Okay, how's this for perspective. Below I have copied a post from a SSCA forum by a poster who is crew on SAR aircraft.
SSCA Forum • View topic - Reliability of EPIRB
by Kamaloha » Tue Feb 10, 2009 12:54 am
John, many thanks for the compliments. I'm not an expert on too many topics and I have learned a great deal from you and others on this forum about a great many cruising subjects. It is nice to be able to give something back. This is one area where I do have a little expertise. I've lectured to national pilot, medevac and SAR conferences on ELT/EPIRB/SARSAT technology, and I've served as our CAP state director of
emergency services. For 23 years I've also made my living as a medevac helicopter paramedic, so I do worry about getting found if I go down. I'd be happy to post a link to some of my powerpoint presentations but they include a lot of photos of crashed aircraft so they are awfully big. They are also quite oriented toward the aviation community and land-based SAR, and a little off-topic for the cruising community.
Hellosailor, you raise some interesting conjectures, but they aren't borne out by the facts. The descriptions I gave are for
current technology and standards, not past tense. The
GPS receiver is still by far the most
power hungry component of an EPIRB; nearly all modern GPSes use either the SiRF or the Rockwell chipset, and those weigh in at nearly a watt of
power consumption. (I confess to not knowing what the chipset my McMurdo GPIRB uses.) By contrast the 121.5 MHz transmitter uses 100 mW (one tenth watt) at a duty cyle of 90%, for an average power
consumption of 0.09 W, and the 406 MHz transmitter uses 5 watts at a duty cycle of < 1%, for an average power consumption of 50 mW. As far as the schedule of how frequently the
GPS acquires a fix, that is mandated by the
certification standard, and the numbers I gave are for the standard as revised in 2008, so it is not based upon obsolete ideas, but upon experience with the new units and the best of
current thinking... so that really is how these things
work.
As I described earlier, we've been able to track the 406 MHz data burst in our aircraft for as long as you've been able to transmit it; the Becker DF receiver was designed at the same time as the transmitters and it is the de facto standard for airborne search. Thus nearly all aircraft that are looking for you ARE looking at the 406 signal, but due to the reasons I described in my previous post it is the "rough cut" signal and we switch to the 121.5 MHz signal for the "finish work" as soon as we are able. As far as WHY the Becker does not display the GPS coordinates directly in the aircraft, that part I can't explain with authority, but I surmise it is because the system is designed such that the air crew is SUPPOSED to chase the homing signal, not the GPS coordinates, so having them displayed directly would tend to lead inexperienced air crews on wild goose chases. I was disappointed the first time I got to chase a real 406 only to discover that the Becker displays nothing useful; no MMSI, tail number, boat name, or coordinates, only the
serial number of the unit, which must be referenced to the national database to get anything useful out of it. Bummer.
However, it is absolutely the case that search aircraft are homing on the 121.5 MHz signal you are transmitting right now, and not on a set of coordinates being encoded from the GPS. You are correct that overwater search crews do take into account drift as best they can. However, you missed my whole point, which is that we already have ten times more accuracy than we need to find you without the added information of the encoded coordinates. The
satellite will locate you to within two miles without those coordinates, your homing signal has a range of at least twenty miles; and all I need to do is be able to receive your homing signal and I will find you. It's as simple as that. Having "more accurate" information to begin the search doesn't change my prosecution technique one
iota - as soon as I can receive that homing signal, that's what I will be following. I'm not going to delay my prosecution by overflying the coordinates if I can receive your homing beacon; I'll be turning straight for you instead. All you need to worry about is me getting close enough to receive it.
Yes, 121.5 MHz EPIRBs are obsolete (and illegal) but that is not because they are harder to find. It was because of the astronomical falsing rate. They were designed to a very low cost and low
reliability standard, and 99.4% of all activations were false alarms, and every false
alarm required a crew to go look for it (actually not quite true, see below).
The SARSAT technology is completely different for the old vs. the new EPIRBs. On the old system (which was turned off completely ten days ago) low orbit "spy" satellites in polar trajectories carried the detection payloads. Since there were multiple satellites the period between overflights was irregular, but it averaged 45 minutes. It took two
satellite passes with a positive signal hit to resolve the location ambiguity. Because of the astronomical falsing rate, initial satellite resolutions were NEVER prosecuted until a third positive pass. Thus a minimum of an hour and a half to two and a half hours elapsed before the first
alarm bells rang to initiate a search.
With the new 406 system, the improved
reliability standards mean that the false alarm rate is now 93%, still far from perfect but a vast improvement. Now there are detection packages flying on geostationary
weather satellites. These are in a much higher orbit which is why they had to go to a five watt signal. However since you are always in view of a geostationary satellite the initial activation time is measured in seconds, not hours. The first steps used are to look up the
serial number in the international MMSI database and call the series of contact numbers you have provided. This identifies nearly all the false alarms quickly without any search assets ever being dispatched - a phenomenal improvement over the old system.
But - and it is a big but - all that technological advancement is only used to improve false alarm detection and initial asset deployment. Once we are in the search area, it is the old fashioned 121.5 MHz homing signal that completes the mission and finds you, and the techniques we use are all but identical to what has been done since the first ELT's were deployed in 1964.
One final topic and I'll go to
bed. You ask what I do or would own. My aircraft ELT is a GPS-enabled unit. I do not own a hiking PLB but if I did it would be a GPS-equipped unit. The reason for both of these is that an aircraft crash in the woods can be damnably hard to spot - it is amazing how an aircraft can just disappear into the trees. Further, once activated these signals will NOT be moving, so therefore the coordinates will represent an accurate position, and it is quite likely that an aircraft will never be dispatched at all; we'll just send in a ground team with an eTrex. After all, while is is nice to overfly a crash, an aircrew can only search and locate, it takes a ground team to search and
rescue.
For the boat, I do own a McMurdo GPIRB. However I bought it before I had ever searched for one, before I took my AFRCC (Air Force
Rescue Coordination Center) search management course, and before I understood the technology. In other words, I was just as taken in by the hype as everyone else. If I were
buying it anew I would not bother with the GPS feature at sea. For individual use I see no need to attach a GPIRB to a lifejacket. I was resisting counseling anyone to do what I do, and you won't get any grief from me if you achieve greater peace of mind by upgrading - attitude is everything in a survival situation and you will want to feel you've done whatever is best in your own circumstances. It is just that the original poster was severely cash-limited, and adding the GPS almost doubled his
price without statistically increasing his likelihood of being found. I don't wish to boast, but we train every month so I've found and watched others find hundreds of transmitters, and our success rate is 100%. If the
radio is transmitting, we will find you, night or day, clouds or sunshine. It's as simple as that.
It hardly needs saying that none of this applies to the SPOT. Even if it did succeed in getting its message through, it got off the train at the first stop. No aircraft could possibly be equipped to home on it, since it only data bursts every five minutes, without a homing signal like an EPIRB/PLB, and I know of no aircraft receivers in existence that will receive and decode it.
Hopefully I've presented the information you need to make a decision you are comfortable with - and will be effective should the need ever arise. Good luck!
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