milies are not pure, there being a great deal of information exchange at the conceptual level despite the secrecy under which these systems were developed. Approximate SS system time lines for several of the research groups tracked here are shown in Figure 2.9. Since the SS concept was devel- oped gradually during the same period that Shannon's work on information theory became appreciated, J. R. Pierce's commentary [96] on the times should be borne in mind: It is hard to picture the world before Shannon as it seemed to those who lived in it. In the face of publications now known and what we now read into them, it is difficult to receover innocence, ignorance, and lack of understanding. It is easy to read into earlier work a generality that came only later. 2.2.1 WHYN Many of the roots of SS system work in the U.S.A. can be traced back to the pioneering of FM radar by Major Edwin Armstrong during the early phases of World War II. The Armstrong technique involved transmitting a sinu- soidally modulated wide-band FM signal, and then heterodyne-mixing the return from the target with the transmitted signal at a frequency offset. When the frequency of the modulation was properly adjusted SO that the round- trip propagation delay corresponded to one modulation period, the output of the mixer was very narrow-band and the one-period difference between the transmitted modulation and that of the replica then gave a measure of the two-way propagation delay to the target. Certainly, this created a band- width expansion and compression methodology, primitive though it was, SYLVANIA AN/ARW-4 HUSH-UP BLADES ARC-50 REX FACSIMILE SYS. NOISE WHEELS NOMAC F9C-A PN SCW CODORAC MAGNAVOX ARC-50 FH 8 TH SS CONCEPTS WIENER SHANNON DS-SS SS SYSTEM DEVELOPMENT THEORY THEORY Figure 2.9. Approximate time lines for the systems and concepts feature in this history. Early Spread-Spectrum Systems since the FM wobbulation was simply a sine wave. Sylvania's Bayside Laboratories on Long Island received the contract in World War II to con- tinue de