In the TI field model, time dilation of a process (e.g., a clock) has only one cause: velocity of the process relative to the TI field. The TI field is directly subject to gravity and is accelerated radially toward the center of mass of a gravitational body (GB). The infall velocity of the TI field at the orbiting satellite produces the time dilation of the satellite clock attributable to gravity. Gravity produces time dilation only indirectly via the mediation of the TI field. If the satellite is in a perfectly circular orbit the orbital velocity vector and the infall velocity vector of the TI field are orthogonal and the squares of their velocities relative to the satellite clock may be added algebraically. If the satellite is in an elliptical orbit the orbital velocity vector and the infall velocity vector are not orthogonal and must be added vectorially. The difference between these two calculations comprises the ellipticity bias, a bias that exists only if gravitational time dilation is caused by the infall velocity of the TI field and not (directly) by gravity.