 For BABS , the only change was to change the broadcast 's transmissions to a series of short pulses rather than a continuous signal . These pulses were sent out when triggered by the AI radar 's signals and were powerful enough that they could be picked up by the Mk . IV receiver within a few miles . On reception , the Mk . IV would receive either the dots or dashes , and the operator would see an alternating series of blips centred in the display , popping out and then disappearing as the BABS antennas switched . The duration of the blip indicated whether the aircraft was to the left or right , and became a continuous blip on the centreline . This technique was known as AI beam approach ( AIBA ) . 
