 The Historical Origins of Spread-Spectrum Communications Figure 2.1. Gustav Guanella, Swiss pioneer of noise-modulated radar and speech privacy systems. (Photo courtesy of I. Wigdorovits.) signal (see Figure 2.2(c)). Delay matching errors are detected by cross cor- relating the internally delayed signal with a 90 degree phase-shifted (across the whole transmission band) version of the received signal. Thus, if the trans- mitted signal is of the form the propagation delay is Tp, and the internal delay is Ti, then the measured error is proportional to This ensemble of phase locked loops, all rolled up into one neat package, possesses a tracking-loop S-curve which looks like the Hilbert transform of the transmitted signal's autocorrelation function. Undoubtedly, Guanella's patent contains possibly the earliest description of a delay-locked loop. Guanella used the same type of error-sensing concept in an earlier patent filed in 1936 [14]. Many of his inventions are cited as prior art in later patents. In addition to accurate range measurement, the noise-radar patent indi- cates improved performance against interference. Guanella evidently did not pursue these intriguing claims, but instead turned his innovative talents to the field of speech scrambling. Emerging Concepts H (-2) (T1-82) (81-22) 10000 AMPLIFIER arine Figure 2.2. Guanella's noise radar patent [13] (redrawn). Part (c) shows a noise source (G1), recording media (Z3) for mechanizing the adjustable internal delay, fil- ters (P1 and P2) whose design controls the measurement characteristic read on meter J. Parts (a) and (b) show two possible characteristics as functions of the time delay mismatch T1 - T2. The characteristic of part (b) was used later in the disclosure to describe delay-lock feedback control of the internal delay. 2.1.2 Developments in Communication Theory In 1915, E. T. Whittaker concluded his search for a distinctive function among the set of functions, all of which take on the same specified values at regu- larly spaced points along the re