The Clohessey-Wiltshire equations model what happens when two orbiting bodies are close to each other. Typically, one spacecraft is fixed as the primary craft, which is called the target, and the secondary spacecraft is called the chaser. The Clohessey-Wiltshire equations model the movement of the chaser relative to the target. In other words, in this frame of reference, the target's position is always considered to be . The functions LaTeX: x\left(t ight),y\left(t ight), and LaTeX: z\left(t ight) give the coordinates of the chaser's position at time LaTeX: t. Likewise, their derivatives and second derivatives give the velocity and acceleration of the chaser relative to the velocity and acceleration of the target. This is useful because the movement of the chaser may be significant relative to the position of the target, even if the movement is small and insignificant relative to the body which it is orbiting. The system of equations is as follows (where all primes denote derivatives with respect to time, ): In the previous equations, is a constant which denotes the mean angular velocity of the target spacecraft. It can be calculated by , where is the orbital period of the target body. Note that the last of the three equations