital elements are valid.
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- The time system it's based on.
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- The frame of reference, usually the Earth-centered inertial frame.
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This information is sufficient to calculate the position and velocity of the satellite at any time, given the right software and techniques. Since the satellite's orbit is constantly changing due to various factors such as gravitational pull, atmospheric drag, etc., ephemerides are typically issued in batches, covering a period of a few days to a few weeks, and updated regularly to reflect these changes.
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It is important to note that ephemerides are issued with an associated accuracy, which is the level of precision with which the ephemeris predicts the satellite's position and velocity. This accuracy can depend on many factors, such as the quality of the observations used to create the ephemeris, the level of knowledge of the satellite's orbit, and the level of precision of the software used to generate the ephemeris. |
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