 The first EC-137 made its maiden flight on 9 February 1972, with the fly-off between the two radars taking place during March – July that year. Favorable test results led to the selection of Westinghouse's radar for the production aircraft. Hughes's radar was initially thought to be a certain winner, simply because much of its design was also going into the new F-15 Eagle's radar program. The Westinghouse radar used a pipelined fast fourier transform (FFT) to digitally resolve 128 Doppler frequencies, while Hughes's radars used analog filters based on the design for the F-15 fighter. Westinghouse's engineering team won this competition by using a programmable 18-bit computer whose software could be modified before each mission. This computer was the AN / AYK-8 design from the B-57G program, and designated AYK-8-EP1 for its much expanded memory. This radar also multiplexed a beyond-the-horizon (BTH) pulse mode that could complement the pulse-Doppler radar mode. This proved to be beneficial especially when the BTH mode is used to detect ships at sea when the radar beam is directed below the horizon.
