Apparatus comprises: a multi-element antenna; and a transmitter configured to broadcasting multiple packets from the multi- element antenna, wherein each packet comprising: a positioning part; and a calibration data part, wherein the calibration data part of a packet comprises: a portion of calibration data; and data indicating a location of the portion of calibration data in a set of calibration data. The transmitter is configured: to switch between different elements of the multi-element antenna in a sequence when broadcasting the positioning part of the packet; and to transmit all of the calibration data part of the packet without switching between different elements of the multi-element antenna.