In late 1937, Maurice Elie at SFR developed a means of pulse-modulating transmitter tubes. This led to a new 16-cm system with a peak power near 500 W and a pulse width of 6 μs. French and U.S. patents were filed in December 1939. The system was planned to be sea-tested aboard the "Normandie", but this was cancelled at the outbreak of war. At the same time, Pierre David at the "Laboratoire National de Radioélectricité" (National Laboratory of Radioelectricity, LNR) experimented with reflected radio signals at about a meter wavelength. Starting in 1931, he observed that aircraft caused interference to the signals. The LNR then initiated research on a detection technique called "barrage électromagnétique" (electromagnetic curtain). While this could indicate the general location of penetration, precise determination of direction and speed was not possible. In 1936, the "Défense Aérienne du Territoire" (Defence of Air Territory), ran tests on David's electromagnetic curtain. In the tests, the system detected most of the entering aircraft, but too many were missed. As the war grew closer, the need for an aircraft detection was critical. David realized the advantages of a pulsed system, and in October 1938 he designed a 50 MHz, pulse-modulated system with a peak-pulse power of 12 kW. This was built by the firm SADIR. France declared war on Germany on September 1, 1939, and there was a great need for an early-warning detection system. The SADIR system was taken to near Toulon, and detected and measured the range of invading aircraft as far as . The SFR pulsed system was set up near Paris where it detected aircraft at ranges up to . However, the German advance was overwhelming and emergency measures had to be taken; it was too late for France to develop radars alone and it was decided that her breakthroughs would be shared with her allies. In mid-1940, Maurice Ponte, from the laboratories of CSF in Paris, presented a cavity magnetron designed by Henri Gutton at SFR (see above) to the GEC laboratories at Wembley, Britain. This magnetron was designed for pulsed operation at a wavelength of 16 cm. Unlike other magnetron designs to that day, such as the Boots and Randall magnetron (see British contributions above), this tube used an oxide-coated cathode with a peak power output of 1 kW, demonstrating that oxide cathodes were the solution for producing high-power pulses at short wavelengths, a problem which had eluded British and American researchers for years.