Distress Signal Emitted By Flat-Screen TV

pinqkandi writes "CNN is a running a story on an Oregon college student's flat-screen Toshiba TV which was releasing the 121.5 MHz international distress signal. He was unaware of the issue until local police, search and rescue, and civil air patrol members showed up at his apartment's door. Apparently the signal was strong enough to be picked up by satellite and then routed to the Air Force Rescue Center in Virginia. Quite impressive - luckily Toshiba is offering him a free replacement."

Comments

Signals

[Anonymous Coward]

For those who are wondering what type of signal this is, check here:

http://www.navcen.uscg.gov/marcomms/gmdss/epirb.ht m

Animah S/V Solaris

406MHz Digital Distress signal

[amigoro]

There's another digital distress signal too. The 406 MHz distress beacon emits both an analogue 121.5 MHz signal and a digital 406 MHz signal. The digital signal carries a code which identifies the beacon while the analogue signal is to enable aircraft to home on location. That digital code can be cross referenced with a database of registered 406 MHz beacon owners held at AMSA which identifies who is in trouble and what type of situation they are in. This enables the search and rescue authorities to tailor a response to the emergency situation.

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Re:Must have been quite powerful

[tonsofpcs]

The 121.5 MHz (as well as 243 MHz) Distress call response is being phased out, and the newer 406 MHz call is becoming a more accepted (and used) standard.

See the official NOAA Press Release (PDF) for deteals.

Re:Must have been quite powerful

[dgatwood]

Or the U.S. Coast Guard press release (HTML) here.

Basically, that frequency was getting way too many false positives, so they're phasing it out. To quote the release, "121.5 MHz false alerts inundate search and rescue authorities. This is another major factor in influencing the decision to stop the satellite processing."

121.5 MHz is in the middle of cable channel 14. Frankly, it's rather surprising that this doesn't happen -constantly-.

Re:Signal Details

[flyboy974]

You'll be waiting ~ 3-4 hours before somebody comes and finds you, if they are really good at it.

First, wait for a satellite pass will notify the Air Force. They will then verify it if they can, contact the FAA for missing flights, etc. The next call goes out to the Civil Air Patrol wing that is responsible for that area. They in turn will normally notify the local authorities who are in charge of S&R. Of course, when you broadcast on 121.5, that sound is audible in every Airtraffic Control center that it can reach.

Once they have done this, they will organize a ground based S&R party and try get a general area of where the signal is coming from. Remember, this is non-directional, so they have to go to a few different places, measure the direction and approx. strength of the signal, and then they will know about where it is. Triangulation sucks, esp. with trees and mountains.

Once they have done this, they'll start their search. Oh, if it's at night and it's not somewhere near them, they'll wait until the morning. Hope you don't keel over at night.

Finally, once they triangulate it, they home in on it. In this case, they homed it to an apartment. Questioned the guy, and went back out into the hallway and confirmed it was coming from there.

So, do you REALLY want a 121.5 ELT locator? I would get one of the new 406mhz ones which are digitally encoded with your information. In addition, some models offer GPS in them that will transmit your GPS coordinates when it sends it. Much nicer and easier to find.

Oh, and I'm not a CAP member any more. But, it was fun while I did it. Not enough time now, but, maybe after I'm done building my airplane I'll have time.

Re:Must have been quite powerful

[DigiShaman]

I just checked the frequency of channels for TWC Austin. (digital cable)

Ch 6 = 85Mhz
Ch 7 = 177Mhz
Ch 8 = 183Mhz
Ch 9 = 189Mhz
Ch 10 = 195Mhz
Ch 11 = 201Mhz
Ch 12 = 207Mhz
Ch 13 = 213Mhz
Ch 15 = 129Mhz
Ch 16 = 135Mhz
Ch 17 = 141Mhz
Ch 18 and above keep climbing past 141Mhz

Notice that Channel 14 doesn't exist and how the lower chanels skip around a bit. But the closest to 121.5Mhz is Channel 15.

The relationship of 121.5 Mhz to NTSC video

[Skapare]

The frequency of the NTSC color subcarrier (the TV color system used in analog video standards in North America and Japan) is defined as exactly 5 MHz times 63/88. That works out to 3.579545454.... (infinitely repeating 54's) MHz. The horizontal scanning frequency is then defined as a 2/455 times the color subcarrier frequency. That works out to 15734.26573426.... (infinitely repeating 573426's) Hz (very nearly the original monochrome horizontal frequency of 15750 Hz). This is where the problem lies. 121.5 MHz divided by 7722 is exactly the same frequency as the horizontal in an NTSC color video signal.

The 7722nd harmonic shouldn't really be that strong, right? Usually not. But the harmonics can get to be very strong overall even at such high orders when dealing with modulating the high voltages needed for the horizontal sweep. There should be some low pass filters that prevent that from getting into the VHF range. But if the filters are absent, or were incorrectly installed, or were damaged somehow, and if some wires formed some resonance near 121.5 MHz (like wires going out to cable, speakers, etc) ... a wavelength of about 2.47 meters or 8.1 feet ... it is possible that harmonic, and a bunch of others near it, could be enhanced and radiated.

The local oscillator in the tuner is a remote possibility. But it would have to be tuned to be receiving a video carrier at 75.75 MHz based on the common satndard of 45.75 MHz for the IF stage in the tuner. But there is no TV broadcast on that frequency in the US ... though I could not rule out there being something on that frequency from a cable system. Still, it wouldn't be an expected place for a TV to tune to. But if the TV has a non-standard IF frequency, the local oscillator getting on 121.5 MHz by some expected channel could be possible. Those leak a lot and it's how the snoops can tell what channel you are tuned to by spying on the RF emitted from your house.

If just this one TV had the problem, then apparently it must be a manufacturing defect or shipping damage (or maybe user damage or tampering). If it were a design problem, I'd sure we'd hear more about it. That probably rules out the CPU clock frequency.

Can affect Air Traffic Control

[p.rican]

as well. Cable companies can be fined enormous amounts for failing to provide evidence of an ongoing leakge control/abatement program. When I was a cable TV tech, my primary job was to drive around the neighborhood looking for the "Leakage". It's called "CLI" or Cumulative Leakage Index. If the leakage was coming from someone's house and we could not gain access to the house to correct the problem, the customer's service was disconnected at the pole and a note was left on the door. The usual culprit was a crappy amplifier and those "high quality gold plated screw on RG6 connectors" you see sold in Radio Shack, Walmart etc.

If you want to do wiring yourself, here's what you should be looking for:

1. (at least) Dual shielded RG-56 coax cable
2. (at least) 80% braid (no copper braid either)
3. RG56 crimper
4. RG56 crimp style connectors. Not the screw-on connectors
5. Splitters with frequency rating of 5MHz-1GHz
6. Install amplifiers in your house preferably before the first split of the coax.

Hope that helps