Future Linear Colliders have hard requirements for the beam transverse position stability in the accelerator. A beam Position Monitor (BPM) with the resolution better than 0.1 micron in the single bunch regime is needed to control the stability of the beam position along the linac. Proposed BPM is based on the measurement of the asymmetrical mode excited by single bunch in the cavity. Four stages of signal processing (space-,time-, frequencyand phase-filtering providing the required signal-to-noise ratio) are used to obtain extremely high resolution. The measurement set-up was designed by BINP and installed at ATF/BNL to test experimentally this concept. The setup includes three two-coordinates BPM's at the frequency of 13.566 GHz, and reference intensity/phase cavity. BPM's were mounted on support table. The twocoordinates movers allow to move and align BPM's along the straight line, using the signals from the beam. The position of each monitor is controlled by the sensors with the accuracy 0.03 micron. The information from three monitors allows to exclude angle and position jitter of the beam and measure BPM resolution.. In the experiments the resolution of about 0.15 micron for 0.25 nC beam intensity was obtained, that is close to the value required.

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