Orbit determination

Orbit determination is the estimation of an object's trajectory and orbital parameters from observations, using a dynamical model to infer motion between measurements.

Astrometric observation: A measurement of a celestial object's position on the sky relative to a reference frame. Angular positions are the basic data from which many orbit estimates begin.
Keplerian orbit: An idealized orbit described by Kepler's laws, with a conic path and a central gravitational force. Its six classical elements provide a common coordinate system for describing estimated orbits.
Spacecraft navigation: The determination and control of a spacecraft's position, velocity, and trajectory. Mission navigation depends on estimating a spacecraft's orbit from tracking measurements.
Initial orbit determination: The first estimate of an object's orbit from a limited set of observations. It establishes a preliminary orbit before a longer observational arc supports refinement.
State vector: A vector specifying an object's position and velocity at a particular time. An estimated state vector compactly specifies the orbit at a chosen epoch.
Orbital elements: Parameters that specify an orbit's size, shape, orientation, and position at a given time. They are a standard way to report the quantities an orbit determination estimates.
Satellite tracking: The observation of satellites over time to determine and monitor their positions and motion. Tracking data provide continuing measurements for updating satellite orbit estimates.
Orbit propagation: The prediction of an object's future or past motion from a known state and dynamical model. Propagation predicts motion from an existing state; determination infers that state from data.
Two-body problem: The idealized motion of two bodies interacting only through their mutual gravitational attraction. It supplies the simplest dynamical model for propagating an estimated orbit.
Coordinate system: A framework of axes and conventions used to assign coordinates to positions and directions. Observed directions and inferred positions require consistent reference frames.

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