  The International Linear Collider and other proposed high energy e+ e-
machines aim to measure with unprecedented precision Standard Model quantities
and new, not yet discovered phenomena. One of the main requirements for
achieving this goal is a measurement of the incident beam energy with an
uncertainty close to 1e-4. This article presents the analysis of data from a
prototype energy spectrometer commissioned in 2006--2007 in SLAC's End Station
A beamline. The prototype was a 4-magnet chicane equipped with beam position
monitors measuring small changes of the beam orbit through the chicane at
different beam energies. A single bunch energy resolution close to 5e-4 was
measured, which is satisfactory for most scenarios. We also report on the
operational experience with the chicane-based spectrometer and suggest ways of
improving its performance.
