Info on "B" Solutions in 406 DF

[Avery]

My squadron participated in a UDF 406 ELT search on one of the islands in the Florida keys. We were told we were searching for, and I hope I got this right: a "B" solution. I'm assuming there was an "A" solution, because the "real" ELT was going off near Ft. Lauderdale. We heard nothing. I've been googling and only find vague references to A and B solutions. Either an answer or directions to find the answer would be helpful.

[sardak]

When Sarsat makes a pass over a signal, the ground computers determine an A and B solution, equal distance either side of the satellite ground track.  This is because the Doppler shift of the signal determines the location where the satellite passed the beacon, but it can't determine which side of the track the signal is on (the antenna is omnidirectional).   Then (taken from the Sarsat documentation):

A subsequent satellite pass on a different satellite track can be used to resolve the ambiguity [is it A or B?].  An estimate of the true (A) and image (B) location can also be calculated by taking into account the Earth rotation when computing the Doppler solutions. However, this ambiguity resolution technique is dependent upon the stability of the transmitted frequency and can only be used reliably with 406 MHz distress alerts.

With 406 beacons, the system calculates the probability of whether the A or B is the most likely location.  On the initial pass, the probabilities are 50/50 (higher math at work there).  With subsequent passes the probability of one solution over the other should increase.  Sometimes the probabilities will flip, i.e. the A solution appeared to the true location, then the system flips to the B side having a higher probability.

A GPS encoded 406 beacon causes the system to generate an E position, which is the position of the GPS coordinates transmitted by the beacon.  Even if the system receives GPS coordinates from the beacon, it still computes an A and B position. The system also computes a merge combining the GPS position with the Doppler position. (Which is interesting, because this potentially dilutes the accuracy of the GPS position).

The messages generated by the Sarsat system, explanation of A, B, E positions and ambiguity; and explanations of high, low and missed passes can be found in this document:
http://www.sarsat.noaa.gov/rccmsgsv18.pdf

Mike

[Avery]

Ah, the little known rccmsgsv18.pdf reference. Thanks, Mike

[BigMojo]

Was this back in January? I remember a phantom EPIRB we had back then that we never got a signal on, and I know they launched a plane down your way the next day...

[Avery]

Mission was on or about 17 Jan this year. We launched a UDF team, with no joy on 121.5 in Islamorada.

[Avery]

Why I'm so interested in "A" and "B" solutions: because I believe, as a pilot, that having a GPS feed to your 406 ELT is the only way to go. Save your money today, buy it first of next year. A 406 ELT without GPS is not the way to go, and if 'ya look at the way the new system works, the Golden Hour may be long past using only a 406 ELT signal. My Son, the pro in the family, actually flies with two GPS fed 406 ELTs. 'Course he flies in Africa, where such redundancy is appreciated.

[BigMojo]

Mission was on or about 17 Jan this year. We launched a UDF team, with no joy on 121.5 in Islamorada.

Yup...that was the one. I was in LA at the time, but I know we had a UDF team and an Aircraft out. Never picked up a signal, I heard the FCC was called in and they couldn't find it either.

[jimmydeanno]

in Islamorada.

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I'm going to have a house there in a few weeks...hopefully...if I'm lucky...
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