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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."
http://www.navcen.uscg.gov/marcomms/gmdss/epirb.ht m
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Corvallis Gazette-Times has more details and a picture of the guy posing with his TV. Apparently, he mostly watches public broadcasting and has acquired a taste for all the quality children's programming it provides, especially "Arthur".
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.
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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-.
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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.
Actually, not that powerful, relatively speaking. A typical 121.5 EPIRB puts out 75-125 milliwatts. Keep in mind this is VHF and the satellites are typically NOAA birds in low-earth orbit expecting a line of sight signal, so 100 milliwatts is very workable.
The issue with 121.5 EPIRBs is all they do is send a warble tone -- no ID, location, nothing. All the processing is done by the rest of the infrastructure, and even then the output is basically a position (still no ID) to within a mile or so -- with people using radio-direction finders narrowing it down more.
The newer 406 MHz EPIRBs have specific user data and location information transmitted in their digital packets, so not only do they know the where, but they also know the who -- so when they get a boat beacon originating at someone's house they pretty much already know it's a false alarm.
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.
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The signals are received by three satellites, to triangulate the position of the transmitter. I don't know what kind of antenna the receiver uses. Bloody big ones, I would think.
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.
now we need to go OSS in diesel cars
Cable Channel 14 does exist, they just aren't using it. The Minions of Satan, I mean Time-Warner Cable, in my area reshuffled the deck awhile back and quit using it as well. Cable channels 2-6 (low VHF) and 7-13 (high VHF) use the same frequencies as their over the air counterparts, but where over the air 14-83 (UHF) is in one continuous block of frequencies (around 470 to 890 MHz), cable uses frequencies for 14 and up that are used for many things other than television as far as over the air use is concerned.
I see even classic Slashdot is now pretty much unusable on dial up anymore.
Even if the ELS frequency is being moved, it is still quite important to keep 121.5 clear, as that is also the standard voice aviation distress frequency. Aircraft voice radios can't tune into 406MHz.
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For those of you who might be wondering what this costs: Replacing an ELT on an aircraft is not like replacing an EPRIB on a ship. You need to ensure the shock switches fire appropriately, and that the unit is mounted such that it will survive a crash.
The last time I explored that option for our airplane we were staring at something around $1200 to do this upgrade. It's hardly chump change.
Further, we need to get our navigation gear coordinated so that the 406 MHz signal has GPS to feed to the world. That's not easy to do for aircraft without panel mounted GPS navigation receivers.
Also, new regulations regarding the pointless ADIZ around Washington DC practically require pilots to monitor 121.5 to respond to an intercept if one happens. If you hear callsign "huntress" on the air and they're operating in your vicinity, remember to be on your very best behavior.
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If you want to do wiring yourself, here's what you should be looking for:
Hope that helps