In analyzing close approaches between a primary and secondary object -- i.e., between a satellite of interest and an orbiting body that presents a threat of collision -- Advanced CAT assigns to each a threat volume, comprising an ellipsoidal shape enclosing the object. Often, the volume represents the degree of uncertainty about an object's position at any given time. At any given instance, Advanced CAT computes the range between the threat volumes of the two objects. A warning is given whenever the range falls below a user-selected threshold. A collision event occurs whenever the range between the two threat volumes becomes zero or negative.
The dimensions of a threat volume ellipsoid are defined by three axes:
- The X axis in the tangential direction, i.e., parallel to the object's velocity vector;
- The Y axis in the cross track direction, i.e., parallel to the orbit normal vector, or the cross product of the radius and velocity vectors; and
- The Z axis in the normal direction, i.e., parallel to the cross product of the X and Y axes.
The X and Z axes are in, and the Y axis is perpendicular to, the orbit plane.
Advanced CAT also provides the option of having the close approach calculation use the range between the objects as the criterion rather than ellipsoid separation.