A state vector is a satellite's exact position and velocity at one instant — six numbers plus a time stamp. It fully defines the orbit: from a single state vector you can compute the orbital elements, and by integrating the forces acting on the craft, predict where it will be at any other time.
Understanding State Vector
Elements or states — two views of one orbit
Orbital elements and state vectors carry identical information but suit different jobs. Elements are human-readable — inclination tells you at a glance how the orbit is tilted, and for an idealised two-body orbit five of the six barely change, making elements ideal for cataloguing thousands of objects compactly. States are machine-friendly: numerical integrators, manoeuvre targeting and rendezvous work all operate on position and velocity directly, and near-circular or near-equatorial orbits that make certain elements ill-defined (the argument of perigee of a circular orbit points nowhere) cause states no trouble at all. Real pipelines convert fluently in both directions, choosing the representation that suits each step.
Why operator ephemerides beat the catalogue
A surveillance radar sees a satellite for a few minutes per pass and fits an orbit through sparse, noisy tracks. The satellite itself, carrying a GNSS receiver, knows its own state continuously to a few metres. Operator-supplied state-vector ephemerides — which can also embed planned manoeuvres that no external tracker can anticipate — are therefore the gold standard input for close-approach screening, and screening systems prefer them over general-perturbations data whenever operators share them. The gap between the two data qualities is one of the strongest arguments for data-sharing in space traffic coordination.