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Old 02 August 2013, 03:07   #11
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There is a GPS test feature which drains battery so should only be used every 6 months. Wonder if that helps when you move major location or just the almanac over time...
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Old 02 August 2013, 03:19   #12
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Completely 'cold' start for GPS with no 'knowledge' of where it is likely to be is usually stated as 15 minutes.
Thanks for the correction as I hadn't looked it up and it was an assumption, hence why I didn't state it as fact. Far better than the previous poster saying 30 minutes though. Floating in the ocean even if prepared would not be a fun experience. In the colder waters that many of us boat in a drysuit is the best exposure protection. Pockets are easy to add to most drysuits, and give a place to store safety gear.
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Old 02 August 2013, 06:29   #13
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Completely 'cold' start for GPS with no 'knowledge' of where it is likely to be is usually stated as 15 minutes. In order to get a position the device needs to receive the almanac over the air. The almanac only "lasts" months so with a PLB/EPIRB it can't be "preloaded". The entire almanac takes 12.5 minutes to be received from the satellite. And this assumes that you maintain visibility of the sky for that whole time. You can potentially save on this time if (i) you have 'in date' almanac files preloaded (not possible on a PLB) and / or (ii) you have some idea where in the world you actually are so the geometry is simplified; PLB's are sold for world wide use and certainly would expect to be in much more than say a 100km radius which might be enough to help.

Smartphones can overcome this headache by (i) downloading the almanac over GPRS/3G/4G etc signal in a fraction of a second and (ii) having a rough estimate of position from its nearest cell towers - but those luxuries don't apply to PLBs.
I guess the SPOT transmitters might have an advantage as they are already 'live' when you hit the button?

Was looking at the Spot, but the annual connect subscription of 99 seems a bit much. Not so sure about the Spot Connect that pairs with your smartphone.
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Old 04 August 2013, 04:10   #14
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Originally Posted by Poly View Post
Completely 'cold' start for GPS with no 'knowledge' of where it is likely to be is usually stated as 15 minutes. In order to get a position the device needs to receive the almanac over the air. The almanac only "lasts" months so with a PLB/EPIRB it can't be "preloaded". The entire almanac takes 12.5 minutes to be received from the satellite.
From what I've read you don't need the almanac to get a fix. The almanac tells it what satellites to expect and makes it quicker to find a new sat when an old one goes out of range. Otherwise it almost uses a process of trial and error to work out whats going on up there and which satellite is talking to it.

Provided you have a multi-channel GPS (12 or 50) you should be able to get a cold fix in 3 minutes or less.

You do need something called "Ephemeris data" which is broadcast for the specific satellite for 6seconds every 30 seconds, but only remains valid for 2 hours. So unless your GPS has been on in the last 2 hours it will take 30-60 seconds for a fix.

The 15minutes quoted is because traditional GPS used a single channel to process all that information, So each satellite was handled one by one. Your modern GPS is a 50 channel unit and can do many things at once.
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Old 04 August 2013, 05:40   #15
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From what I've read you don't need the almanac to get a fix. The almanac tells it what satellites to expect and makes it quicker to find a new sat when an old one goes out of range. Otherwise it almost uses a process of trial and error to work out whats going on up there and which satellite is talking to it.

Provided you have a multi-channel GPS (12 or 50) you should be able to get a cold fix in 3 minutes or less.

You do need something called "Ephemeris data" which is broadcast for the specific satellite for 6seconds every 30 seconds, but only remains valid for 2 hours. So unless your GPS has been on in the last 2 hours it will take 30-60 seconds for a fix.

The 15minutes quoted is because traditional GPS used a single channel to process all that information, So each satellite was handled one by one. Your modern GPS is a 50 channel unit and can do many things at once.
The data is not transmitted on multiple frequencies like you imply. Multichannel receivers aren't receiving multiple radio frequencies they are just using a sort of parallel processing to crunch the very complicated mess of a signal that they receive. The reality is the entire almanac is still only transmitted every 12.5 minutes so if you are relying on getting it 'from the sky' then you have to wait for that long. It MAY be possible to piece together bits of the almanac from signals sent from different sats if they were out of sync - but I've never seen this suggested anywhere - and my experience of turning on 12 / 50 channel gps after long periods of 'off' are that it does take a lot longer than 3 minutes to get a fix.

There are confusing terminologies used by different people. So some people say 'cold start' but this assumes a valid almanac and date etc, whilst others genuinely mean totally first start (sometimes also called a factory start).

There is a long explanation here: How many GPS channels make sense? - Electrical Engineering Stack Exchange

and some wider reading here: GPS explained: Composition of the Data Signal


I'm not aware of any of the PLB makers clearly stating a TTFF which suggests to me that its not that impressive, and there is little difference between the cleverness of the software/hardware!
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Old 04 August 2013, 05:52   #16
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PLB purchase and use

I carry a McMurdo 210 Fastfind with GPS. When I say carry, I mean it's clipped to my LJ and in a pocket - all the time when I'm boating or hillwalking.

It's a few years old and if I was buying again, I'd look for something that floated when deployed. My unit has to be held with the antenna upright, so that's 30 minutes doing that while trying to focus on sea survival as well. Bear in mind, as skipper one may have a crew in the water too and it all needs doing.

My advice would be don't fret overly about the GPS warm up times. It's largely academic. When the PLB is deployed, it transmits a 406MHz squawk immediately which alerts the relevant authorities in your registered country. At this point, they begin to ramp up for a rescue. They will have an area to focus on - probably ten miles square or so. The GPS part of the process has nothing to do with this. They will attempt to contact the owner/other contact by telephone and obtain any information possible to determine if it is a false alarm. Most EPIRB hits are false alarms. They will proceed with a rescue if no information proves a false alarm. By this time they will be launching all appropriate rescue units and directing them to the general search area. When your PLB/EPIRB sends a precise location obtained from the GPS satellites, the CG will get this and update the rescue air/craft.

In addition to this, your PLB will also be transmitting a homing beacon on 121.5MHz. Many lifeboats can use this for homing in on you once they are in your general area.

The best PLB is the one you have in your pocket. If you deploy it, they will find you...
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Old 04 August 2013, 06:08   #17
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I also have a Mcmurdo fing. I've had it surgically implanted, you can never be too safe, eh Willk?
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Old 04 August 2013, 06:31   #18
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I've had it surgically implanted, you can never be too safe, eh Willk?
That's a bit extreme - did you use a scalpel or Vaseline? When that antenna deploys, the helo crew are gonna think you're REALLY pleased to see them

I'm not ashamed to carry my PLB or advise others to do the same.
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Old 04 August 2013, 06:46   #19
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I'm not ashamed to carry my PLB or advise others to do the same.
What a strange comment, why would you be?

I too have mine strapped to my buoyancy aid so I don't go on the water without it.
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Old 04 August 2013, 06:52   #20
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What a strange comment, why would you be?
Either you haven't read the quote that I was responding to, or you're unable to recognise the sarcasm therein...
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