The bomber would overfly an Initial Point where it would be picked up and locked onto by a tracking radar (I can't remember the radar type, but it was mounted on a mobile unit). The radar track was fed to a plotting table on another mobile unit. Prior to weapon release, the aircraft would transmit a tone which stopped at weapon release. This produced a spike on the plotting trace from which, using weapon forward throw and aircraft groundspeed an impact point could be plotted relative to the target position.