The NA48 experiment at the Cern SPS aims at measuring the direct CP
violation parameter 
with
a precision of about 2 
10 
[1] by comparing the relative
decay rates
of short- and long-lived kaons into two neutral or two charged pions:
The principle
of the experiment consists in collecting the four relevant decay modes concurrently
in the same detector, using almost collinear 
and 
beams
distinguished by tagging the proton which produces the particle.
In this way, differences and variations in detection efficiencies for
and decays become unimportant, as do rate dependent effects
introduced by accidental activity in the detector elements.
High-rate capability and high-resolution detectors for the identification of the charged and neutral pion modes and for the proton tagging system are necessary to provide excellent background rejection and to keep the systematical uncertainties at the desired level.