Frequency agility is a capability of electromagnetic transmitters, principally radio and radar but possibly laser, to change their operating frequency in a rapid manner. While radars incorporating both receiver and transmitter may not need to synchronize their frequency changes with other radars, radio networks will fail unless all transmitters and receivers that need to communicate with one another know the frequencies available for use, and the schedule or events that cause a frequency shift.
The simplest application might involve no more than having a set of backup frequencies to use if one or another is jammed or unusable due to natural phenomena. Indeed, there are civilian applications, such as cellular radio that shifts to find a frequency that is not busy, in a frequency division multiple access network.
In an environment where there is a significant concern about electronic intelligence collection, or a threat of tactical electronic warfare, the frequency shifts may need to be very rapid. Rapid frequency shifts require accurate time synchronization, as might be provided by GPS, which, in turn, ties to a pseudo-random number generator that selects the next frequency to be used.