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The position of an event in the PSPC is determined from the amplitudes of signals on the two mutually perpendicular cathode grids, K1 and K2 (Fig. 3.1). The cathode grids are divided into 25 (for cathode K1) and 23 (for cathode K2) cathode strips. Each strip consists of seven or eight individual cathode wires linked together and connected to a charge-sensitive preamplifier. Typically an event will induce a signal on 3 to 5 strips, although this is not the case for events with pulse heights corresponding to keV (see Sect. 3.5.4). The signals from each preamplifier processed and have their signals digitized by means of pulse shapers, peak detectors, and analog-to-digital converters (ADCs). After verifying the validity of an event, the position is calculated by a centre-of-gravity calculation and assigned an position. (A detailed description of the method of position determination and its ground calibration measurements can be found in [Briel et al.1988].