Path delay, its variations, and some implications for the field use of precise frequency standards

In order to assess the practical utility of high-performance frequency standards in field applications, the effect of the propagation medium must be taken into account. After a brief summary of noise processes, a discussion of the performance of current frequency standards is given, together with an estimate of the relative cost, reliability, and necessary system support for each type. Literature references on atmospheric physics are given to allow determinations of those limitations which the earth´s atmosphere and ionosphere impose on the transfer of precise frequency and time. Formulas are presented to estimate the effects of changes in temperature, pressure, humidity, and frequency on the propagation path of electromagnetic waves. Current methods used to compute the path delay for HF, VLF, Loran-C, and other precise frequency transmissions are given. Based on this material, we outline needs for future frequency control capabilities.