a system of modulation and demodulation for the transmission of binary data in the presence of multipath . the binary data modulates the phase of the carrier and the same binary data , after a delay of &# 34 ; δ &# 34 ; bits , also modulates the frequency of the carrier . following reception , the binary data is obtained either by frequency demodulation or by a method of coherent phase shift demodulation wherein the mark or the space frequency is selected for detection of each bit in accord with the bit received &# 34 ; n &# 34 ; bits previously . the system selects between the demodulation methods to obtain one which operates satisfactorily in the instant electromagnetic environment .