er to pro- duce an advance copy of the sequence without waiting a full period. In an effort to completely rule out this possibility, Caltech graduate student Lloyd Welch was hired in 1955 to study the generation of sequences which avoid the cycle-and-add property by resorting to nonlinear recur- sions [171]. Although laboratory system work continued to use linearly generated PN sequence for test purposes, final designs were to be based on nonlinear generators. 2. Initial jamming tests revealed weaknesses in the SCW system when con- fronted by certain narrow-band jammers. Most of these were due to problems in the mechanization of the multiplications required in the PN scrambler and correlator descrambler. For example, if the descrambler effectively mechanizes a multiplication of the jamming signal by a con- stant plus the receiver's PN sequence replica (the constant representing a bias or imbalance in the multiplication/modulation process), then the multiplier output will contain an unmodulated replica of the jamming signal which has a free ride into the narrowband circuitry following the descrambler. The sure cure for this problem is to construct better bal- anced multipliers/modulators since the processing gain achievable in an SS system is limited by the "feedthrough" (or bias) in its SS multipliers. In the mid-1950s, JPL was able to build balanced modulators which would support systems with processing gains near 40 dB. 3. Another major concern of system designers was the decibel range and rates of variation of signal strength, as a result of missile motion and pulsed or intermittent jamming. At the circuit level, the two approaches to controlling signal levels in critical subassemblies were automatic gain control (AGC) and limiting. The AGC approach suffers from the possi- bility that its dynamics may make it susceptible to pulse jamming, while limiters, although instantaneous in nature, generate harmonics which might be exploited by a jammer. The eventual design rule-of-thumb was The Historical Origins of Spr