Radial Acceleration is defined as the acceleration of the tracking servo across the disc or in a radial direction. If the track contains a deviation, the head must speed up to maintain tracking. Obviously at 540RPM or 1X this is less difficult than in a 40X drive with speed up to 8000RPM. Often the result is the tracking servo loses tracking, has to slow down and, at worst, loses its data. The ability to measure RA as well as other radial parameters like Radial Noise and Eccentricity are essential in all CD-ROM replication.