Fast wire scanners have been used for some time to measure the transverse emittance of circulating beams in the SPS over a wide range of intensities for both fixed-target and collider operation 11, 21. The precision obtained with these devices is of the order of a few percent for the emittance and a few tenths of a millimeter for the beam centre. Although in general this is quite adequate, in the particular case of the proton--antiproton collider operation of the SPS it is desirable to improve the precision substantially in order to measure slow phenomena such as intrabeam scattering or beam cooling over a reasonably small time interval. In order to achieve this, a new wit-e scanner has been developed, based on a linear movement instead of the rotating mechanism used previously. The device moves a 30 pm carbon fibre through the beam at. a speed of 0.3 m.s-I. an order of magnitude slower than normally used for the fast wire scanners. The position of the fibre at any time is measured with a high precision linear optical transducer with an accuracy of 4 pm.