storey includes three optical modules (OMs) [@OM], each housing a 10-inch photomultiplier [@PMT] and a local control module that contains the electronics [@frontend; @DAQ]. The OMs are orientated 45$^{\circ}$ downwards in order to optimise their acceptance to upgoing light and to avoid the effect of sedimentation and biofouling [@biofouling]. The length of a line is 450 m and the horizontal distance between neighbouring lines is 60-75 m. In one of the lines, the upper storeys are dedicated to a test system for acoustic neutrino detection [@amadeus]. Similar acoustic devices are also installed in an additional line that contains instrumentation aimed to measure environmental parameters [@instrumentation]. The location of the active components of the lines is known to better than 10 cm by a combination of tiltmeters and compasses in each storey and a series of acoustic transceivers (emitters and receivers) in certain storeys along the line and surrounding the telescope [@alignment]. A common time reference is maintained in the full detector by means of a 25 MHz clock signal broadcast from shore. The time offsets of the individual optical modules are determined in dedicated calibration facilities onshore and regularly monitored in situ by means of optical beacons distributed at various 