There are many practical systems in existence that, provide continuous estimation of radar signal parameters. A large number of these belong to a class which performs this function by measuring variations in the energy of the demodulated received signal. A general noise fluctuation analysis of this class of signal parameter estimators is presented. The general results are applied to three systems that estimate the propagation delay of a radar signal. Specifically, the three systems represent existing radar altimeter concepts. Comparative noise fluctuation results are plotted for the three systems as a function of aircraft ground speed, altitude and other relevant system parameters.