Changes in signal strength from one frequency to another

[DCMon]

Heard same thing

I listen to air traffic here in Oregon and notice this change in strength when changing frequencies. Sometimes the signal is much better after the change and sometimes it is so weak I can barely hear the plane. I have noticed this for a long time and thought maybe it had to do with antennas on the planes. Not all planes do it and I have heard it on planes all different directions from my location.
One example a flight was handed off from Seattle Center to Oakland Center. I could barely hear the plane before the hand off and afterwards it was loud and clear. The switch took place about a hundred miles south of me and the plane was flying away from my location. Frequencies 135.150 to 119.975

 

[ATCTech]

Regarding regular ATC TX/RX operation versus cross-coupling, the controller by default hears all traffic on any frequency assigned to the sector and the controller's transmissions are heard on each of those frequencies where he/she has ATC transmit enabled. (You can listen to a frequency without having transmit capability 'on' but you can't transmit without receive enabled.)

Coupling has no impact on that basic function. When frequencies are coupled, any signal received on any coupled frequency is automatically rebroadcast on all other frequencies selected in the coupling scenario. This function requires the controller to have transmit (Push To Talk) enabled for each of those frequencies as well.

What we don't ever want to happen is the controller's transmit signal go out on the air on frequency "A", be received by the ATC receiver for frequency "A" (which it always will as the ATC transmitters and receivers are always close to each other) then sent back to the switch and rebroadcast on frequency "B". The controller's voice on any frequency will always be "live from the headset", never, ever a rebroadcast from a second frequency. Among other potential problems that could cause, the most destructive possibility is when path delay between two remote sites causes an endless PTT loop. Our old system was a bit of a bear for that sometimes. This one is much better as the coupling is controlled internally by the switch and reacts much more quickly and accurately.

Ok, now coupling step by step.

When an aircraft transmits on frequency "A" the switch sees the receive signal coming in and using the software coupling scenario that's been invoked also sees that controller PTT is NOT active on that frequency, meaning this is an external signal source (the aircraft or a vehicle on the airfield) and thus it is determined to be a valid coupling situation. Any *other* frequency selected for coupling at that ATC position/sector has it's PTT automatically applied by the switch and the incoming receive signal from frequency "A" is sent out on the transmit circuits to frequency "B". Now, frequency "B"'s ATC receiver also picks this up and sends it back to the switch, BUT the switch already has PTT applied on frequency "B" while the aircraft continues to transmit on frequency "A". This prevents any possibility of simultaneous coupling "in reverse". Pretty smart, but also very basic logic. You will never hear frequency "A" retransmitted to "B" *then* "B" to "C" etc. The coupled retransmit audio will always be a single "hop" from the incoming source frequency to each coupled frequency in parallel, not in series. It should be obvious now to that when the controller himself transmits the switch knows PTT is applied on all of the assigned frequencies and therefore inhibits coupling of any kind during the ATC transmission.

Of course if an aircraft then talks on frequency "B" after "A" goes quiet the whole situation reverses automatically, meaning the aircraft on "A" now hears the aircraft on "B". (It always works in both directions, *never* just "A" to "B" but not "B" to "A".)

The caveat to this is that a controller may have 5 frequencies assigned to him but only 3 of them are selected for coupling. That will confuse the casual listener as he will hear some traffic being rebroadcast and some not. At the same time, no one position can create/invoke more than one coupling group at any one time, so you can't couple "A" and "B" and separately couple "C", "D" and "E" from the same ATC position.

Does that make it any clearer? It's so second nature to me that I might be unintentionally glossing over some of the intricacies. Just ask if there's anything I can reword for you.

Bob

 

[AirScan]

Bob,

Ok, now coupling step by step.

Thanks for the excellent technical description.

Novascotion,

In the second premise ...This is the premise that I am frankly doubting

If you want to hear what cross-coupling sounds like there is a feed over at LiveATC that covers a Montreal Center high altitude frequency in Timmins, Ontario, 133.975. Depending on the time of day this frequency could be cross-coupled with 6 to 20 frequencies.

Unlike Moncton that vary what frequencies are being cross-coupled, Montreal always seems to have the cross-coupling function enabled with all the frequencies the controller is using.

An aircraft transmission that is being cross-coupled to 133.975 from another frequency has a distinct sound and will always be heard at the same level as the controller. It becomes quite easy to tell what aircraft transmissions you are hearing via cross-coupling vs directly from an aircraft that is on 133.975.

YUL ACC (Noranda Sector) ...

Airport Detail: YUL | LiveATC.net

 

[ATCTech]

Just a quick note on ZUL versus ZQM centers. Moncton (and ZQX Gander) dynamically realign airspace for each sector based on the published North Atlantic preferred routes at least twice per day (westbound influx in the morning and eastbound rush afternoon/evening) to accommodate the traffic volume. They essentially fine-tune the sector widths for the number of parallel tracks they require and subdivide the frequency assignments accordingly. Montreal and west don't tweak the sector's requirements like that but rather combine and collapse sectors into one another as traffic rises and falls while maintaining the same boundaries.

Bob

 

[novascotian]

Thanks to all of you for the suggestions, comments and the great description of cross coupling. If I have this correct, and my local Moncton frequency (133.95) was part of it, I would be hearing traffic on the other frequencies being retransmitted out to the aircraft that are on 133.95. The only thing is, there isn't anyone on 133.95 except the controller, so I assume they could and would not have it coupled, though even in the very few times I have heard aircraft on this frequency I have not heard retransmissions of other aircraft, so either they never couple this site, or else Moncton in general does not use it at all.

 

[ATCTech]

I just spoke to my counterpart at Moncton Center and he has no record of any ongoing problems with 133.950, and as a matter of fact watched it being used while we were chatting. It may never (or at least rarely) be coupled to anything else, but all I can suggest is that you're in range of our ground station but out of range of the aircraft they talk to, despite it being a high-level frequency.

In a word - weird.

Bob

 

[majoco]

To illustrate my question, I will use the Sydney, NS frequencies for Moncton: 132.75, 133.3, 133.7, 125.25, plus the Halifax one on 133.95. Yes I know that Moncton sometimes joins others to the north as well into the group. I find however that in quiet times or depending on the traffic flow, they often do not use 133.3 and 133.7 at all, and actually combine the sectors into just 132.75 along the spine of NS and 125.25 offshore, but that is another story.

Yes, but the previous controller knows the frequencies that are in use and directs our aircraft to use the frequency for those sectors at the changeover - our pilot is doesn't know or care that his sector controller is also controlling aircraft in other sectors (or has combined three sectors) and the controller may be transmitting his signals on three or more frequencies - perhaps there is nobody listening in those sectors - but our aircraft doesn't need to hear the signals from other aircraft in other sectors - they should be well out of our way.
Understandable that in a bit of crowded airspace then it is very helpful to all to be able to hear the other aircraft in your sector - just to say "I see you" when the controller says "you're number three in the sequence" when the other aircraft is coming in from the side. The controller is entitled to get a bit snarky because you're only supposed to be talking to him but it does grease the wheels.
As I said before, I have never heard aircraft signals being retransmitted on other frequencies and I see no need for it.

 

[novascotian]

Re the comments from ATCTech... I am sure it is correct that there are no problems with 133.95 on any technical level.. the signal out seems very good... but there is no way that me not hearing any aircraft on it means they are simply out of range. I not only listen but also watch on planefinder... and whenever the controller speaks to an aircraft I usually look for it on the screen, and I find it... if it is fairly close to me I will be hearing it respond on another of Moncton's frequencies that is joined with 133.95, but again I want to emphasize that the air side is never ever retransmitted on 133.95, at least when I have been listening. If the aircraft is not heard I usually find it is well up north over the Gulf of St. Lawrence or even farther north, or it is well over to the northeast and close to Gander's area. Neither of these areas would be served by a Halifax station. The Halifax site, with its one frequency, looks like it is intended for aircraft that are off the central part of Nova Scotia, well out into the Atlantic, but when I compare with planefinder, I never have encountered any aircraft on VHF in that area that are not on 125.25 or sometimes another Sydney-based frequency, or if a bit westwards, on one of the Yarmouth ones. I can hear traffic at least 200 miles away, and once they are out past that into the Atlantic they are on New York ARINC HF. So sure, the actual 133.95 works okay but the fact remains, no aircraft seems to be on it, yet Moncton transmits on it daily. It is sort of like they forgot about it, and it just sits there simulcasting with 125.25 and others. The sites at the two ends of Nova Scotia seem to give adequate coverage. One thing maybe is traffic that is high level but not very high. like maybe FL300... maybe they would need this site located in between the two others.

But hey, don't tell your buddy that it is on but no one is listening, except me!! I don't want them to turn it off... it is my key to hearing the ground side of things...!!

 

[AirScan]

novascotian,

What about the Moncton low altitude frequency in Halifax on 135.300, do you monitor that one very often ? Last time I was in Halifax I monitored it briefly and noted that it was simulcasting with other frequencies but as far as I can remember I did not hear any cross-coupling on it either.

 

[ATCTech]

I'll have 133.950 NOTAMed out of service so you won't have to wonder any longer... <GRIN>

Bob

 

[novascotian]

Instant anxiety rush to read 133.95 out of service.. before reading it properly.... thanks for the early morning excitement ATCTech!

As for the low level Moncton PAL at Halifax on 135.3, it is on its own through the daytime and always or almost always simulcast, as I no doubt mistakenly call it, with many of the other low level frequencies, and again there is never ever any of this retransmission of the aircraft side of things. For example I often hear the controller coming over 135.3 but he or she is speaking to an aircraft in northern New Brunswick, I guess on 123.7. Too far for me to hear the aircraft on that frequency, and I certainly do not hear it retransmitted on 135.3.

My opinion is, as someone who has listened for years and years to Moncton Centre is that retransmission doesn't happen here at all and for sure (i.e.not just an opinion) not through the two Halifax peripheral sites.

 

[AirScan]

novascotian,

My opinion is, as someone who has listened for years and years to Moncton Centre is that retransmission doesn't happen here at all and for sure (i.e.not just an opinion) not through the two Halifax peripheral sites.

Thanks for the report.

There is a LiveATC feed located in Moncton that is monitoring the low altitude frequency 124.400. At certain times during the day and In the late evening and overnight hours the one controller is working all Monctons low altitude airspace.

Airport Detail: YQM | LiveATC.net

It is clear that 124.400 is cross-coupled with some but not all the other low frequencies. From handoffs I can confirm 124.400 is cross-coupled with 124.300, 135.65 and 123.700 but I have not heard it cross-coupled with 135.300 (when the controller is clearing working that sector), which is consistent with what you are hearing in Halifax. It appears from what I've heard so far that the frequencies in New Brunswick and P.E.I. are cross-coupled while the ones in Nova Scotia are not.

 

[AirScan]

Some observations after further listening on 124.400 in Moncton (via LiveATC).

It appears that Moncton's low altitude airspace (FL280 and below) is basically divided into 2 areas and covered by the following frequencies.

Area 1 (New Brunswick and P.E.I.) (frequencies cross-coupled)

124.400 - Moncton
135.650 - Charlottetown
124.300 - Fredericton/Saint John (Clarendon)
135.500 - Saint John (Clarendon)
123.700 - Miramichi
134.250 - Charlo (Dalhousie)
132.500 - Caledonia Mountain

Area 2 (Nova Scotia and Grindstone, Quebec) (frequencies not cross-coupled)

135.300 - Halifax
118.600 - Sydney
123.900 - Yarmouth
134.350 - Grindstone

During normal traffic volume during the day Area 1 and Area 2 are split and worked by 2 separate controllers.

During periods of light traffic volume during the day and the overnight shift Area 1 and Area 2 are combined and worked by 1 controller.

All frequencies in Area 1 are cross-coupled while none of the frequencies in Area 2 are. This does not change when Area 1 and 2 are combined.

Area 1 might be further subdivided as traffic requires ?

Not sure what the purpose of 135.500 is as it appears to be co-located with 124.300. The published Jeppesen approach plates for Fredricton (YFC) and Saint John (YSJ) only list 124.300 and I have not heard 135.500 in use yet.

If anyone can verify the above or provide more info. it would be appreciated.

 

[novascotian]

While of course I have never heard the cross coupling, it does look the breakdown of the areas is the way I see, or hear, things as well. Funny how Grindstone is with NS and essentially Charlottetown, that is part of the NB group, lies right in between.

Also keep in mind, for completeness, that Halifax Terminal shuts down at night, and 119.2 becomes another member of the simulcast, but not coupled, group. In fact I think this means that two identical signals are coming from the same location when this is in effect, 119.2 and 135.3. I assume they do this in the interest of consistency when aircraft are arriving at, or leaving from Halifax, even if who they are talking to changes.

I am thinking that I have heard 135.5 being used but I cannot be certain. I have always wondered why somedays ATC sends aircraft to 124.3 and sometimes 132.5, and sometimes 135.5.. I mean different from one day to another for aircraft heading west towards Saint John and beyond, from Halifax.

 

[AirScan]

I have always wondered why somedays ATC sends aircraft to 124.3 and sometimes 132.5, and sometimes 135.5.. I mean different from one day to another for aircraft heading west towards Saint John and beyond, from Halifax.

I noticed that as well. Same with arrivals into YHZ from the west (ie: YYZ, YOW, YUL) sometimes on 123.90 and other times on 124.30. I noticed when Area 1 and 2 were combined they were using 124.30 and when they were split they were using 123.90. Might be a coincidence, or that's how they configure the frequencies with the sector ?

 

[ATCTech]

Talking to a former Gander controller last night who told me Moncton is basically divided High East/High West and Low East/Low West for en route traffic, 4 specialties in other words. The actual sectors of course fall within those specialties as they do in all our centers. I presume the various TCU airspace (terminal) control zones would be their 5th specialty.

Frequencies are grouped by specialty and then divided among the sectors within those specialties. As traffic decreases the sectors assume the frequencies of their other sectors, but not those of another specialty.

In Toronto we have 8 specialties and about 30 radar displays. Mid day everything is open and the frequencies are assigned accordingly. Overnight we collapse to one sector open per specialty and all the frequencies assigned to the specialty are handled by 1 controller. The frequencies never go to other specialties in normal operations. I'll promise Moncton does the same thing. If you want specifics I do work with a former Moncton controller as well, but I didn't see him last night.

Bob

 

[novascotian]

Bob, what you say goes completely with my observations, at least in the high level part. "Specialties" is a new term to me, and I guess part of the "insiders' " terminology, so thanks for that.

I am wondering if the high and low have the same boundary between their east and west.. For the high it does appear to be a line more or less from around Sheet Harbour and angling slightly west of North to cross over eastern PEI and then up maybe a bit west of the Magdalen Islands and then I don't know. Of course it is hard to tell because I notice that aircraft are sent to the next sector before crossing the boundary. I am basing this statement on assuming that the boundary between Boston and Moncton runs pretty close along he border and its extension south...and they always seem to switch over a bit west of this when eastbound, and a bit east of it when westbound... so assume that is the way it is done generally.

Would be great to see a sector map for CZQM low level... I have never seen one... and I suppose what I have for high level is out of date.

Anything you can pass along re the Moncton ACC area would be welcome, other than as I said something that will lead to 133.95 being eliminated or not used as much.

 

[AirScan]

Bob,

Thanks for the insight. It would appear that the Halifax Terminal would be part of the low altitude specialty as it is combined on the overnight shift. I've noticed as well they never combine high west with high east, even on the overnight shift.

Would be great to see a sector map for CZQM low level... I have never seen one... and I suppose what I have for high level is out of date

I've been trying to track down a contact at Nav Canada to obtain sector charts for all Canadian ARTCC's. So I'd be interested in the above as well.

There are some over at Canairradio but they have not been updated in some time.

http://www.canairradio.com/YQM.jpg

Area Control Centre - Sector Maps