I live close to the edge of the ARTCC region covering my area. There is no way I could talk on VHF to the control center at normal GA altitudes; it might be possible from airliner or bizjet cruise altitudes, but I'm not sure even in that case. How do pilots who are outside radio range talk to ARTCC?
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$\begingroup$ So their radio has coverage of at least their entire geographic area? $\endgroup$– SomeoneMay 3 at 21:06
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$\begingroup$ I can't talk on VHF ham radio to some 1/10th of the distance to ARTCC. $\endgroup$– SomeoneMay 3 at 21:06
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2$\begingroup$ "How do pilots..." implies that aircrew have some ability to influence effective range. Maybe better to rephrase to ask how the network of transmitters ensures full coverage in all sectors of ARTCC responsibility. $\endgroup$– Michael HallMay 3 at 22:59
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$\begingroup$ and your HAM radio may not even be capable of tuning the aviation band frequencies. ATC and aircrew don't generally like kids on the ground busting in and starting to chat... $\endgroup$– jwentingMay 4 at 6:30
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$\begingroup$ Also, ham radio is ground-to-ground communication (usually). Aviation comms are air-to-ground, which increases the effective range dramatically. $\endgroup$– randomheadMay 4 at 13:21
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2 Answers
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In an ARTCC, there are multiple sectors controlling their own unique slice of airspace. Each sector will have at least one frequency. If the ARTCC has 40 sectors, it will have at least 40 individual frequencies. When the controller transmits, his/her voice is digitized and sent over data lines to a Remote Communications Air to Ground (RCAG) site (commonly called a rag site) that is located in or near the overlying sector's airspace. When pilots transmit to ATC, the process is reversed, resulting in an analog voice in the controller's headset. All ARTCC frequencies go through a rigorous analysis to ensure the frequency coverage matches the sector's lateral and vertical limits.
If all sectors in an ARTCC are open, the frequency or frequencies assigned to a sector are used only for aircraft in that sector. As traffic volume is reduced, sectors can be combined so that one controller is using two or more frequencies. Aircraft will hear the transmissions of the controller to aircraft in a different sector, but will not hear the responding transmission from the aircraft since it is on a different frequency. On the midnight shift, it is not uncommon to have 3 or more sectors combined, along with their assigned frequencies.
Although every sector will have at least one dedicated frequency, the voice switch allows assigning different frequencies to sectors that would not normally use them. Usually this is done in response to a frequency outage. There are also discrete frequencies used for military operations in special use airspace. Additionally, all sectors have access to guard frequency (121.5/243.0) to communicate with aircraft that are in distress or have lost radio contact.
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$\begingroup$ A chum who used to fly across the border into the US from Canada (Cornwall Ont, near Massena NY) told me he was required to contact Boston Center before flying across, and to do that he had to climb several thousand feet to get LOS. Maybe ten years ago. Dunno how it works now. $\endgroup$– John KMay 4 at 3:33
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$\begingroup$ Remote Communications Air to Ground (RCAG) site that is located in or near the overlying sector's airspace — copying this for emphasis @Someone. The actual radio antennas are geographically diverse and may be nowhere near the physical ARTCC building. It may in fact be true that even an airliner on one side on the side of the Center's airspace would be unable to communicate on a frequency used on the other side of the airspace. $\endgroup$ May 4 at 13:24
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$\begingroup$ Most ARTCC RCAG sites are co-located with other FAA equipment, such as VORs, radar sites, and air traffic control facilities. This allows for maintenance personnel to be able to service multiple pieces of equipment without having to travel as much. $\endgroup$ May 4 at 16:53
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$\begingroup$ And to add some context to @RetiredATC 's excellent answer, a typical ARTCC will have 50 to more than 100 of these remote transceivers. The number of transceivers needed for a geographic area is (a) the nature of the terrain and geographic size of the center's airspace, and (b) the traffic density. You can only support so many concurrent conversations on a single frequency, so transceivers can be spaced more closely together in areas traffic volume dictates more frequencies be available. $\endgroup$– Max RMay 5 at 4:13
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ARTCCs don't have just one transmitter. Each one has multiple stations, connected to the central hub via wire, microwave links, fiber optics, etc. This is also why each center has multiple frequencies, because adjacent stations can't be on the same frequency or there will be interference, even if both stations are transmitting the exact same thing.
answered May 3 at 21:13
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$\begingroup$ That makes sense! Is the same thing transmitted over all of the stations, or are they separately controlled? $\endgroup$– SomeoneMay 3 at 21:15
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1$\begingroup$ @Someone In general, it's usually the case that the same thing is transmitted over all stations. I'm sure it's possible to isolate one station and use it individually, but most controllers usually don't bother. $\endgroup$ May 3 at 21:18
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1$\begingroup$ @HiddenWindshield This would almost never be the case. A center could easily be managing hundreds, and quite often thousands of simultaneously flights, with easily 50 or more controller stations operating 50 or more sectors through 100-200 distinct transmitters. Any given transmitter has one and only one controller talking on it, and only aircraft in that controller's sector under that individual controller's supervision would be on that frequency. It would be mathematically impossible to manage air traffic if transmissions for 500-1000 aircraft were "simulcast" on every frequency. $\endgroup$– Max RMay 5 at 4:03