  We show that properties of the beam position monitor (BPM) system designed
for the measurement of transverse and energy beam jitters can be described in
terms of the usual accelerator physics concepts of emittance, energy spread,
dispersions and betatron functions. Besides that, using the Courant-Snyder
quadratic form as error estimator we introduce the scalar objective function
which can be used as design criteria of a BPM measurement system with needed
properties.
