406 MHz ELT search

[wrench]

I see browsing the site that in the past there has been some confusion about the usefulness of 121.5 MHz ELT emergency beacon freq. A few months ago I went on a ground search for a 406 MHz emergency beacon. It turned out to be inadvertent triggering and all was well in this case. But I can tell you from this and other experience that if we did not have the homing ability of the 121.5 MHz companion signal from the 406 MHz equipment the ELT battery may have run dead on the unit before we ever found it. We were given Lat, Lon coordinates by SAR derived from a 406 MHz ELT beacon transmission to start the search. But we actually found it 3 miles away (typical).

As a side note, CAP aircraft are equipped for 121.5 MHz homing but were not used in this search. PLB's and EPIRB's I believe transmit on both 406 and the 121.5 MHz frequency when triggered also.

My observations from this type search

1) The 121.5 MHz is monitored by many commercial and some of us GA types while flying. We often still get reports of 121.5 triggering from these sources and was verified by commercial aircraft on this occasion also.

2) We get a very small search area IF your 406 MHz ELT has the ability to output a GPS signal to SARSAT. Typically within a few hundred meters, but not all 406 MHz ELT's have this ability!    406 MHz ELT's without the GPS output ability have a much larger search area. Our target in this case did not have GPS output thus a longer search time to find it.

3) In heavy ground or wooded cover the 121.5 may be the only reason SAR will find you. Especially for PLB's.

4) Our search area had mostly cell coverage but in many areas there was none especially in the low areas of the hilly terrain. Cell phone may or may not be a good backup but there are lots of recreational places in our state where coverage is nil.

5) If you have one of these devices, a 406 ELT or PLB or EPIRB, registration rules for the device's need to be followed to protect us all. I like many don't like a bunch of additional rules but to protect us and our passengers it can be very useful in this case.

 

My observations on 121.5 homing, I hope this helps.

 

[yvesg]

wrench, I have a question: Why was the 406 MHz homing not possible? It is because you guys do not have the equipement to do so? (Directional receiving antenna and S-meter I assume)

Or is it because the 406 MHz signal pattern does not allow homing? (Could be the case if the signal is sent in short bursts or some  wideband stuff I am not sure about)

Thanks

Yves

[Piloto]

Homing is possible on 406 MHz but requires a more sophisticated receiver than that for 121.5 MHz. Ideally you decode the 406 MHz reported position instead of homing into the signal. Satellite reception of a 406 MHz beacon requires a clear view of the antenna. If the antenna is blocked by the airframe it would be impossible for the satellite to receive the signal. But the 121.5 MHz signal can be pick up by a nearby searcher even if blocked. On maritime service the 406 MHz beacon has bigger batteries and floats upright with the antenna exposed in the clear. If you are travelling over water it helps on rescues to bring along a handheld marine radio. Most boats are on standby on channel 16 for emergencies. After all if you ditch you need a boat, not a plane. Check with Sully.

José

[wrench]

1 hour ago, yvesg said:

wrench, I have a question: Why was the 406 MHz homing not possible? It is because you guys do not have the equipement to do so? (Directional receiving antenna and S-meter I assume)

Or is it because the 406 MHz signal pattern does not allow homing? (Could be the case if the signal is sent in short bursts or some  wideband stuff I am not sure about)

Thanks

Yves

We currently don't have 406 homing equipment. As you suggest it may not be feasible. 

[Jerry 5TJ]

The 406 MHz ELT transmission is a short message repeated every 50 seconds.   It wasn't designed for conventional DF.  

Thanks for the operational summary, Wrench.  You made it clear why a GPS connection to the ELT is very useful.  Without GPS coordinates the satellites can Doppler locate an ELT but 3-5 miles is about the accuracy limit for a several pass solution.

I can see how a 121.5 signal from a modern ELT helps local searchers.  

 

 

[Flip]

Jerry is correct that a 406 MHz ELT or EPIRB sends out a 'ping' every 50 seconds. It is not a continuous signal which means the DF needle snaps to the relative bearing of the signal (let's say the 3 o'clock position) and locks into that position until the next ping is received.  During the ensuing 49 seconds, no matter how you maneuver the searching aircraft, the needle will point to the 3 o'clock position (like a dog chasing his tail).  This is why the continuous 121.5 signal is important to help us find you.

We launch out towards the gps position provided by your ELT (please register and connect you ELT's to a gps source!) or the derived SARSAT position and DF on the 406 signal until we get close enough to pick-up the 121.5.   It is as easy as following the head of the needle at that point.  If the needle swings, we went to far.

I can't speak for the CAP, but the USCG helicopters can DF on any VHF-AM (121.5), VHF-FM (marine Ch 16), UHF (243.0) or 406 frequency.

Trivia: there are actually 6 different 406 MHz frequencies used by ELT's and EPIRB's

Flip

U.S. Coast Guard MH-60T pilot 

Edited September 13, 2016 by Flip

[carusoam]

Great educational post, wrench & flip!

Nice work vehicle too, flip.  

Best regards,

-a-

[1964-M20E]

Thanks for the information.

 

I will ask my local CAP squadron commander what their capabilities are.

 

I just had the 406 installed in my plane and it is connected to my GPS.  I believe it also has the 121.5 but I will have to double check.  If I know I have an emergency and activate it manually using the dash mounted controller can I assume that ever 50 seconds when it sends out is burst of information it is updating my lat and long coordinates?

[Jerry 5TJ]

2 minutes ago, 1964-M20E said:

..... If I know I have an emergency and activate it manually using the dash mounted controller can I assume that ever 50 seconds when it sends out is burst of information it is updating my lat and long coordinates?

Yes, it transmits the current GPS coordinates.  If GPS connection ends, the ELT uses the last valid GPS coordinates it received.