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tokens: P. Bastien et al. 2017, in preparation). The OMC 1 region was observed 21 times between 2016 January 11 and 25 in a mixture of very dry weather (Band 1; ) and dry weather (Band 2; ) under JCMT project reference numbers M16AL004 (BISTRO) and M15BEC02 (POL-2 commissioning). In order to determine the behavior of rms noise in our observations as a function of integration time, we measured the standard deviation on the Stokes Q and Stokes U values in a region with relatively constant signal in both the Stokes Q and the Stokes U maps, located between OMC 1 and the Orion Bar. This region, centered at approximately , was chosen because it was relatively flat, moderately unpolarized, low in emission, and away from the brightest sources, and because there was no region entirely without signal in the central part of the map. Figure 2 shows how the noise integrates down in this 21-repeat (∼14 hr) POL-2 observation. Figure 2. Mean measured rms noise in Q and U as a function of time (in minutes) at a single off-source position. The data points show the scatter on the individual measurements, and the line is the running mean. A behavior consistent with is seen, as in the ideal case. The polarization noise in Figure 2 is seen to integrate down close to , as in the ideal case. The scatter of individual measurements reduces satisfactorily as the data are subsequently combined. We find that there is no evidence of any “noise floor” in long integrations. From this plot we see that this data set has reached 2.1 mJy beam −1 rms noise in 13.5 hr. An rms noise value of ∼2 mJy beam −1 was set as the target value for the BISTRO survey. Appendices A and B list a series of tests that we carried out to confirm the repeatability of our measurements and to demonstrate consistency with previous data. 4. First Data from the Survey Figure 3 shows a polarization map taken with POL-2 of the OMC 1 region of the “integral filament” in the Orion A molecular cloud, with half-vectors rotated by 90° to trace the B-field direction. Only vectors with a signal-to-noise ratio of 3 or greater in polarization fraction are shown (i.e., ). The Orion A molecular cloud is a well-resolved and well-studied region of high-mass star formation (e.g., Bally 2008 ; O’Dell et al. 2008 ). It is the closest region of high-mass star formation, located at a distance of 388 ± 5 pc (Kounkel et al. 2017 ). The half-vector lengths show the percentage polarization, with a 5% scale bar in the corner to give the calibration. The underlying image is an 850 μ m total intensity map of the same region taken using SCUBA-2. Figure 3. Polarization map of the OMC 1 region of the “integral filament” in Orion A, with half-vectors rotated by 90° to show the B-field direction. The Orion Bar can be seen in the southeastern part of the map. Half-vectors with are shown. The background image is a SCUBA-2 850 μ m emission map taken using the standard SCUBA-2 DAISY mapping mode. The half-vector gray-scale is chosen for contrast