Characterization and concepts of time-frequency dissemination

Fundamental considerations that arise in designing a time or frequency dissemination system, are discussed and some dissemination methods are surveyed. A section on "Signal design for time and frequency dissemination" briefly summarizes radio propagation characteristics, discusses time signal format design, and considers how noise affects time and frequency signals. We point out fundamental techniques of time and frequency dissemination and describe similarities between systems for time dissemination and navigation. The use of synchronous satellite transponders and commercial television systems for time dissemination is emphasized because of their great promise. No attempt is made to cover every existing dissemination system; some systems are treated elsewhere in this Special Issue on Time Frequency. The concluding section gives three categories of users according to their required accuracy and shows typical systems which can provide desired accuracies. Various dissemination techniques are charted and evaluated in terms of salient characteristics such as accuracy, geographical coverage, cost factors, and others. To alleviate an existing frequency spectrum problem, we suggest that designers of communication and navigation systems consider opportunities for including time and frequency dissemination in their systems.