EPIRB's - GPS Options

[BobJ]

Greetings all,

I'd been planning to rent an EPIRB again for the SH Transpac, but after the recent tragedy in the DH Lightship I'm thinking of buying one to have aboard permanently.

There is a big price difference between the EPIRB's with internal GPS and the units with a GPS interface. The latter use a cable and infared signal from your separate GPS to send position coordinates to the EPIRB, and then the EPIRB flashes a light to confirm that the coordinates are loaded.

The good thing is that since they're always loaded, the first transmission from the GPS-interface type EPIRB includes the GPS coordinates, whereas the more expensive units with internal GPS have to first be turned on and then must acquire the GPS data from the satellites. Also the GPS interface units are smaller and lighter.

The downside to the GPS interface units is that if the unit is separated from the GPS source it will transmit the last GPS position it had, which could be a problem if your GPS and cable didn't make it into the liferaft with you.

For an EPIRB with either type of GPS input, I would want a Class II so I can decide when I want to turn it on (vs. Class I's which activate when submerged).

Any thoughts on this before I order one? Thanks!

 

[Alchera]

I have the ACR unit with the internal GPS. More expensive, as you say, but I don't have to worry about cables, or verify that the interface is working correctly and that the unit is picking up a valid GPS position. Plus, I want it to continue to update my GPS position even while trailing behind the liferaft on it's tether. That would be difficult to do even if you were able to carry a handheld GPS to plug it into.

In my case, all my GPS data flows along a NMEA bus on the boat which could easily go out if there was a serious problem that affected the electrical system. The distance between the position at which the problem occurred and the position at which the boat started to sink could be significant.

- Mark

 

[Viking Sailor]

I also have a ACR unit with the internal GPS. I can't recommend this unit. A few months ago a boat sank and their new ACR unit never worked and their old unit worked for about 3 hours. So, I got out the instruction manual for my unit and tried to do a self test. It failed! I call ACR and they had me return it for repair. The unit was repaired free of charge but they didn't replace the battery. Thus, I now have to spend $300+ to get a new battery in the next few months. Also, the manual said that I can only check the operation of the GPS functionality of the unit once as it draws down the battery. Before I send my unit back to ACR for a new battery, I intend to try and find another unit that has a replaceable battery. Or, one that a least doesn't cost me $300+ for a dumb battery.

 

[AlanH]

A good read, which in some ways is dated in that it compares 406 EPIRBS to 121 EPIRBS, but who carries a 121 any more? (there's one on Ankle Biter, believe it or not. I gotta take that thing off.) Note that PLB's operate on 121 mhz as well as 406 mhz, just like the usual "big" 406 mhz EPIRBS.

http://www.boatus.com/boattech/epirb.htm

http://www.painswessex.com.au/News_media/WhyGPS.htm

And this independent test on EPIRBS and PLB's done by McMurdo
www.equipped.org/McMurdo 406 MHz Beacons Test Report HiRes.pdf

A nice summary by friends in New Zealand (find the chart at the bottom)
http://www.nztbf.org.nz/epirb.htm

Little sales blurb on an ACR product: http://www.mypilotstore.com/mypilotstore/sep/3382

Moe from McMurdo from a re-seller....look for the little "Location Accuracy Detection Time Diagram" at the bottom that compares units.

http://www.airseasafety.net/epirb.html

****************

I'm going to cut and paste some quotes from various sources on EPIRBS.

from the Boat US link:

The location of a transmitting 406 beacon can be determined within approximately three miles by the first satellite pass, and to within one mile after three satellite passes.

That "three miles" is really a square that's three miles on a side. After three passes that gets more specific to a square that's one mile on a side.

From the Pains-Wessex site:

The system is designed to retransmit a distress message received on 121.5MHz or 243MHz immediately to a ground receiving station (known as a LUT - Local User Terminal). Consequently, unless both the distress beacon and a LUT are within the area of coverage of the satellite at the same time, the distress call will not be received. The 406MHz systems carried on the satellites, however, have the capacity to store messages for rebroadcasting when a LUT is contactable, thus ensuring that all 406MHz distress calls received by a satellite are transmitted to a LUT for action.

This LEOSAR array ensures that a satellite crosses over every place on Earth, and that the average delay before this happens is 45 minutes. In the case of 406MHz transmissions, there may be another delay (again, 45 minutes average) until the message is rebroadcast to a LUT.

Because these satellites are moving with respect to the Earth, the received signals are subject to a Doppler shift in frequency; this shift is used by the satellite to calculate the location of the signal source. Thus a distress transmission from a standard 406MHz EPIRB is detected within 90 minutes and is located to an accuracy of 5km.

**********

5 km equals very roughly three miles, which is what the Boat US article says.

So all this means that the 406 mhz signal needs to be picked up by a LEOSAR satellite, and then transmitted to a LUT. The more-or-less maximum delay time between when you set off your EPIRB and when that signal is initially picked up by a LUT is 90 minutes. In fact, nearly always, it's a whole lot less than that. By the time two more satellites have passed over, meaning at worst 180 minutes more, your position has been transmitted to within a mile. That whole scenario, absolute *Worst Case* means 270 minutes, or four and a half hours. In fact, it's very likely to be a whole lot quicker than that. The chart here: http://www.airseasafety.net/epirb.html
says that average time from setting a standard EPIRB off until the SAR center gets the information is about an hour.

Notice that the limiting factor here is the transmission of data to the LUT.

OK, with a GPS-equipped 406 EPIRB, the GPS signal acquisition is really fast, like 3-4 minutes. You know how long it takes for a handheld to get a good fix. Something I don't understand is how the GPS-enabled EPIRB gets a message to the Search and Rescue guys quicker than the non-GPS EPIRB, becasue the EPIRB STILL has to send it's data to the LUT, right? Hmmmmm...maybe not.. Anyway, while the unit itself knows where it is to within a hundred yards, it still COULD take 90 minutes for that data to get to the LUT....as Worst Case. In fact, it doesn't, it usually takes 5-10 minutes and the accuracy is really good.

http://www.airseasafety.net/epirb.html
...and here it claims that the average time it takes to get your data to the SAR is 2-15 minutes.

Also note that a regular 406 EPIRB with a good battery will broadcast for 72 hours or more. That's important when you're 1,000 miles away from land. A GPS-enabled 406 EPIRB will do the same. However, a PLB (personal location beacon) which uses 406 and GPS technology only broadcasts for 24 hours (worst case) to 36-48 hours.

Lots of stuff here. I looked into this because Joan wanted me to, BTW. ANYWAY, here's my two cents.

For crossing an ocean to Hawaii, or going to Mexico, or something like that, the 90-minute average time delay associated with a standard 406 mhz EPIRB is irrelevant. In fact, by the time the Matson ship that the Coasties call to come pick you up, gets there the satellites will have gone over you 10 - 20 times, they'll know exactly where you are within a mile, and they'll even have a track of your drift. OK, OK, well...that's unless you're really lucky and there's a ship 5 miles away when disaster strikes. Anyway, it's likely that Matson ship can't get to you in 90 minutes from when your alarm goes off, so having it take 90 minutes to get a message to the SAR rather than 10 minutes makes no difference at all.

But when you're going to the Farallones, the Coasties can be there to haul your butt out of the water within an hour of getting the message from the LUT. In that scenario, the best thing to have would be a PLB physically ON you, so that you can't be separated from it. The increased precision of the GPS is nice, the speed of signal acquisition is great, and the fact that it can't broadcast for 72 hours is not nearly as important, since you're close to shore. They're small and light and they cost less than a "big" epirb, though the batteries don't last as long, so I hear..

Upshot.... IMHO... for the two cents my opinion is worth.

For crossing oceans, get a "regular" 406 mhz EPIRB and save a few bucks over the GPS-enhanced version. If you want to spend the money, there's no downside to having the GPS version, but also not a huge increase in safety.

For going to the Farallones, or blasting down to Monterey or Santa Cruz, get the PLB that operates on 406 mhz with an integral GPS.

BEWARE of the SPOT SATELLITE MESSENGER
http://www.traditionalmountaineering.org/News_Spot_PLB-Plus.htm

This uses the globalstar phone network. It also isn't really an EPIRB in that it doesn't send a message to SAR groups....the price is great and it'd be good for a backpacker, but not for us crazies.

All of the above is my humble opinion...

 

[Libations Too]

Alan,

For what it is worth, in '06 when I was just beginning to get Libations Too ready for coastal sailing I looked critically at my safety equipment. Beyond PFDs, jack lines, and tether I did not have much. Like everyone else, I was on a budget and wanted to get the most from my first purchase. My evaluation was similar to yours, and I came to essentially the same conclusion (at least for coastal sailing). I ended up purchasing an ACR PLB, and keep it on my PFD at all times...even when just sailing in the Bay. When I filled out the registration I made it clear that this PLB was one that I carried on my PFD...implying that if it went off I was in the water and wanted help ASAP.

It has turned out to be relatively easy to live with. I sometimes hit my elbow on it and am reminded that it is there, but otherwise I am not much aware of it. One potential problem for me since I wear glasses is being able to open it and activate it while in the water without my glasses. I open it each spring and review the start sequence just in case I have to do it without the benfit of my glasses.

I was happy with this first purchase and have since added all of the other safety items required for the LongPac...all of which just make good sense for anyone venturing far out the Gate.

 

[BobJ]

Lots of good info - thanks.

I took a PLB in '06 but wasn't real confident about it (and it's a piece of gear you want to be confident about). From what I read, it wasn't clear that a signal from it would get to the Coast Guard right away. My recollection was that because it wasn't a EPIRB, the call would go through other agencies first, until they figured out it was registered to someone with a boat. Also, it's only supposed to run for 24 hours vs. 48 for an EPIRB and as one of you wrote, it could take awhile for a vessel to reach you. So this time I'm getting the real deal.

The info about the big ACR unit is also interesting/disconcerting. That was my dream unit - I figured I'd have to settle for something less expensive. Maybe I'll look again at the McMurdo line.

Any thoughts about the Class I (automatic) vs. Class II (manually-deployed)? I don't want the thing going off until I tell it to, or do I?

 

[Libations Too]

Bob,

I thought the PLB and EPIRB signals went through the same network...no difference in terms of who recieved the signal or who managed the response. Do you recall where you learned that they were handled differently? Anyone else know?

(Assuming both PLB and EPIRB are 406 MHz units)

 

[tiger beetle]

Bob,

I thought the PLB and EPIRB signals went through the same network...no difference in terms of who recieved the signal or who managed the response. Do you recall where you learned that they were handled differently? Anyone else know?

(Assuming both PLB and EPIRB are 406 MHz units)

The difference I can determine between the two is the battery life, as previously noted 24 hr for PLB and 48 hr for EPIRB. A possible difference is the antenna mechanism (I do not see an external antenna on the PLB).

The 406 PLB and EPIRB, according to NOAA SARSAT, utilize the same satellite constellations (GEO and LEO satellites).
http://www.sarsat.noaa.gov/emerbcns.html

Both units should also broadcast a 121.5mHz low power signal for purpose of radio direction finding when Search and Rescue folks are in the area. Check the individual unit you're interested in to verify that it has this feature.

Tiger Beetle has an ACR RapidFix 406 Category II (manual activation) EPIRB stowed near the companionway - this EPIRB accepts a GPS cable and I did not install the cable (the cable was too short to reach from the GPS to the EPIRB storage location). The battery will be replaced in May at a cost of approximately $300. I asked Sal at Sal's Inflatables how long the battery replacement takes and he said it's either two days if the EPIRB case passes pressure testing to confirm watertight seals, and two weeks if the case fails and must be sent back to ACR for repair.

If money were no object than an EPIRB with on-board GPS makes sense; given the price differential I went with the less-expensive EPIRB without the built-in GPS.

I've never been a big fan of float-free Category I EPIRBs as you have to stow them on deck in a relatively large canister to protect the unit from physical damage. That's why I purchased the smallest Category II unit I could find and placed it belowdecks near the companionway.

- rob

 

[BobJ]

Bob,

I thought the PLB and EPIRB signals went through the same network...no difference in terms of who recieved the signal or who managed the response. Do you recall where you learned that they were handled differently? Anyone else know?

(Assuming both PLB and EPIRB are 406 MHz units)

The PLB I used in '06 was borrowed from a friend who wore it during the ARC. He told me about the routing of the emergency signal and it made sense, since the PLB's are used by hikers, skiers, etc. It's the human element in the routing I'm concerned about. I assume once they track the signal to the ocean they turn it over to the Coast Guard but being the government, how long might it take for that "hand-off"?

Since no doubt some bells went off when I said "borrowed," I will add that with my friend's permission I re-registered the unit in my name and in the "remarks" section of the registration, listed the name and description of my boat. I also amended my FCC Ship's Station License to list the PLB. But which if any of my remarks got into the computers of the appropriate agency?

Rob reminded me of my other concern with the PLB (a FastFind Plus w/GPS) - its antenna is a little folding thing that pops up when you activate the unit. In the brochure it shows the activated unit sitting on a rock and it says to keep the unit upright with antenna exposed. I was concerned that if I was in the water, I might not be able to do that. I'm also not sure about how water-resistant the unit is, especially the antenna. I could do further research but would rather just get a unit that is designed for use on a boat.

 

[AlanH]

It's good to note that both PLB's and 406 EPIRB's also emit low power 121 mhz signals for homing. In fact when the 47 foot Coast Guard cutter is coming out to get you, they're homing in on the 121 mhz signal. So all the 406 mhz signal has to do in terms of accuracy is get the cutter close enough that it picks up a solid 121 mhz signal. 1 mile is easily close enough for that, seeing as over-flying aircraft at 20,000 feet will pick up your 121 mhz signal. So again, is the extra-precise positioning data of the GPS-enhanced units really a big deal?

OK, OK, the Matson ship that's picking you up 1,000 miles out probably doesn't have a 121 mhz receiver, but by the time the Matson ship is on the scene, looking for your liferaft, the satellites have flown over you 5-20 times and the accuracy of the signal is within a mile... plenty precise enough for locating a liferaft at sea. In this case, the extra accuracy of the GPS-enabled unit might be a little bit nicer, it's just a matter of whether you want to pay for it.

I thought it was interesting that during EPIRB tests off Santa Cruz, signal strengths were increased in a crowded liferaft by holding the unit off of the floor so that it wasn't surrounded by bodies.....obviously not an issue for us.

I still don't understand how/why the PLB and the GPS-enabled EPIRBS get a signal/message to the SAR folks so much faster than the non-GPS enabled EPIRBS. I believe that all 406 mhz signals are routed to the LUTs, and so the same delays in terms of satellite positioning should apply to both. If anybody knows the answer to this, I'd love to find out.

 

[Viking Sailor]

The way it works is that the message sent by the GPS enabled EPIRB contains your position. So, from just one satellite pass the rescue folks know where you are. For EPIRB's without access to GPS position info it takes multiple satellite passes to determine your position. Thus, it could take many hours before a rescue attempt is started.