ZOB Hi alt freqs.

xms3200

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Anyone out there know what XC & SC & asterix next to the yellow highlighted frequencies on the ZOB chart mean...just curious and dying to know.

Thanks for all replies!!!
 

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alphazulu

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XC is probably "cross coupled"
Weak explanation, but as I understand something with ARTCC controllers and the use of their sector frequencies
I am sure someone will jump in with (much) better information
 

ATCTech

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Cross-coupling has been around in our systems since the 1980s. It allows multiple, ATC-selected combinations of frequencies in that specialty to "rebroadcast" to the other coupled frequencies assigned to the specialty so aircraft that need to be on geographically distanced PALS (RCAGs in FAA-speak) hear one another while under the jurisdiction of a common controller position. As you might guess in a country the size of Canada we have some very large sectors. During low-traffic periods when sectors combine it's still necessary for aircraft to use those remote comm sites because range wouldn't allow reliable signals to more distant facilities. With cross-coupling the controller transmits on all the frequencies he's assigned and when the designated aircraft responds, all other aircraft on the coupled frequencies hear the reply. This prevent "stepped on" problems and for all traffic to know the controller is indeed talking to multiple aircraft spread across a wide area rather than wondering why they're only hearing one side of any given exchange. It works very well. Our tower systems even have that functionality Air/Ground/Clearance Delivery positions can all combine to one controller overnight and aircraft, tugs and airside vehicles don't have to change frequencies based on time of day or position on the airfield.

My avatar is a shot of a Canadian ATC voice switch panel I posted here many years ago to explain with actual photos what the controller does and sees to enable coupling.
 
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AirScan

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Anyone out there know what XC & SC & asterix next to the yellow highlighted frequencies on the ZOB chart mean

Yeah that's my chart I put together, I should have included a legend on what my various notes mean. The XC does mean frequency "cross-coupling". In this case 132.825 has been observed cross-coupled with 125.075 when the two sectors are combined, ZID77 and ZID87.

Just to add to ATCTech's great explanation, my understanding is in the US they only have the capability to cross-couple two frequencies together, unlike Canada where multiple frequencies can be coupled. It also seems to be a controller option, some use it while other don't (for some reason).

I wonder if "SC" means simulcast, meaning same controller but no coupling.

That's exactly what it means. The * beside 135.375 indicates it's only used when the sector is open or as required. So in this case when ZOB46 is combined into ZOB45, 120.325 covers both (for aircraft) but the controller can still be heard "simulcasting" on 135.375. The chart is a bit dated so it's possible the two frequencies can now be cross-coupled which would make the SC redundant. I just have not been able to confirm it yet ?

BTW in the sector adjacent to the east, ZOB58, the R beside the box indicates the frequency is rarely used but the controller can still be heard simulcasting (SC) on 121.075. The arrow pointing up beside ZOB59 means 119.725 is used to cover the sector above.

The intent of the chart is to show typical configurations during the day shift. The chart boundaries are based on official sector charts. The frequencies and notes are based on observations from various sources so may not be up to date or cover all possible configurations. As this information is always changing any updates heard or feedback on accuracy always appreciated. When I have time I'll post an updated chart with a legend explaining the notes.

AS
 
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spanky15805

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Airscan, are you saying that the two different frequencies are using the "system" (rag-artcc-rag) as a repeater in the cross coupling mode?
 

AirScan

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Airscan, are you saying that the two different frequencies are using the "system" (rag-artcc-rag) as a repeater in the cross coupling mode?
Not really the same thing, but my knowledge of "repeaters" is limited. That's a question for ATCTech.
 

alcahuete

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It's kinda silly to note simulcasting, because frequencies are ALWAYS simulcast when sectors are combined. You don't turn frequencies off, because you want to be able to hear aircraft calling on every frequency you are operating. Some pilots look at charts to get the frequencies, some (particularly VFR) go off of memory or apps, etc. Not only is it something you don't want to turn off, most (if not all) FSOPs mandate leaving it on. So SC should be on every frequency on the chart. ;)

Airscan, are you saying that the two different frequencies are using the "system" (rag-artcc-rag) as a repeater in the cross coupling mode?
That's exactly what happens. If you are working combined, for example, and using 121.000 and 121.100. Aircraft transmitting on 121.000 would be heard on 121.100 and vice versa. ATC would transmit on both. It's a fantastic feature.


Just to add to ATCTech's great explanation, my understanding is in the US they only have the capability to cross-couple two frequencies together, unlike Canada where multiple frequencies can be coupled. It also seems to be a controller option, some use it while other don't (for some reason).
More than two frequencies can be cross-coupled. The limitation is that frequencies with multiple RCAG sites cannot be cross-coupled. I have no idea as to the technical reason behind it, only that it can't be done. That's the main reason you don't see cross coupling used very often at the ARTCC level in the US. And yes, it is selectable by the controller.
 

ATCTech

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I'm not really certain what the question about acting as a "repeater" specifically refers to. In a positive control ATC environment, there's no requirement for aircraft to communicate with each other on ATC frequencies. Cross-coupling does in effect create a repeater though, so you are correct in calling that but it's not by design for air to air use. The whole concept of "RRTU" as we originally called it was designed by my collegues in Quebec decades ago as they have a few huge sectors where multiple remote sites were required to cover a single sector's airspace. ATC was tired of aircraft stepping on each other because they didn't know someone else was already talking to the controller on a different frequency so our Engineering folks devised an external box that all the ACC (ARTCC, sorry guys!) frequencies could be routed through and combinations of coupling could be configured by the technical group at ATC's request. That morphed into a software function of the newer voice switch itself, allow virtually endless combinations to be created by the controller right at his/her touch screen based on what areas he/she was working and what the traffic load would balance out best with between remote frequencies.

There's another aspect to this I'll point out. Think about hand-offs between sectors and even more between ARTCCs. Controllers become very habitual in procedures. So, when ARTCC "X" sector "1" hands flight off to ARTCC "Y" sector "9" it's instinctive to always say "American 1, contact "Y" center now on 123.575" because it's "always" 123.575 the hand-off goes to. Let's say traffic is low at ARTCC "Y" so they've already consolidated sectors and the controller wants to use frequency 119.900 because of low traffic volume but has deselected 123.575. With cross coupling, he can retain ALL the specialty's frequencies and not care who gets handed off to him on what would be the "usual" frequencies during busier periods as he has 2, 3 or more frequencies combined as if they were 1 as since everybody is hearing everybody else.
 
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ATCTech

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Reference multiple RCAG sites not able to be crossed-coupled, I can think of one possible technical reason that our system already takes into account. I won't go into the entire description here but it has to do with another discussion in this forum about "PTT delay" between different RCAG sites. Think of it this way. Retransmit (coupling) can only be allowed to happen when an aircraft transmits, obviously. However, remember that when ATC transmits that signal too is picked up by the remote receiver and the controller's voice audio & the receiver squelch signal come back to the ARTCC switch. If local ATC position PTT is NOT active on that frequency the switch assumes it's an external source causing the squelch to be open (read: aircraft is transmitting) so the switch activates PTT on the OTHER coupled frequencies. When local ATC PTT is active, it doesn't, nor does it need to force PTT on.

When cross-coupling, knowing round trip "local ATC PTT dropped time delay to remote RX squelch signal dropped at the voice switch" is critical to prevent endless-loop TX/RX lockups. Our switch software has a value we test and set on every frequency during routine maintenance that measures and sets a software value to prevent this from happening. We refer to it as "squelch delay" and it prevents the voice switch from assuming a squelch open signal coming from a remote site with NO ATC PTT signal applied at the local voice switch as being the aircraft transmitting until the delay time lapses. A timing diagram makes this easier to envision but it is a critical software setting in the switch. Because each frequency has it's own software value it allows any frequency to be cross-coupled to any other no matter the individual loop delay value. We're talking differences of tens to maybe low hundreds of milliseconds in round-trip delay here just to be clear.
 
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ATCTech

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I had the opportunity to provide a tour to a group of FAA techs at CZYZ back in the 1980s. Even then our 1970s-designed Rockwell-Collins voice switch, which was RF FDM (300 RF channels, SSB, between about 7 and 10.7 MHz) that ran on 75 Ohm coax cable blew their minds. We had a whopping 32kB of memory and 256kB single-sided 12" platter hard drives on DEC pdp-11/35s (fully redundant main/standby systems with dual RF cable plants) and performed initial O/S and database loads from magtape. Later we updated to loading the 11/35s from DEC 386 CPU PCs via serial ports so IPL (initial program load) went from a couple of minutes to maybe 15 seconds, and no tape heads to clean!. The FAA guys couldn't believe we could go from power-on to single-side full ATC voice-comm operation in less than 1 minute and have dual-redundancy back in less than 2 minutes. They were cold start IPL to single system in about half an hour back then.

And to blow our own horns a bit, we were trained to repair every piece of that hardware with the exception of the DEC core memory boards (no IC chip memory!) to component level, plus all of the R-C hardware outside the DEC central equipment. The block diagram on the equipment room wall was about 20' long and as a joke our training school added an infinite-loop "kitchen sink" circuit to it after one of the techs quipped "this system has everything but a kitchen sink" during a training session at our school. On paper it then did until the day it was retired in the early 2000s, kind of a 1980s version of today's software "Easter Eggs".
 
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AirScan

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alcahuete,

It's kinda silly to note simulcasting, because frequencies are ALWAYS simulcast when sectors are combined.[
It's some of the of the ultra-high frequencies that were not being heard when combined with the high sector below, usually ones that were not used very often. Chicago 128.400, Minneapolis 133.075, Boston 118.825, New York 124.775 are some that come to mind. I'll check again when I'm in those areas.

More than two frequencies can be cross-coupled. The limitation is that frequencies with multiple RCAG sites cannot be cross-coupled.
Thanks for the info. My understanding was based on what a New York Center controller had told me, but that was a few years ago. I still have not heard a US controller cross-coupled to more than one. Can you give me a sector that normally does this ?
 

alcahuete

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Thanks for the info. My understanding was based on what a New York Center controller had told me, but that was a few years ago. I still have not heard a US controller cross-coupled to more than one. Can you give me a sector that normally does this ?
I don't have a specific example off the top of my head, but I know when we looked into adding a bunch of cross-coupling at LA Center, we ran into issues because most sectors have multiple radio sites. However, without that limitation, we would have been able to cross-couple entire areas (5-6 sectors), which was the goal.
 

ATCTech

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I don't have a specific example off the top of my head, but I know when we looked into adding a bunch of cross-coupling at LA Center, we ran into issues because most sectors have multiple radio sites. However, without that limitation, we would have been able to cross-couple entire areas (5-6 sectors), which was the goal.
Hi alcahuete,

Just curious, do you mean most sectors operate different frequencies at different sites (very common here too) or are you guys using equipment to operate the same frequency from multiple sites simultaneously? We do the latter around Hudson Bay here in Canada to cover a huge body of water with 3 remote sites triangulated for coverage, all on the same frequency, where all 3 receivers feed to Edmonton ACC (ARTCC) and based on the signal quality of any given aircraft transmission (and I *think* now ADS-B position data) the closest transmitter is automatically used for the reply from ATC. Frequentis told us that it's a popular solution in the mountains of south-central Europe as well.
 

alcahuete

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Hi alcahuete,

Just curious, do you mean most sectors operate different frequencies at different sites (very common here too) or are you guys using equipment to operate the same frequency from multiple sites simultaneously?
Hi,

Same frequency, multiple radio sites. They operate exactly as you mention, with diversity algorithm, where the first signal received is the one it goes with. We have to select the transmitter site manually. A lot of the sectors either cover a large distance, or are in mountainous terrain, so multiple radio sites are needed.
 

spanky15805

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ATCTech. Makes sense, now. I don't foresee the FAA heading in that direction. They have enough trouble with the diversity algorithm, I can't imagine "it" being able to actually select the "correct" rag site, like alcahuete mentioned, "it" selects the first signal to make the trip to the artcc not the most intelligible signal.

alcahuete, does the algorithm "look" at mains, standbys and buec's or mains and standbys?
 

alcahuete

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ATCTech. Makes sense, now. I don't foresee the FAA heading in that direction. They have enough trouble with the diversity algorithm, I can't imagine "it" being able to actually select the "correct" rag site, like alcahuete mentioned, "it" selects the first signal to make the trip to the artcc not the most intelligible signal.
Yeah...not going to happen. Like you say, we have a hard enough time getting it to work as is. Kinda sad. ha ha!

alcahuete, does the algorithm "look" at mains, standbys and buec's or mains and standbys?
It will take whatever you have selected. All mains, all standbys, all BUECs, or a combination thereof. Or you can turn it off entirely and get the wonderful barrel effect (LOL), or run single site, etc.
 
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