rite the characters at the receiver synchronously vary in rate of opera- tion (see Figure 2.5). Thus, attempts to jam the system with periodic sig- nals, which might achieve the "cumulative effect" at the receiver output, will be unsuccessful. In a sequel patent filed six weeks later [69], it is specified that the facsimile pulse modulation should have a low average duty cycle, be characterized by steep wavefronts, and have high peak-to-average power, in order to attain superior protection. This time-wobbling system is obviously an early rela- tive of modern TH-SS systems. Concurrently with these efforts, deRosa COV- ered similar applications in the field of radar by filing what may be the first patent on random jittering of pulse repetition frequencies [70]. Another study of protected communications was launched when ITT submitted Proposal 158A to the NDRC for consideration. Although the original proposal only suggested the use of redundancy in time or frequency as a possible AJ measure, a 1944 report [71] stated with regard to jamming The enemy can be forced to maintain a wide bandwidth if we use a coded fre- quency shifting of our narrower printer bandwidth SO that it might at any time occupy any portion of a wider band. This clear suggestion of FH-SS signaling was not explored further in the last year of the contract. Several different tone signalling arrangements were con- sidered for communication to a printer at rates on the order of one charac- ter per second. Synchronization of these digital signalling formats was accomplished in open-loop fashion using precision tuning forks as reference clocks. These forks are temperature compensated over a wide range and are mounted in a partial vacuum, SO that their rate is not affected by the low barometric pressures encountered at high altitudes. Their accuracy is of the order of one part in a million, SO that once the receiving distributor was phased with the transmitted signal, it remained within operable limits for two hours or more. A differential gear mechanis