aircraft and Navy blimps. With a gyro-stabilized mount, the SG could detect large ships at 15 miles and a submarine periscope at 5 miles. About 1,000 of these sets were built. ASG was designated AN/APS-2 and commonly called "George"; some 5,000 of these were built and found to be very effective in submarine detection. A compact version of the SG for PT boats was designated the SO. These were introduced in 1942. Other variants were the SF, a set for lighter warships, the SH for large merchant vessels, and the SE and SL, for other smaller ships. The Navy also adopted versions of the Army's SCR-584 (without the M-9 unit but with gyro-stabilizers) for shipboard search radars, the SM for fleet carriers and the SP for escort carriers. None of these were produced in large quantities, but were highly useful in operations. The BTL developed the SJ, an S-Band supplement for the SD meter-wave radar on submarines. The antenna for the SJ could sweep the horizon to about 6 miles with good accuracy. Late in the war, the improved SV increased detection ranges to 30 miles. L-Band Airborne Early-Warning The most ambitious, long-term effort of the Rad Lab was Project Cadillac, the first airborne early-warning radar system. Led by Jerome Wiesner, about 20 percent of Rad Lab staff would ultimately be involved. Designated AN/APS-20, this 20 cm (1.5 GHz), 1 MW radar weighed 2,300 pounds including an 8-foot radome enclosing a spinning parabolic antenna. Carried by a TBF Avenger carrier-based aircraft, it could detect large aircraft at ranges up to 100 miles. The airborne radar system included a television camera to pick up the PPI display, and a VHF link transmitted the image back to the Combat Information Center on the host carrier. The system was first flown in August 1944 and went into service the following March. This was the foundation of the post-war Airborne Warning and Control System (AWACS) concept. X-Band In 1941, Luis Alvarez invented a phased array antenna having excellent radiation characteristics. When the 3 cm magnetron was developed, the Alvarez antenna was used in a number of X-Band radars. The Eagle, later designated AN/APQ-7, provided a map-like image of the ground some 170 miles along the forward path of a bomber. About 1,600 Eagle sets were built and used by the Army Air Forces primarily over Japan. The same technology was used in the ASD