121.50MHz interference

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Hooligan

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Occuring right now (7:10PM local Sunday) & starting at least 2.5 hours ago when I first noticed it, the SF Bay area is being blanketed with interference on the emergency frequency of 121.50MHz.

I was up in the Marin Headlands all day (without a radio on) but first noticed the signal a few minutes after crossing into SF from the Golden Gate Bridge on the Uniden BC-796 hooked up in my car. Signal was full-scale with audio characteristics also indicating it was very strong. At first, I thought it was an unusual case of EMI from a bus behind me, then maybe from within my car, but I picked it up on 4 different radios, and the signal did start to fluctuate as I headed S to my home in central San Mateo County.

I'm now picking up the signal in my living room, using a Yaesu VX-5 handheld & GRE-PSR-500.

The audio kind of sounds like FSK/RTTY and changes pitch a couple times each minute. I'm not hearing it on 243.0MHz, and it's definitely not an EPIRB, though the poor guys at USCG Sector San Francisco gave out a Notice to Mariners on 16 about "a 121.5MHz EPIRB being audible in the SF area..."


While many airports in the SF Bay area have a receiver for 121.5, most of them don't have transmit capability, so that if a plane were to declare an IFE on it, 20+ different airports wouldn't try to reply back. This 121.5 signal is definitely coming from an FAA Remote Communications Outlet or military Ground-Air Transmit/Receive site high above average terrain.

My educated guess based on signal strength during my drive home is the emitter is at the FAA/JSS site at the old Mill Valley Air Force Station atop Mt Tam, the separate FAA RCO around the 'mid-peak' area of Mt Tam, or an FAA/NORAD/Navy site near Half Moon Bay. The Monument Peak area of the East Bay Hills above Fremont has an RCO but I think a signal from there would have been weaker for me in SF. Likewise RCO at Black Mountain wouldn't have come in so strong for me in SF.


My associate 'Portable-John' in the Concord area hears it about half-scale, my associate 'BS' in Discovery Bay hears nothing using his VX-5.

An FAA Electronics Technician friend of mine wasn't familiar with the noises being transmitted on 121.5, and he called Oakland ARTCC to discuss it around 5PM.

I'm listening to Civil Air Patrol freqs & the FAA 'C3' LMR channels now for any related comms.


This sort of thing happened in Southern California earlier this year & that signal lasted for a couple days.



Tim
 

avtarsingh

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i locked out 121.5 and 243.0 out of my scanners because it seems there's always some kind of interference or weird noise on it

weird
 

inigo88

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Not hearing anything from 5 miles north of the Mt Tamalpais "Middle Peak" FAA/JSS site. Mt Tam completely blocks my line of sight to Half Moon Bay, most of SF and the south bay. I can pick up some transmissions from Monument Peak but not others, and then I start receiving east bay stuff clearly north of Monument and Wiedmann around Oakland and Grizzly Peak.

I should definitely have line of sight to both the radar site and RCO on Mt Tam middle peak, and I'll keep an ear out for it.

Hope that helps narrow down your search,

Inigo
 

Hooligan

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The 121.50 emissions were heard at least as far N as Santa Rosa, which does tend to support the guess that the source was one of the two FAA sites on Mt Tam, but in the past, a similar wide-coverage signal was traced to a faulty system aboard a UAV operating out of Creech AFB Nevada.
 

inigo88

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I've always been surprised by the amount of line of sight I can get to SF and the south bay area from Petaluma. I agree that Santa Rosa is quite a stretch though. Feel free to PM or email me if it happens again, Mt Tam is VERY close.

A similar wide-coverage signal was traced to a faulty system aboard a UAV operating out of Creech AFB Nevada.
Any more story behind this? Maybe this time it was an RQ-170 out of TTR. :)

Regards,

Inigo
 

Hooligan

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The problem is back, & on the air as of right now.

My posse & I are hopefully going to head out & do some basic RDF work later today, trying to rule out Oakland ARTCC & Monument Peak in Fremont, Scarpet's Peak near Half Moon Bay, etc.

We're also going to be watching for patterns that could indicate the emitter is coming from an aircraft, perhaps one doing a fairly tight orbit & thus not having a lot of signal strength fluctuation.

In theory, ZOA Center in Fremont should be the only local *FAA* site with a high-profile 121.50 transmit capability. The FAA transmitters are capable of 100% duty cycle, but also have time-out timers, so who knows...


/-/oolie
 

Hooligan

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We seem to have found that when the signal is off 121.50MHz, a similar signal pops up higher up in the VHF spectrum on a freq in the federal band usually just used by military. We're trying to confirm the correlation.

We were able to rule out some locations, but it sure does seem like it's a low ERP transmitter with a lot of HAAT to the antenna. Mt Tam is still looking like a good possibility, though the new freq would be very curious & isn't something we'd expect to be coming from there, so if not Tam, the thought goes back to a UAV doing a pretty tight orbit NW or N of SF & loitering for a day or two at a stretch.

USCG continues to make broadcasts about a 121.50MHz EPIRB being heard in the SF Bay area but it doesn't seem like CAP has been tasked with it, nor are FAA techs busy on their LMR system.
 

Norman

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I am also hearing that interference on 121.5 mhz. Last heard yesterday, 12-15. Been hearing it for over a week now, off and on. Location here is W/O Placerville. I am not familiar with the signal that I am hearing.
Norm.
 

inigo88

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Strange, I was starting to feel sure it was Mt Tam but I'm not hearing it on 121.5 MHz or 138 - 144 MHz (AM). Does anyone have a recording of what it sounds like and what we should be looking for? What I heard yesterday was just a high pitched tone that sounded almost like a birdie in the scanner, not any kind of digital mode.

Thanks,

Inigo
 

inigo88

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I finally hear what you guys are talking about. DEFINITELY a digital mode on 121.5 Mhz AM. LOUD AND CLEAR! I'm 90% sure it's coming from the Mt Tam RCO. :)

The "clicky" sounding format reminds me of a slowed down version of the Mobitex used by the RAM Mobile Data network up in the 930 MHz range. At first I even thought it was a pilot with a stuck mic making fart noises. :D

Inigo
 
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Hooligan

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shoulda mentioned in a prior post that the signal I've been hearing off & on on 121.50 since yesterday isn't exactly the same sort of noise that was being generated a week ago Sunday. Then, it rotated thru a couple different types of FSK-type sounds. Now, it's more of just an open carrier with a little bit of a whine to it.

It's been there again today, off & on. The other freq that it seems to be associated with is 139.70MHz, which has had a carrier (with about a 1000Hz whine to it, like we hear with many aircraft transmissions) up for much of the past 24+ hours now. We haven't confirmed this completely, but when the wide-coverage signal is off 121.50, 139.70 seems to have it, and vice-versa.

121.50 is pretty self-explanatory -- lots of places have transmitters for it. 139.700MHz is a little different though. It's in federally-owned spectrum used for tactical air & tactical/land mobile radio. NORAD Western Air Defense Sector ("BIGFOOT" & "DEER HUNTER") use UHF channels like 364.20MHz Air Intercept Control Common & 271.0MHz to talk to military aircraft in the Bay Area, from a Ground/Air Transmit/Receive site located on the ridgeline above Half Moon Bay, not Tam, and I don't think Tam or anywhere else in the area should have any sort of remote base on 139MHz. Tam has the ARSR-4 RADAR at the old Mill Valley Air Force Station, which also has FAA Back Up Emergency Communications VHF/UHF stuff for ATC, but the primary FAA Radio Communications Air/Ground site on Tam for Oakland Center is at the 'middle peak," as Inigo noted. FAA transmitters at RCAG/RCO sites normally put out 10 watts & may or may not have a time-out timer programmed, whereas the military-related sites such as the 121.50MHz transmitter at Scarper's Peak is usually 50 watts.

What we (read:Inigo!) need to do is hit the two FAA sites atop Mt Tam & confirm/deny the 121.5/139.7 mess coming from there, by getting as close as legally possible (you can walk right thru the middle of the Middle Peak RCAG), removing the antenna from the radio & see if you still get the signal, and then try tuning off-freq too to see if you get adjacent-channel QRM.

I don't know whether or not the FAA or FCC is really concerned about this, but the 121.50MHz problem should be considered a high-priority issue since ARTCC & ATC have to be annoyed with aircraft constantly advising them if they're aware of a signal on 121.50, then plenty of aircraft & ground stations with 121.50MHz guard receivers are going to be muting them these days & USCG can't be happy with all the reports of a 121.5MHz EPIRB. Plus the signal can mask any legit emergency comms or beacons on 121.50MHz.

The Radio Reference SF Bay Area Radio Geek Emergency Response Team needs to take charge of this investigation & restore peace & tranquil static to international emergency frequency!


Tim
 

inigo88

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Hehe gotcha, and for what it's worth NORAD WADS does use one of the two Mt Tam RCAG sites. "BEAVER CONTROL" (San Diego FACSFAC) is only in Half Moon Bay.
 

wolter

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from a Ground/Air Transmit/Receive site located on the ridgeline above Half Moon Bay, not Tam, and I don't think Tam or anywhere else in the area should have any sort of remote base on 139MHz.
There have been problems on Montara Mountain for about a week. News reports link it specifically to an AT&T cell phone outage, but maybe it's also related to this?
 

WayneH

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There have been problems on Montara Mountain for about a week. News reports link it specifically to an AT&T cell phone outage, but maybe it's also related to this?
The FAA doesn't have anything on top of Montara and if there was some glitch with a two-way transmitter I wouldn't receive it as well as I do in the Valley about 50' above sea level. I have been able to receive Tam but it depends on propagation conditions.

Still active with no level change (from yesterday) as I type this. Signal flutter indicates to me it's quite a distance away.
 

inigo88

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We went hiking up in the ruins of Mill Valley AFS on Mt Tam this afternoon, and brought the scanner along. No signals on 121.5, 139.7 or 243.0 in the vicinity of the radar site. Did not make it all the way to Middle Peak to check, but I definitely would have heard the signal if it was active because of the amazing line of site from up there.
 
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Hooligan

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Location of the offending emitters has been confirmed as Mt Tam. I haven't heard the mess on 121.5 lately but it continues frequently on 139.70.

While high-power (relatively speaking -- 50 watts) unintentional broadcasts on 121.5 is everybody's problem, the two problematic transmitters are both military-controlled. If you want to use it as a propagation beacon, 139.70 should also be 50 watts into an antenna that's probably about 3dB gain.
 
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