Mis-use of Satellite Messengers (inReach, SPOT) - Page 4 - ClubTread Community

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post #31 of (permalink) Old 05-31-2016, 03:05 PM Thread Starter
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I'll get those details to you.

We've had other incidences where the older spots with the easily triggered 'help' buttons would get pushed accidently.. >:-(

To add to the rigmarole the coordinates we get from these call centres in the Dallas are sometimes quite far from where the subject actually is, due to terrain and signal strength..

We got one from the middle of a lake, so had to get boats, swiftwater people mobilized, etc.. Then it turns out he's nowhere near water, bumped it by mistake..
Unbelievably frustrating that they either don't send, or the call centre doesn't send, the estimate of the signal error. I've tried my best to talk to the various manufacturers about this, and got no results.

It's as if they designed it without consulting SAR at all.

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post #32 of (permalink) Old 05-31-2016, 03:10 PM Thread Starter
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The SPOT location algorithm uses your back track and a little of the forward track to rectify its current location. That process does not occur in real time. That's why it seems to be on-the-money when you look a the message location.

It's performance under an instantaneous SAR-send would leave you surprised at how poorly it resolves, often hundreds of meters off. This is the issue with SAR often locating SPOT calls a long way from where people actually are. Further, it's almost unheard of for a PLB to be complicit in a false alarm.

A PLB needs no such thing. It powers out a very strong high resolution and low resolution signal of your location, to two satellite networks (GEOSAR and LEOSAR) and it works within metres of your location in almost every case and nearly instantly. No battery to change, no subscription fee, what's not to like.

Like I said, SPOT's are cool and useful. However, EPIRB/PLB's are far more robust at getting SAR to you in any situation.
PLB doesn;t work quite as well as that.
A lot of PLBs don't have a GPS. Those that do have the following issues.
Because it's off most of the time it needs to download the almanac (no assisted GPS) and it can take 20 minutes to derive the first GPS signal.
Even when it gets a lock, it truncates the accuracy of the fix in order to transmit the information over the low bandwidth link to the satellite.

This was the experience we had a few years back
http://blog.oplopanax.ca/2011/08/per...s-perspective/

HOWEVER, all that being said I still recommend a PLB over a SEND for most people - since you only use it in an emergency and it is a registered, internationally recognized standard for emergency rescue.
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post #33 of (permalink) Old 05-31-2016, 05:05 PM
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Wow! The variation in PLB points is not good at all.

Oplopanax, your blog is great.....very informative.
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post #34 of (permalink) Old 05-31-2016, 08:49 PM
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PLB doesn;t work quite as well as that.

This was the experience we had a few years back
http://blog.oplopanax.ca/2011/08/per...s-perspective/

HOWEVER, all that being said I still recommend a PLB over a SEND for most people - since you only use it in an emergency and it is a registered, internationally recognized standard for emergency rescue.
That is a very cool blog, really nicely documented. What was the brand of PLB involved in that situation. For sure a PLB with an internal GPS is essential. I've never seen a higher end PLB or EPIRB (mcmurdo fastfind) take longer than few minutes to download the ephemeris/almanac data but that's not saying it couldn't take longer. I recall, in my marine radio training, the US coast guard evaluations backed that up. I'm trying to find some documentation on that.

In EPIRB/PLB testing i've been involved with (marine mostly) the truncation errors never amounted to more than 20m of resolution, but in that particular situation in your blog perhaps there was extensive obfuscation of the GPS satellites? Did they actually turn off the PLB?

At any rate, it makes total sense to always carry some small pen flares (we do).

Again, very cool blog!

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post #35 of (permalink) Old 06-01-2016, 03:08 PM Thread Starter
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That is a very cool blog, really nicely documented. What was the brand of PLB involved in that situation. For sure a PLB with an internal GPS is essential. I've never seen a higher end PLB or EPIRB (mcmurdo fastfind) take longer than few minutes to download the ephemeris/almanac data but that's not saying it couldn't take longer. I recall, in my marine radio training, the US coast guard evaluations backed that up. I'm trying to find some documentation on that.

In EPIRB/PLB testing i've been involved with (marine mostly) the truncation errors never amounted to more than 20m of resolution, but in that particular situation in your blog perhaps there was extensive obfuscation of the GPS satellites? Did they actually turn off the PLB?

At any rate, it makes total sense to always carry some small pen flares (we do).

Again, very cool blog!
Can't remember the model of PLB, possibly McMurdo

They were in very heavy rain, in a mostly east - west valley so GPS signal was probably as bad as it was going to get. This also affects the speed at which the ephemeris will download, as well as the time to get a fix. The other consideratons are where they placed the PLB, of that I am not sure.

I've been using the wrong word to describe the cold start, time to first fix process by describing it as "downloading the ephemeris". When a GPS has been off for a long time it needs to download the almanac which is transmitted every 12.5 minutes, and can takes 12.5 minutes to download. I'm guessing the worst case to download this is about 15 minutes if you miss the beginning of a transmission and your algorithm can't mash two almanacs together.

If the signal is weak or there is an error in the download it could take longer of course.
https://en.wikipedia.org/wiki/Time_to_first_fix

You're right about the truncation, it would amount to approximately 20m - in this case that would still have given us uncertainty as to which bank of a confluence of three creeks they were on!

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Last edited by oplopanax; 06-01-2016 at 03:10 PM.
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post #36 of (permalink) Old 06-01-2016, 11:04 PM
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Can't remember the model of PLB, possibly McMurdo

They were in very heavy rain, in a mostly east - west valley so GPS signal was probably as bad as it was going to get. This also affects the speed at which the ephemeris will download, as well as the time to get a fix. The other consideratons are where they placed the PLB, of that I am not sure.

You're right about the truncation, it would amount to approximately 20m - in this case that would still have given us uncertainty as to which bank of a confluence of three creeks they were on!
I'd think all the signalling devices manufacturers should take heed of the kind of situation you describe in your blog, which is an excellent precis of a real event.

It would be interesting to hear from the Dept of Defence, who register and oversee these matters. There are perhaps metrics kept at CFB trenton on these things, though they're probably classified :-(

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post #37 of (permalink) Old 06-02-2016, 02:20 PM Thread Starter
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I'd think all the signalling devices manufacturers should take heed of the kind of situation you describe in your blog, which is an excellent precis of a real event.

It would be interesting to hear from the Dept of Defence, who register and oversee these matters. There are perhaps metrics kept at CFB trenton on these things, though they're probably classified :-(
I think the main issue, which is addressed via education and discussions like these, is that the companies over sell the capabilities of all devices.

From the declarations of GPS accuracy (which I dealt with here http://blog.oplopanax.ca/2012/11/your-gps-is-lying/ ) to either over stating, under specifying or otherwise not managing people's expectations on how a device that you need to reply on works.

Somewhere in the mix of advertising and profit seeking there's a break in the responsibility these companies have in properly informing people about the technology they are selling.

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post #38 of (permalink) Old 06-03-2016, 10:55 AM
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Originally Posted by oplopanax View Post
I think the main issue, which is addressed via education and discussions like these, is that the companies over sell the capabilities of all devices.

Somewhere in the mix of advertising and profit seeking there's a break in the responsibility these companies have in properly informing people about the technology they are selling.
good point. I've got some contacts in coast guard/defence that are into these things in a very official way. I'm going to press them a bit and see if there's a way to get access to their findings

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