, while correlation at IF à la Basore, with the correlator output being the envelope of the bandpass- filtered IF signal, would require SS carrier sync error to be bounded by the reciprocal of the SS carrier bandwidth. Green then designed and built (see Figure 2.20) a digitally controlled SS carrier generator in which five stagger-tuned resonant circuits were shock- excited by pseudorandom impulse sequences which in turn were generated from fifteen stored binary sequences of lengths 127, 128, and 129 (see Figure 2.21). The resultant signal had a long period and noise-like qualities in both the time and frequency domains, yet was storable and reproducible at an electronically controlled rate at both ends of a communication link. The propposed SS carrier synchronization procedure at the receiver was quite similar to then contemporary tracking-radar practice, progressing through search, acquisition, and track modes with no change in signal structure. Tracking error was sensed by differencing correlator outputs for slightly dif- ferent values of clock oscillator phase. Based on Green's results which indi- cated that an SR system was feasible, and on jamming tests which confirmed TR system vulnerability [140], Group 34 resources were turned toward pro- totyping an SR system. This marked the end of TR system research at Lincoln Laboratory. The Historical Origins of Spread-Spectrum Communications Figure 2.20. These two racks of equipment constitute the transmitter and receiver used to carry out the experimental portion of Paul Green's secret Sc.D. dissertation. The SS carrier generators occupy the upper half of each rack, with the plug boards allowing the operator to change the structure of the 15 stored binary sequences. Later in the F9C system, these plug boards were replaced by punched card readers. (Photo from [139], courtesy of M.I.T. Lincoln Laboratory.) 2.2.8 F9C-A/Rake The prototype SR-NOMAC system developed for the Army Signal Corps by Lincoln Laboratory was called the F9C. Its evolution to a final deployed con