of the Tizard Mission was a major step forward in the evolution of radar in the United States. Although both the NRL and SCL had experimented with 10–cm transmitters, they were stymied by insufficient transmitter power. The cavity magnetron was the answer the U.S. was looking for, and it led to the creation of the MIT Radiation Laboratory (Rad Lab). Before the end of 1940, the Rad Lab was started at MIT, and subsequently almost all radar development in the U.S. was in centimeter-wavelength systems. MIT employed almost 4,000 people at its peak during World War II. Two other organisations were notable. As the Rad Lab began operations at MIT, a companion group, called the Radio Research Laboratory (RRL), was established at nearby Harvard University. Headed by Frederick Terman, this concentrated on electronic countermeasures to radar. Another organization was the Combined Research Group (CRG) housed at the NRL. This involved American, British, and Canadian teams charged with developing Identification Friend or Foe (IFF) systems used with radars, vital in preventing friendly fire accidents. Metric-Wavelength After trials, the original XAF was improved and designated CXAM; these 200-MHz (1.5-m), 15-kW sets went into limited production with first deliveries in May 1940. The CXAM was refined into the SK early-warning radar, with deliveries starting in late 1941. This 200-MHz (1.5-m) system used a "flying bedspring" antenna and had a PPI. With 200-kW peak-power output, it could detect aircraft at ranges up to 100 miles, and ships at 30 miles. The SK remained the standard early-warning radar for large U.S. vessels throughout the war. Derivatives for smaller vessels were SA and SC. About 500 sets of all versions were built. The related SD was a 114-MHz (2.63-m) set designed by the NRL for use on submarines; with a periscope-like antenna mount, it gave early warning but no directional information. The BTL developed a 500-MHz (0.6-m) fire-control radar designated FA (later, Mark 1). A few went into service in mid-1940, but with only 2-kW power, they were soon replaced. Even before the SCR-268 went into service, Harold Zahl was working at the SCL in developing a better system. The SCR-270 was the mobile version, and the SCR-271 a fixed version. Operating at 106 MHz (2.83 m) with 100 kW pulsed power, these had a range up to 240 miles and began service entry in late 1940. On 7 December 1941 an SCR-270 at Oahu in