A state vector is a set of data telling exactly where the shuttle is in its orbit in space. From a state vector, ground controllers can tell how fast the shuttle is moving, what country it is over at any given time, and predict when it will pass over any ground site. They can also predict things like is the shuttle in darkness, can the shuttle see the moon, or can the shuttle see another vehicle, like the MIR space station. J-Track, our java spacecraft tracker, uses state vectors to calculate spacecraft positions.

Much of the replanning of the mission events uses a state vector as a starting point. For example, if an activity requires video downlink from the shuttle, that means that the shuttle has to have one of the two TDRSS satellites in view of its antenna. To predict whether the shuttle can see either of the TDRSS satellites it is necessary to know where the shuttle and the two satellites are.

State vectors also allow the ground to predict things like when and where the shuttle can land. Ground controllers use the state vector to predict which orbit, or rev, the shuttle will be over the appropriate landing site. For example, if KSC is the preferred landing site controllers need to predict when at the end of the mission the shuttle will be within range to land at KSC. If an appropriate opportunity does not exist, an adjustment can be made to the state vector to "bring in" an appropriate landing site. An orbit adjustment, or burn, is a firing of the OMS engines to raise, lower or adjust timing of events, such as landing opportunities.

State vectors are as important to the ground support personnel as they are to the shuttle itself. The shuttle needs to know where it is too. For the shuttle to maneuver to a new orbiter attitude (the direction the shuttle is pointing) it needs to know where it is in space so it can calculate the most efficient way to get to the new orbiter attitude. During any shuttle flight, updated state vectors are uplinked to the shuttle on a regular basis.

State vectors can come in many forms; Cartesian, Spherical Polar, Classical Elements, 2-Line Element, to name a few. The form of a state vector is just a way of identifying which set of orbit parameters is being used to define the orbit. All of these forms have one parameter in common, time. It is necesary to relate all the parameters of the state vector to a particular time point. Without this, predictions on where the shuttle will be in the future are impossible. Most of these forms use seven (7) parameters, including time, to define an orbit (except 2-Line Element).

Many of these forms also can be represented in several different reference systems; Mean-of-1950, Mean-of-2000, Mean-of-Date, just to name a few. So, for example, a particular state vector could be cartesian Mean-or-1950 or cartesian Mean-of-Date. These systems define which year of reference to use for the vernal equinox (the time when the sun crosses the plane of the earth's equator in spring). Over the centuries, the position in the sky of the vernal equinox slowly changes. Since many of the orbital elements are measured with respect to this point, it is important to reference them to the year for which they were derived.

Last Updated September 20, 1995 Contacts