Frequency coupling is a method for air-ground voice communication in which more than one frequency is used but stations using one of the frequencies can receive messages sent on the other(s).
Sometimes it is necessary for a single ATS sector or unit to use more than one frequency simultaneously. Examples of such situations include:
Temporary collapsed area control sectors due to sector closure. Generally, when a sector is closed and its airspace is absorbed by another sector, the adjacent sectors and units are informed, that ATS will be provided on a new frequency.
Temporary combining of a tower and approach unit during less busy hours.
Provision of service to aircraft with different communication capabilities. While 8.33 kHz communication channel spacing is a standard for ATS communications there are still aircraft that are not equipped with such radios (e.g. some general aviation aircraft which can only use 25 kHz channel spacing and some state aircraft that are only UHF equipped).
The benefit of using two frequencies simultaneously is that there is no need to instruct pilots to make frequency changes in the above situations. This reduces communicaiton workload. Additionally, the risk of loss of communication is reduced as frequency changes are known to be a major contributor to such occurrences.
There are two basic methods to use multiple frequencies simultaneously:
Separate use. The controller simply swithces two (or more) frequencies on and their transmissions are received by all aircraft than are on any of these frequencies. When a pilot transmits, the controller receives the message (and can determine on which frequency it was sent). Aircraft on the same frequency will also receive the message. However, pilots using the other frequency will not which would:
Increase the likelihood of overlapping transmissions (as pilots on one of the frequencies have no way of knowing whether someone is transmitting on the other one).
Frequency coupling. The controller swithches the two (or more) frequencies and performs the "coupling" function of the VCS. If a message is received on one of the frequencies, it is automatically re-transmitted on the other(s) by the ground station. Thus, pilots are able to hear all communication as well and the downsides of the previous method are eliminated. This function is available in most modern VCSs.