 June 1944 [82]. ITT also participated in these World War II guidance programs, notably with a system called Rex [63]. One patent, evidently resulting from this work and filed in 1943 by Emile Labin and Donald Grieg [83], is interesting because it suggests CDMA operation in pulse code modulation (PCM) sys- tems by slight changes in the pulse repetition frequency. In addition, the patent notes the jammer's inherent problem of trying to deliver its inter- ference to the victim receiver in synchronism with the transmitted pulse train. However, the notion of multiplicity factor or spectrum spreading is not mentioned. A third guidance system for the control of VB's and GB's was proposed by the Hammond Laboratory. The Hammond system used a complicated modulation format which included a carrier wobbled over 20 kHz to pro- tect against tone interference, and FM control signals amplitude modulated onto this frequency-modulated carrier [63]. More notable in this history than the system itself is the fact that Ellison Purington in 1948 came close to describing a TH and FH carrier for a radio control system in a patent appli- cation [84] (see Figure 2.6). The actual details describe a TH-SS system with control signals coded into the transmission using frequency patterns. Magnetic or optical recording "on a rotating member driven by a constant speed motor" was one suggestion for the storage of different time hopping patterns, while another possibility mentioned involves delay line generation of pulse train patterns. Control keys are hidden in the way that the patterns Emerging Concepts farta f25 , 125 f2, t2 Time p CONSTANT PULSE TYPE SPEED TRANSMITTER MO TOR AMPLIFIER RECORD 13 PULSE TYPE TRANSMITTER RECORD 14 AMPLIFIER RECORD 15 PULSE TYPE AMPLIFIER TRANSMITTER (Z) f3 CONTROL DESIRED Figure 2.6. Purington's time-frequency chart definitely suggests a hybrid TH and FH system. However, the illustrated transmitter uses the assignment of TH patterns to fre- quency tracks as a method for signal encoding. (Redrawn from [84].) The 