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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. $\endgroup$ 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? $\endgroup$ 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. $\endgroup$
    – user
    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 appropriate 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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It is not likely due to meteorological conditions affecting the physics of the radio waves (as others have mentioned the difference between the top and bottom of the aviation band is not very large, and ATC has no easy way of tuning a frequency on the radio).

In the center environment I am not sure why a certain area would be on one frequency one day and another frequency on another day, unless both the main and backup equipment for the usual frequency was inoperative and they were using an entirely separate standby frequency. Some sectors will have these and some might not. Center sectors are very static, and even if two or more sectors are combined controllers will generally have you use the "proper" frequency for your area. This will ensure proper radio reception and make things easier if the sectors need to be decombined.

In the terminal environment, however, which airspace is owned by which controller (departure, initial approach, final approach, etc) can be highly variable depending on runway configuration. At a smaller Class D or class C there might only be one frequency for the entire TRACON, or an east/west or north/south split that doesn't change. But at a Class B TRACON the controller overlying your tiny airport in the morning might be different from the controller overlying it in the evening.

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  • $\begingroup$ For the pain of stepping on / being stepped on aircraft on the "other frequency" that one can't hear, it's the perpetual gripe in the cockpit "why don't they just put everybody on one frequency" - but when the transmitters are far enough apart geographically & not easily re-tuned, I can see how that's not a great option. Didn't consider that the remote sites can't change frequencies the same way we do... it would save a lot of frustration & "everybody stand by..." calls if they could! $\endgroup$
    – Ralph J
    Feb 14 at 4:12
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    $\begingroup$ @Ralph all very true. I don't know if this is the case but I can easily imagine the permanent ATC transmitters and especially antennas being precisely tuned to the proper length for a specific frequency, while aircraft antennas have to work over a broader spectrum and won't be a perfect fraction of a wavelength except for certain frequencies. $\endgroup$
    – randomhead
    Feb 14 at 4:31
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In the Center, time of day makes a difference, and so does traffic flow. With less traffic flow, sectors will be combined, and the high altitude area around BFF will go from 134.57 to 135.02, because it's combined with the next sector to the east, and 135,02 has better coverage over the combined airspace (there's only one main/standby set for 134.57 at Cheyenne (CYS), with 1 BUEC (Back Up Emergency Communications) at Crawford, NE.).

135.02 has the Crawford (QHX) site, a site at O'Neill (ONL), one at Rapid City (RAP),and backups at Ainsworth (ANW) and Sundance (SUC). On the mid, 135.02 is used for the Rapid City, Casper and Crazy Woman areas, as well, where 133.67 would be used during the day. This gives coverage for high altitude aircraft from Rock Springs (OCS) to CZI to RAP and east, down to ONL, to Denver (DEN) (you probably knew that one), and back to OCS.

It's a matter of convenience for both the controller and the pilot. Combined airspace means fewer frequency changes, fewer frequency changes means fewer things that can go awry.

Disclaimer: Frequency antennae, in spite of their identifiers, are not necessarily at or near their associated airports. Most are near associated VORs.

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  • $\begingroup$ Isn’t it possible to couple several frequencies in the states? What you describe as a „matter of convenience“ sounds extremely inconvenient to me because you have to change working frequencies all the time. At our unit we just use all the normal frequencies with combined sectors as well which is in fact the most convenient way: less frequency changes, aircraft are on correct frequencies when opening sectors straight away, no necessity to tell adjacent units your new frequency… $\endgroup$
    – pcfreakxx
    Jul 13 at 10:44
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    $\begingroup$ You only change the working frequency in regards to what you assign the pilot. You still monitor and broadcast on the others. For instance, with the BFF high sector, when it combines into the TDD (Thedford) high sector, the TDD sector has 134.7 selected, and broadcasts on it. The tricky part (it's part of the job) is to know if an aircraft is on 134.7 in a combined situation, and get them switched to 135.02, so they can hear other aircraft. $\endgroup$
    – atc_ceedee
    Jul 13 at 15:56
  • $\begingroup$ When the sectors are decombined (usually during a slow period before the early push comes through) it's a matter of assigning 134.57 to the wstbound aircraft that are in the BFF sector airspace. No need to change the eastbounds, which are flying into the TDD sector. $\endgroup$
    – atc_ceedee
    Jul 13 at 16:06
  • $\begingroup$ Yes I know all of what you mentioned (expect your local UK sectors and freqs ofc) however you didn’t answer my question. Can you couple frequencies or will you just use a single one? You were talking about changing the working frequency while still monitoring and broadcasting on the others. So what’s the whole points of that then when you could just keep all the aircraft on the usual frequency of the sector they enter first? $\endgroup$
    – pcfreakxx
    Jul 13 at 16:30
  • $\begingroup$ @pcfreak, when you say "coupled frequencies," are you talking about a rebroadcast system where, say, a pilot transmitting on 134.57 will also be heard by pilots listening on 135.02? To my knowledge we do not have that capability in the US. $\endgroup$
    – randomhead
    Jul 13 at 17:41

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