SD, SDa, SD-1, SD-2 (XAS)
This submarine air warning radar completes the list of long-wave air search sets originally designed for US service. It was intended to be omnidirectional, on the theory that its function was to alert the submarine to the need to crash dive. At 2.65m it used the longest waves in US naval radar, but the peak power of 100kW (8.5 microseconds, PRF 60) meant little in conjunction with a poorly defined beam, and gave a range of only 20nm on a bomber at 10,000ft assuming an antenna 40ft above the water. Accuracy and resolution were 1000yds and 1500yds respectively. Sixty sets were produced between late 1941 and mid-1942. All were field modified to SDa standard, the J-type vertically polarized antenna being replaced by a horizontally polarized U-type, and video amplification for IFF being added. These antennas were retractable. SD-1, of which 20 were built (beginning late 1942), had a fixed mast, and SD-2, of which 60 were delivered from 1942 on, was a new production version which included provision for IFF in its indicator unit. An unusual feature of the original design was a duplexer permitting alternate operation of the submarine's radio transmitter. RCA.