ZLA Sector 31- can hear pilots but not the controller.

[airtrafficjam]

Hi,

On the ZLA Sector 31 feed, I can only hear the pilots talking. No communication from the ATC can be heard. Is the source for the feed out of range from the ATC?

[Chananya Freedman]

 this is a frequency i am not familiar with. i will try to do some more digging. 

[airtrafficjam]

Yes, please let me know if you find out the cause of the issue.

[RonR]

The most likely cause is that the feed site is too far away from the ATC transmitter site.  Have you heard the controllers on this feed before or has it always been pilot-side only?

[airtrafficjam]

In the last 3 weeks that I have been listening to this feed, initially, I could hear communication both ways. Then it changed to where I could only hear the pilots. Now, I can hear mostly the controller, maybe sometimes the pilots, but a lot of heavy static.

I don't know if the location of the transceiver changed. Are feed providers regularly checked for quality?

[airtrafficjam]

It also appears that during less busy hours, Sectors 09, 12, 30, and 31 (at least) are combined and handled by the same controllers. However, weirdly, I can hear the controller's communication on the Sector 09 frequency but a lot of times pilots are responding to him on the Sector 30 frequency. How does that happen?

[Bob2]

Good questions about what you are hearing, and the reasons are not obvious.

Many/most frequencies are available at more than one transmitter/receiver site.  The controller can select which site to use, probably now with a tap or two on a touch screen.  Reasons for changing include broken or degraded equipment, temporary interference from external sources, routine (e.g. daily) operational tests, airplanes at low altitudes on opposite ends of a large sector, and reducing the number of frequencies needed for adequate coverage when sectors are combined.  In addition to all the other details, controllers need a mental model of how frequency coverage works in their sectors, especially in mountainous areas because VHF is line of sight.

This document discusses some aspects of communications, and defines terms such as RCAG, RTR, and RCO:

https://www.faa.gov/documentLibrary/media/Order/FAA_Order_JO_6500.28.pdf

If you do a general search on RCAG you can probably find information about current Center sites.

I have a list of sites that is about 20 years old.  At that time, ZLA31, 126.775, had sites at Julian and Yuma.  Those may have moved or there may be additional ones now.  If I mouse-over the archive access link on the LiveATC page, my browser shows a link ending in "ksan_zla31", which suggests that the feed provider is near San Diego.  My wild guess, 20 years ago, would be that if the Julian site is in use you can hear the controller.

A controller can transmit and receive on one or more frequencies as desired.  Some sectors are so large that more than one frequency needs to be used even when operating as a single sector.  If combined sectors cover a small geographical area, it might be possible to have everybody on one frequency.  At the time sectors are combined or split, the controller can send an internal message to surrounding sectors telling what frequency or frequencies to use.

There would be lots of possible configurations, and your observations about 09+30 and 25+28 are not surprising to me, and now hopefully not to you.  Another thing to note is it takes some time for signals to get from the feed provider through the internet to you, and this can vary from a few seconds to 30 seconds or more.  If you are listening to multiple frequencies used by a single controller, you might hear an airplane response on feed "A" before you hear the controller give the instruction on feed "B".

Bob

[airtrafficjam]

Thanks for your response! This is a lot of good information! Yes, the Julian RCAG site (ZLA 31) seems operational and the sector boundaries have likely not changed much based on what I am hearing on the feed and how the aircraft are moving.
My sources:
https://www.faa.gov/sites/faa.gov/files/about/office_org/headquarters_offices/ato/ZLA-ARTCC.pdf (Page 9)
https://www.radioreference.com/db/aid/2249

For the 25+28 feed, you can occasionally hear the pilots responding to the controller. My guess is, based on what you said about VHF being mostly LOS, the feed source does have LOS with the RCAG radio sites but does not have good LOS with all aircraft in the air (especially those farther out over the ocean), causing the controller to always be heard but pilots to be heard infrequently.

For Sector 31 (Julian), you can almost always hear the pilots talking but the controller can hardly be heard and there is static. Maybe the feed source is just badly positioned to capture high-quality communication for the Julian RCAG.

I didn't fully understand how one controller would communicate with combined sectors over multiple frequencies. Would they use 2 frequencies to communicate with different parts of their 'combined sector'? Also, last night, when I noticed that 9, 12, 30, and 31 were combined, I could hear the controller on 9, 12, and 31, but the pilots could only be heard on 30 (again not always), and no, it did not seem like the delay example for 2 feeds that you gave holds here. I could hear the controller speak on 9 followed by the pilot responding on 30 within a second. Is this just LOS not always being satisfied causing trouble?

[airtrafficjam]

On a related note, on the radio reference page, you will find that the Julian table lists 3 sectors- a low 12, and two high 31, and 40. Each of those has a VHF and a UHF frequency. Would those be used by commercial/private aircraft in the normal course of operations or will those be used exclusively by the govt./defence/law enforcement?
https://www.radioreference.com/db/aid/2249

[Bob2]

If you only heard airplanes on 30 it might be because the site for 30 is adequate to cover the combined airspace, or at least the parts of the airspace that happened to have airplanes during the time you were listening.  I think it's hard to draw conclusions about the T/R configuration(s) when portions of the airspace may not have any airplanes.  You might gain some insight by listening on multiple nights while watching flight tracking web sites.

Usually civilian uses VHF, but there might be civilian operators doing work for the military that need both.  Military may be either, probably with fighters leaning toward UHF only, and transports leaning toward both.