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If you have experience of getting flight following or flying instrument in the US, you might have noticed that approach and center controllers alternate between two or three frequencies for the same control area. For example, I get 118.1 from the tower for flight following one day, and the next day I get 126.0 for flight following for exactly the same route of cross country.

Why don't they just use one frequency for the same control area? Wouldn't that make their job much easier?

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    $\begingroup$ Might be dependent on atmospheric conditions as certain frequencies will work better in certain conditions. $\endgroup$ – SMS von der Tann Aug 9 '17 at 23:32
  • $\begingroup$ @SMS von der Tann Never thought about it. Their frequencies are all VHF, and different atmospheric conditions still make a difference in term of the quality if transmissions? $\endgroup$ – lemonincider Aug 9 '17 at 23:34
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    $\begingroup$ There may be one controller working several areas one day and separate controllers on busier days, hence more frequencies needed $\endgroup$ – TomMcW Aug 10 '17 at 0:52
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    $\begingroup$ What happens when there are a lot of aircraft in the area, and several want/need to communicate at the same time? $\endgroup$ – jamesqf Aug 10 '17 at 5:46
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    $\begingroup$ @SMSvonderTann The difference between 118.1 MHz and 126.0 MHz is sufficiently small that atmospheric conditions should not cause a major difference in reception given identical equipment over identical paths, and VHF is almost exclusively line of sight. You might expect to experience differences with changes of frequency of several tens of percent or more, but not normally with a <7% difference. The major exception to this would be if the chosen frequency happens to be right in the vicinity of an absorption band in the atmosphere, but I'm quite certain there is no such near the VHF air band. $\endgroup$ – a CVn Aug 10 '17 at 9:36
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This situation is due to usually 2+ sectors being worked. Each sector will use its own frequency when split. When these sectors get combined, one of 2 things will happen. 1) The controller will just work with both sectors frequencies keyed up and be listening on both and transmitting on both and have everyone go to the approrpriate frequency for the sector they're in or near. 2) The controller wants everyone on one frequency and has everyone going into those sector's that they are working to be on just one frequency. Frequency congestion and capability(if the transmitters/receivers are different locations for the different frequencies) are also factors.

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