lation techniques (for resolution), occurred to me, and the WHYN concept was born. At least two classes of navigation systems were studied, the first being a circular-navigation, two-ground station system in which the range to each sta- tion was determined separately. The second technique measured the rela- tive delay between two identically modulated ground station transmissions (see Figure 2.10). Measured accurately, this information located the receiver on a hyperbolic curve. The acronym WHYN, coined by Harvey for this lat- ter system, stands for Wobbulated HYperbolic Navigation. From the receiver's viewpoint, the WHYN system might be considered a primitive form of TR-FM-SS communication, with information contained in the rel- ative signal delays. The Historical Origins of Spread-Spectrum Communications MULT. FREQ. Fc1+AFc1 DELAY MIXER CHANGER (ADJ.) MULT. NARROW FILTER (ADJ.) UTILI- TRANS- ZATION MASTER MITTER RECEIVER DIVIDERS AND n MULTIPLIER m TRANS- MITTER SLAVE HETERO- OSCILLATOR Fcz +0FC2 Figure 2.10. Harvey's WHYN system concept is shown here with a plane flying along a hyperbolic curve. Knowing the appropriate AT1 setting for its destination, the aircraft could employ this curve-following tactic to navigate, using only two ground-station transmissions. A third station would be required to provide unam- biguous location information. Note that the mixer-i.f. amplifier circuitry acts as the correlating mechanism in the aircraft's receiver. (Diagram abstracted from [107].) Early Spread-Spectrum Systems The Bayside engineering team, headed by Harvey, Walter Serniuk, and Meyer Leifer, and joined in 1947 by Nathan Marchand, felt that an FM sig- nal with a more complex modulation than Armstrong's would satisfy requirements. The concept was bench tested via analog simulation with per- fect guidance signal synchronization being wired in. Using multiple tone modulation under a maximum frequency deviation constraint of 10 kHz, no simple multitone FM modulation satisfying the contractual constraints could be found