  A high-resolution, intratrain position feedback system has been developed to
achieve and maintain collisions at the proposed future electron-positron
International Linear Collider (ILC). A prototype has been commissioned and
tested with a beam in the extraction line of the Accelerator Test Facility at
the High Energy Accelerator Research Organization in Japan. It consists of a
stripline beam position monitor (BPM) with analogue signal-processing
electronics, a custom digital board to perform the feedback calculation, and a
stripline kicker driven by a high-current amplifier. The closed-loop feedback
latency is 148 ns. For a three-bunch train with 154 ns bunch spacing, the
feedback system has been used to stabilize the third bunch to 450 nm. The
kicker response is linear, and the feedback performance is maintained, over a
correction range of over $\pm$60 {\mu}m. The propagation of the correction has
been confirmed by using an independent stripline BPM located downstream of the
feedback system. The system has been demonstrated to meet the BPM resolution,
beam kick, and latency requirements for the ILC.
