Some Relations between Speed of Indication, Bandwidth, and Signal-to-Random-Noise Ratio in Radio Navigation and Direction Finding

Rate of phenomenon change and required speeds of indication are quite slow in many navigational and direction-finding systems, particularly those for long ranges. Therefore, the actual total required electrical bandwidths are also quite narrow, probably never greater than 100 cps or so, and in most cases much less than this. Even with complex wave forms, such small total bandwidths are possible if there can be designed a discontinuous-type bandpass filter having a multiplicity of very narrow pass bands occurring at the steady-state Fourier components of the complex signal; i.e., a "comb" filter. One practical method of producing such a discontinuous pass band is described briefly. In view of the interest in new modulation schemes which give an output signal-to-noise ratio which is better than the input carrier-to-noise ratio, it is pointed out that all such systems have improvement thresholds, and many navigational systems provide satisfactory information at output signal-to-noise ratios lower than these threshold values. When this is the case, single-sideband and doublesideband amplitude modulation produce the most sensitive systems. When postdetection bandwidth is very much narrower than predetection bandwidth, many navigational systems will perform satisfactorily even though the carrier-to-noise ratio at the input to the final detector is appreciably less than unity. When this is so, the phenomenon of "apparent demodulation" is encountered.