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epirb gps data

Old 04-21-2003, 10:34 AM
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Default epirb gps data

there was an excellent post here 2 months ago about epirb features that i just re-read, but i have a question about the apparent time delay of gps location information for epirbs.

1) i've read that for the acr rapidfix (and it seems for the globalfix, also), that the data may be up to 20 minutes old because that's how often the unit refreshes position data. does this worry anyone that position data may be 20 minutes old? (running along at 20+ knots you would be almost 7 miles away from that position by the time the epirb actually activates)

2) it seems the pains wessex with internal gps takes 3 minutes to come on, but then it has the current position and broadcasts it. so which is more desirable, longer delay with more accurate position, or faster transmit with older data?
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Old 04-21-2003, 02:25 PM
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Default epirb gps data

I have been researching EPIRB's lately. I think that I previously read that same post and that it sort of implied something it did not mean to imply. Sorry, this is so long, but here is what the ACR web site says:

The RapidFixâ„¢ 406 and GlobalFixâ„¢ 406 are the next generation of 406 MHz EPIRB. Because they transmit your LAT/LON coordinates to SAR forces, they add a whole new dimension to the satellite-aided search and rescue system. The RapidFixâ„¢ 406 is connected to your onboard navigation electronics through an NMEA 0183 interface and provides a position fix within seconds of being activated. The GlobalFixâ„¢ 406 has an internal GPS receiver that initializes within minutes. Both save precious time when it matters most.

Both 406 MHz EPIRBs use GEOSAR satellites that are in geostationary high-earth orbit and can instantly relay emergency transmissions. With your location known from the moment the first signal is received–a matter of seconds with the RapidFix™ 406 or a matter of minutes with the GlobalFix™ 406–your signal can be quickly routed to the closest appropriate rescue agency.

A GPS-enabled EPIRB eliminates the waiting time required for the traditional low-earth orbiting LEOSAR satellites to obtain a fix. On average, the waiting time is about 46 minutes. Those extra minutes can be critical in saving lives, particularly in the case of a medical emergency or a rescue in frigid waters. In a recent test off the coast of Miami, a GPS-encoded signal was routed through GEOSAR and the USMCC to the appropriate SAR facility in less than 2 minutes. Rescue craft were launched within 10 minutes. Conceivably, in a near-shore situation, help could be on the scene in the time it previously took for a LEOSAR to obtain a fix. That's significant progress in the lifesaving process.

A Choice of Technologies

The RapidFixâ„¢ 406 connects to an active NMEA 0183 data source (your vessel's primary GPS), and updates its current position data every twenty minutes. You can initiate a self-test to confirm it has current position data. From the moment you turn your GPS on at the dock until you need the RapidFixâ„¢ 406 in an emergency, the RapidFixâ„¢ 406 is ready to transmit LAT/LON coordinates along with your UIN within seconds of activation. It is best to choose a mounting location in an area convenient for routing the interface cable. RapidFixâ„¢ 406 can be used with a hand-held GPS.

The GlobalFixâ„¢ 406 is completely self-contained. Upon activation, it transmits the 406 MHz signal as any 406 EPIRB does and will continue to do so as long as the EPIRB is on. Its amazing internal 12-channel parallel GPS engine begins acquiring coordinates. It is capable of a cold start initialization within 1.5 minutes; this time lapse is even less in ideal conditions. GPS signals can be difficult to acquire during a cold boot, especially in less than ideal conditions. The GlobalFixâ„¢ 406 will try to gain a position fix for 15 minutes, then will conserve battery power for 20 minutes, then will begin trying again and will keep trying intermittently until it gets a fix. GlobalFixâ„¢ 406 incorporates the latest in GPS technology. There are none better. Once acquired, GlobalFixâ„¢ 406 incorporates your coordinates in the 406 MHz signal. It is convenient to store in a life raft or emergency gear bag. Just turn it on in an emergency. There is no installation or set-up necessary.

In other words, I think the Rapidfix can be 20 minutes off. The GlobalFix and the Paines-Wessex have to boot up (I assume like your regular GPS does in "acquiring" satellites once turned on, but without the benefit of the numbers you set telling it roughly where you turn the unit on).

One post said that in rough water the GlobalFix can have a tough time locating and locking in on a signal. So, if it does not lock in for 15 minutes, you will have been 'in the water' for 35 minutes before it will try to lock in again.

However, I think I will purchase the Cat II GlobalFix. I like the idea of having it in the ditch bag and not having to worry about when (or if) it picked up the last signal of my location. If I have time to grab the bag, then I have it. Otherwise I have to grab the bag and the unit that is (probably) connected to a hard mount. I can also take it on other peoples boats. Lastly, you have to rely on two units to work correctly, rather than one.

If the Rapidfix does not have a signal when I go in the drink, then it sounds like it will never send an accurate GPS coordinate. However, I think it and the GlobalFix send in 406 MHz and 121.5MHz, the latter being the "homing frequency."

My understanding is that the homing frequency generally brings searches within 12 miles, which I read means a search area of 144 sq. miles.

ACR claims their GPS units can be accurate within 0.5 nautical miles in five minutes.

If you buy a 406 unit without the ability to accept GPS (either on its own or through another unit) ACR claims their units are accurate within 2 nautical miles within 1 hour. I think this is because the accuracy increases with a second satellite pass.

My guess is that the Paines Wessex with internal GPS would take about the same time to boot up as the GlobalFix with internal GPS. But that is a guess. I guess it comes down to a decision on manufacturer.

If I am off on this, someone please tell me (as if I needed to ask
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Old 04-21-2003, 08:05 PM
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Default epirb gps data

yeah, that sounds about right to me, and your interpretation is about the same as mine it seems...i much prefer the rapidfix because i don't have to wait for it to come one...wondering if it will come on. but the fact that it is up to 20 minutes behind really bugs me...did you see anything that would indicate that you can do the self test which would cause it to re-initialize the position? then, assuming you had the time to do so, you could at least get it to send your most current position...
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Old 04-21-2003, 08:50 PM
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Try this page:

http://www.acrelectronics.com/rapidfix/rapidfix.html

Then chose view product manual online. Then read 3.7.4 on page 21. I think you may be able to do that.

You just made my decision more difficult.
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