successor units were built in the USSR. The RUS-2 was sponsored by the PVO and intended for early warning. The GAU still wanted a gun-laying system capable of supporting the anti-aircraft batteries. Upon arriving in Moscow, the radio-location group of the NII-9 continued working for the PVO on this problem, returning to Burya, the experimental microwave set built earlier. Within a few weeks, a team led by Mikhail L. Sliozberg and with the cooperation of NII-20, developed a bi-static CW set designated SON (acronym for Stancyja Orudijnoi Navodki — Gun Laying Station) using a 15-cm (2.0-GHz) magnetron. In early October, the experimental Son set was tested in combat by an anti-aircraft battalion near Moscow. The performance of the radio-based Son was poor as compared with that of the existing optics-based Puazo-3, a stereoscopic range-finder that Oshchepkov had earlier improved. The project was discontinued, and no further attempts were made to use magnetrons in radio-location sets. After this failure, NII-9 was sent elsewhere and was no longer involved in radio-location activities. A portion of the radio-location group, including Sliozberg, remained in Moscow working for NII-20. Shortly after Germany invaded the USSR, a delegation of Soviet military officers visited Great Britain seeking assistance in defense hardware. From their intelligence sources, the Soviets were aware of Britain's gun-laying RDF (Range and Direction Finding) system, the GL Mk II, and asked for this equipment to be tested in the defense of Moscow. In early January 1942, Winston Churchill agreed to send one of these systems to Russia, but with the provision that it would be totally secured under British officers and operated by British technicians. When the ship carrying the equipment arrived at Murmansk, a seaport off the Bering Sea above the Arctic Circle, there was a winter storm and unloading had to wait overnight. The next morning, it was found that the entire GL Mk II system – mounted on three trucks – had disappeared. The British Embassy made an immediate protest, and after several days the officers were informed that the equipment had been taken to Moscow for security. It indeed had gone to Moscow – directly to NII-20 and Factory 339, where intelligence experts gave it a total examination and Sliozberg led a team in quickly reverse-engineering the hardware. In mid-February, the NII-20 announced that it had developed a new radio-location system designated Son-2a. It was essentially a direct copy of