ts modulation. Automatic control circuitry generally is included to align the receiver's clocks for proper demodulation of the incoming signal, although some systems have been built in which ultrastable clocks are initially aligned and then are allowed to drift in a free-running mode until communication is concluded. Proper operation of the correlation computing circuits generally requires control of the symbol clock epoch to within a small fraction of the correlation interval's duration T. Similarly, it is necessary to adjust the SS generator clock's ticks to within a small frac- tion of the reciprocal of the SS modulation's short-term bandwidth, i.e., the bandwidth of the energy spectrum of the SS reference waveform within a correlation interval. Section 1.5.3 will indicate that the SS generator clock error for DS and FH systems must be a small fraction of T and Th, respec- tively, to maintain correlator operation at nearly maximum output signal lev- els, as required. Frequency synchronous operation of correlation detectors requires that the phase drift between the incoming carrier (excluding SS modulation) and the receiver's local oscillator, over a correlation interval, be a fraction of radian, i.e., the quantity in Figure 1.3 may be assumed nearly constant dur- ing the correlation computation. (Phase synchronism of the local oscillator requires, in addition, that be near zero.) The bandpass correlator is a fre- Energy Gain Calculations for Typical Systems quency synchronous device requiring that the input to its narrowband filter be centered in its passband to an error tolerance of a fraction of the recip- rocal of the correlation time. Output threshold crossing techniques, similar to those used in radar detec- tion, are an alternative to MF output sampling in Figure 1.3(b), and may have higher tolerance to synchronization errors than SR/DS systems. However, any realized tolerance to synchronization errors implies a potential weak- ness to repeater jamming. 1.4 ENERGY GAIN CALCULATIONS FOR TYPICAL SYSTEMS Th