Page "government" Paragraph 616

The same restrictions apply after local sunset in the case of class 3, stations operating on regional channels, which after that time are required to operate under nighttime restrictions in order to protect each other.

With respect to nighttime assignments, the degree of skywave service and interference is determined by skywave curves ( figs. 1 and 2 of sec. 3.190 of the rules ) giving average skywave values.

These curves were derived by an analysis of extensive skywave measurement data.

It was recognized that skywave signals, because of their reflected nature, are of great variability and subject to wide fluctuations in strength.

For this reason, the more uncertain skywave service was denominated `` secondary '' in our rules, as compared to the steadier, more reliable groundwave `` primary service '', and, for both skywave service and skywave interference, signal strength is expressed in terms of percentage of time a particular signal-intensity level is exceeded -- 50 percent of the time for skywave service, 10 percent of the time for skywave interference.