 Only one receiver was available , which was moved between aircraft for testing . A transmitter of the required power was not available in portable form . Bowen decided to gain some familiarity with the equipment by building a ground @-@ based transmitter . Placing the transmitter in Bawdsey 's Red Tower and the receiver in the White Tower , they found they were able to detect aircraft as far as 40 to 50 miles ( 64 – 80 km ) away . 
