Another Doppler System!

[Major Lord]

Although I am not a great fan of Dopplers for ELT work, a lot of people have been discussing DF'ing the 406 signal too. I came across this Doppler tracker that shows your position on a Google Map and automatically draws bearing lines to the transmitter ( or at least, it signal, which are not quite the same thing!) The thing I though was cool was the low price: $299.00 bare bones ( your computer, assembly, GPS, and radio required)  www.kn2c.us

Major Lord

[starshippe]

   i have had good results from doppler units in years past, and i think the rho theta units use a form of doppler detection. i am also interested in the capability to df on 406. i'm still trying to get a sniffer that will cover 406.

   it would be good to hear from someone who has some experience with this unit. apparently, they are mainly interested in the ham bands, although the processing would of course be the same.

   i do not see how the software would be able to draw lines of position toward the signal without an input from the heading of the vehicle, and the article doesn't mention that. underway, the heading could be derived from the course, but i don't see that mentioned either.

   considering the differences between this unit and the sniffer, it seems like the sniffer would more easily be taken "off road."

   can anyone give us some first hand information on how well this unit operates?

thanks,

bill

[Major Lord]

Some of the other Doppler types use a Flux-gate compass to derive your heading info, or I suppose, as you suggest,  you could derive heading from your GPS course track, which seems a little "iffy" . More importantly, it looks like it might be dependent on having a live internet link to access the Google Maps, but perhaps there are other static mapping programs. Perhaps by driving to the top of the highest terrain feature and parking, you may be able to get a bearing to a 406.025 beacon, with sufficient patience.....There are not a lot of choices in narrow band, super sensitive FM receivers that will operate in the 406  data beacon range either! I have personally tried the Ramsey and the Seimac marine Dopplers, but found working weak signals challenging, no doubt due to the high losses in switched antenna arrays and trying to do phase shift analysis with minimal signal strength. I am tempted to put aside my Doppler bias and pick one of these bad boys up, mostly to see if I can marry one to an APRS transmitter.  I just started producing Fox Hunt transmitters this year for Ham's, but ironically, the only Fox Hunt's I have been on have been ELT missions! I figured I had better go out there and learn the DF game from the ham perspective before I go hog wild and build stuff that's useless.....

Major Lord

[starshippe]

   you might need some luck to go along with that patience, as the elt may not be operating on 406.025.

   shown below is table h-2 of c/s t.012, the 406 mhz channel assignment table.

   the first two usable channels, b and c, were used until the beacon population density forced manufacturers to start using channels f and g. as i understand it, cessna, for instance, has been shipping aircraft with elts using these channels as of the middle of last year. once these channels become populated, channels j and k will be used, etc. these elts and epirbs are becoming very popular, acr received a five year contract for five thousand of them for the coast guard. 

   the becker df units will receive only channels b and c. the rho theta units will scan the entire ~50 khz band.

   i'm trying to find out if the manufacturers, or afrcc, or someone, will give us the actual transmitting frequency, as should be able to be determined from the beacon i.d. code. if we knew this, those of us flying with beckers could put it in as the 406 band practice frequency. this would give us a fighting chance for a successful mission. there is a minor problem with tuning increments, but the receiver would still detect the signal, and respond as usual.

   with more and more of the elts/epirbs carrying embedded gps'es, especially the newer ones, i agree that we will be getting fewer and fewer calls. i would still like to have all the information that i can get when, and not if, i get that call at 2 a.m.

thanks,

bill



H - 2 C/S T.012 - Issue 1 - Rev.6
October 2009
Table H.2: Cospas-Sarsat 406 MHz Channel Assignment Table
Chan. #
Centre Freq. (MHz)
Status for Type Approval of New Beacon Models
Date open Date closed
Comments
Table approved by the Cospas-Sarsat Council at the
CSC-43 Session – October 2009 (see Note 1)
406.007 Not available SARP-2 limitation
406.010 Not available Doppler shift limitation
------- ---------------- ------------------------------------------------
406.019 Not available Doppler shift limitation

A 406.022 C/S orbitography / reference Reserved for System beacons
B 406.025 1982 1 Jan 2002 Open for beacon models submitted for TA before 01/01/02
C 406.028 1 Jan 2000 1 Jan 2007 Open for beacon models submitted for TA before 01/01/07
D 406.031 Reserved, not to be assigned
E 406.034 Reserved, not to be assigned
F 406.037 1 Jan 2004 1 Jan 2012 Open for beacon models submitted for TA before 01/01/12
G 406.040 1 Jan 2010 TBD Open for beacon models submitted for TA after 01/01/10
H 406.043 Reserved, not to be assigned
I 406.046 Reserved, not to be assigned
J 406.049 TBD TBD Available for future assignments / New developments
K 406.052 TBD TBD Available for future assignments / New developments
L 406.055 Reserved, not to be assigned
M 406.058 Reserved, not to be assigned
N 406.061 TBD TBD Available for future assignments / New developments
O 406.064 TBD TBD Available for future assignments / New developments
P 406.067 Reserved, not to be assigned
Q 406.070 Reserved, not to be assigned
R 406.073 TBD TBD Available for future assignments / New developments
S 406.076 TBD TBD Available for future assignments / New developments

406.079 Not available Doppler shift limitation
------- ---------------- ------------------------------------------------
406.088 Not available Doppler shift limitation
406.091 Not available SARP-2 limitation
Notes:
(1) Planned assignments may change if the Cospas-Sarsat Council determines that the beacon population
in an active channel differs from the projected population.
TA Type approval
TBD To be determined
- END OF ANNEX H –

[sardak]

i'm trying to find out if the manufacturers, or afrcc, or someone, will give us the actual transmitting frequency

AFRCC can give you the CSTA # = Cospas-Sarsat Type Approval #. You have to specifically ask for it. With it you can find the exact beacon frequency.

Go to the Sarsat homepage: http://www.cospas-sarsat.org

On the drop-down menu under the "Beacons" tab, look for "Type Approved Models" and under that, "By Type Approval Number"

Go down the list to find the CSTA #. The report for each CSTA # will give the 406.xxx transmit frequency, and other information such as 121.5 homing signal power and whether the beacon has an internal, external or no nav device.

On the same drop-down menu under Beacons is a Beacon Decoder Program. If you have the beacon hex code, you can translate it on this decoder. It provides the same information AFRCC has.

I've been on missions where we've tuned the DF to the specific beacon 406 frequency. Whether it made finding the beacon any easier is open for discussion.

Mike

[starshippe]

   be jubilant, my feet! ok, i'm borrowing from mark twain, but he understands.

   this information is entirely too cool. its exactly what i was looking for. thanks much. i did see two freqs listed for the first one i looked up, but the following half dozen showed one freq.

   when u were on ur 406 mission, did u ever hear the 406 burst? what kind of df did u have, a 517 or a 600? if a 517, did u use the scan function?

   i've got the decoder, and don't see too much use for it so far.

thanks again,

bill

[Major Lord]

Well, I suppose I could build a receiver with a 1 MHZ bandwidth centered on 406...........I don't find the idea attractive...................I think we are more likely to be able to DF the 121.5 beacon, since the lower frequency will make up for its lower power to a degree by having less path loss.....plus it should be on continuously, and the unsporting nature of this does not bother me a bit. Being able to look up the beacon info for frequency would be awesome, but especially with PLB's, they may never have been registered. We still might have a little work left to do!

Major Lord

[starshippe]

   u would only need a receiver with a 60 khz band width, centered on 406.050. the frequency of the low channel is 406.025, and the high channel 406.076. right now i don't think there are any channels being used higher than 406.037.

   thats the freq range i was hoping to get the sniffer to do. i have a 406 beam. i think the 0.6 second 406 data burst direction could be determined by swinging the antenna while the burst is transmitting. if the receiver responded with the changing tone, i think that after a few beeps, u would know the direction to the signal.

thanks,

bill


   

[Major Lord]

It should not be too hard to build a FSM type sniffer, with a notch on 406, but a simple diode detector is not going to provide exactly stellar performance......I think we would have to build a real receiver to have much luck.

Major Lord

[starshippe]

   yes, i was speaking of the vk3yng mk4 sniffer, demonstrated here on u tube....

      Using the VK3YNG Mk4 sniffer and home made tape measure Yagi.

   as the beam is turned, the varying signal strength is indicated by the pitch of the tone. it does autoranging and shows the current strength "octave" on the display.

thanks,

bill

[sardak]

Being able to look up the beacon info for frequency would be awesome, but especially with PLB's, they may never have been registered.

The beacon doesn't need to be registered to get the frequency. The type acceptance number, which is used to find the frequency, is transmitted as part of the basic ID of the beacon, which is independent of registration.

when u were on ur 406 mission, did u ever hear the 406 burst? what kind of df did u have, a 517 or a 600? if a 517, did u use the scan function?

I haven't flown the 406 missions, I've been the IC. The aircrews have heard the burst. We've flown 406 missions and trainings with both 517s and the 600s. I've told the crews to use the scan function when available.

As expected, for crews paying attention, the 406 is detected farther out than the 121.5. The problem is that DFing the 406 is more work, and there's nothing to hear, so it's not until the sweep tone is heard on 121.5 that the real DFing starts, and the 406 is ignored after that. The wing has an operational 406 training beacon, but since there is only one, most training takes place with the more prevalent VHF practice beacons.

A common complaint about the Beckers is that they're "deaf." I suspect this is because, at least in part, they're not on the correct frequency.

Mike

[Mark_Wheeler]

I've been MO on a 406 ELT search in the vicinity of Victorville Airport. 406 Plot put it about 3-5 miles east of Victorville. Going up the Cajon pass I have the DF-517 on scan (It was a first generation ELT that had one of the 406 frequencies we can track). I heard the burst as we're going up the pass and by the time we get in the vicinity of the SARSAT plot we pick up a signal on 243.0 but nothing on 121.5. We DF it to an Airbus A-300 that is being scrapped out on the center of the airport in Victorville. We only heard it on 121.5 within 40 feet or so of the actual ELT.  System worked as it is supposed to and was very easy to find.

However, I've had friends on 406 EPIRBs that have been damaged and only give off the 406 signal. Much harder to find because you have to shoot bearings every 55 seconds when it transmits and if you're not in the right position, you just have to wait for the next burst.

Mark   

[starshippe]

   good info, thanks guys.

   the csta may not be transmitted as part of the beacon i.d. i found an older epirb, still in the box, floating in a local river a couple of months ago and ran its i.d., adcd0658451401, thru the decoder. in the format it was using, bit 43 indicated whether or not the csta was included later in the i.d.. in this case it was not. bit 43 would show up as a "2" value in the fifth digit from the right, yielding a 2, 3, 6, 7, a, b, e, or f. im assuming that the manufacturer would have to be contacted in these cases.

   i also ran the beacon i.d. of a much newer plb that i have, and it showed its csta as 199, which was just right. hoperully all of the newer models will have the csta.

   the 406 bursts should come at random times, between 47.5 and 52.5 seconds apart.

   that 3 to 5 mile satellite guess at the position of the elt is about the same as i am used to seeing with the 121.5 beacons. its surprising that a commercial airliner's 406 elt would not have embedded gps position. do u still have the beacon i.d.?

   the "deafness" of the becker/rho theta units on 121.775, and possibly 121.5, may be at least partially attributable to interference from local fm broadcast stations. it is so bad here that we have to instruct air crews to go a certain distance west of town before paying any attention to the df indication. in other directions we have to be almost on the elt before being able to df on it. 
   i have not seen this interference as bad on 121.5 as 121.775. i think this is because if images coming in at 2*10.7 mhz from the receive frequency.

*** added....
   there should not be a need to do much df'ing on 406. the first burst should allow the aircrew to bring the aircraft to bear on the signal. subsequent bursts should just verify that the signal is still in front of the aircraft. once the initial turn is made, the aircraft should fly directly over the elt.

thanks all,
bill