ING FILTER SPACE SIGNAL DELAY LINE Figure 2.24(a). This two-delay line version of Rake shows how signals arriving via different path delays are recombined for MARK and SPACE correlation detection. In practice, a single delay line configuration was adopted. REFERENCE OUTPUT 435 kcps 9 kcps MULTIPLIER 20 kcps + NOISE MULTIPLIER 2 kcps 9 kcps 9-kcps CRYSTAL OSCILLATOR SIGNAL (common to all taps) II kcps 11 kcps RECEIVED PROPORTIONAL + NOISE SIGNAL ADDER CRYSTAL MIXER 455 kcps 11 kcps FILTER COMMUTATOR Wmin = 1/10 cps MULTIPATH OBSERVATION 2 kcps 9 kcps MULTIPLIER MULTIPLIER 20 kcps + NOISE SPACE SPACE REFERENCE OUTPUT 435 kcps 9 kcps Figure 2.24(b). The tap unit diagrammed here includes a long-time-constant crystal filter whose output signal envelope is proportional to the combining weight (aj). This processing corresponds to that shown in the dashed box in (a). Rejection traps to eliminate undesirable cross prod- ucts are shown by X's. Early Spread-Spectrum Systems Figure 2.24(c). This Rake rack contains thirty tap units, two helical magnetostric- tive delay lines, and a commutator chassis. (Diagrams redrawn from [155], photo cour- tesy of M.I.T. Lincoln Laboratory.) uncertainty [148]. (Readers of this early literature should note that to pre- vent disclosure of the actual F9C SS signal structure, all unclassified discus- sions of Rake, e.g., [156], invoked m-sequences for signal spreading. In addition, mod-clock MARK-SACE modulation was never mentioned in this open literature.) The F9C-A production contract was let to Sylvania Electronic Defense Laboratory (EDL) at Mountain View, CA, in 1955, with Judd Schulke act- ing as Project Engineer for the Signal Corps, and Bob Berg as Lincoln Lab's representative, resident at EDL. By December 1956, the first training man- uals had been published [159]. Originally 16 F9C-A transmitter-receiver pairs were scheduled to be made, but funds ran out after production of only six pairs. The first installation was made for Washington, DC, near Woodbridge, VA/La Plata, MD. Worldwide 