Forum Index » General Lightweight Backpacking Discussion » Pressing the help button: Drama on andromea


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David Thomas
(DavidinKenai) - MLife

Locale: North Woods. Far North.
Re: High Latitude and SPOT on 11/05/2013 18:35:03 MST Print View

Sorry, I was sloppy in my wording. Of course GPS is GPS. But as Dena details, the outbound communication by SPOT and by PLBs are handled by different satellite networks. My SPOT often knows where it is. But it can't reach those low-angle communications satellites very reliably.

My understanding is that the ACR PLB:

http://www.rei.com/product/815753/acr-electronics-resqlink-406-gps-personal-locator-beacon

also pings the polar-orbitting constellation of Russian satellites. Which is good, because we can see Russia from our house. So that is what I carry. Looks like I'll be doing a road trip to the Arctic Ocean in January. I deactivated and PIF'ed my SPOT, but it would almost be fun to bring one and leave it on track mode on the Dalton Highway to see how it does.

One of the above sentences is sarcasm.

Edited by DavidinKenai on 11/05/2013 18:37:26 MST.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Re: High Latitude and SPOT on 11/05/2013 18:44:10 MST Print View

The Globalstar 32 satellite network is also a "birdcage" configuration -

Globalstar

All things be equal, it would Seem like uplinking the Help request would be well covered.

Which brings me back to the conclusion that it is the SPOT/Globalstar infrastructure that is the problem, not the acquisition the initial fix or getting the initiating 911 to a satellite.

Edited by greg23 on 11/05/2013 19:00:22 MST.

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: Re: Re: High Latitude and SPOT on 11/05/2013 18:46:11 MST Print View

All of this is making my head spin. Glad I don't own a Help Button.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Re: Re: Re: High Latitude and SPOT on 11/05/2013 18:50:08 MST Print View

Blame it on Dena - "The location accuracy issue is one of the reasons I use a PLB instead of a SPOT."

That's what got me started. And I although there may be one, I don't think it's related to lattitude.

In the meantime, if you push that button, leave the unit ON!

Edited by greg23 on 11/05/2013 18:57:28 MST.

Rex Sanders
(Rex) - M

Locale: Central California Coast
Drama on andromea: Satellite info on 11/05/2013 19:09:35 MST Print View

Time for correct information on satellites at the location of this incident.

GPS satellites are in medium-altitude earth orbits (MEO) at 20,200 km (12,600 mi) altitude, designed to provide global coverage with at least 6 satellites visible at any time. As others have pointed out, fewer satellites may be visible for many reasons, but you only need 3-4 for a pretty good fix. More at www.gps.gov

SPOT devices send GPS positions through Globalstar satellites, and do not use Doppler ranging. 32 Globalstar satellites are in low-altitude earth orbits (LEO) at 1,400 km (870 mi) altitude, which could provide global coverage. Globalstar restricts coverage for economic and legal reasons.

The location of this incident (Columbia Icefields in Jasper National Park), is very much inside Globalstar's simplex data coverage area, with a downlink station a few hundred miles away in High River, Alberta. For Globalstar, that's about as good as it gets.

PLBs use two different satellite systems in different ways. Geostationary satellites "hovering over the equator" at 35,786 km (22,236 mi) relay GPS fixes from PLBs to ground stations. LEO satellites can compute a PLB position independently using Doppler ranging. More at http://www.sarsat.noaa.gov/emerbcns.html.

So theoretically, as far as the satellites are concerned, a SPOT emergency beacon at this location should have no trouble getting a GPS fix and sending that position accurately through the SPOT system.

In practice, we've seen many reports of SPOT signals not delivered, or delivered with wildly inaccurate positions. Combined with operational misunderstandings by users, and SPOT devices have a poor reputation for reliably reporting positions.

I believe that many SPOT problems which are attributed to user error, actually originate with the poor design of these devices. Why should you have to Memorize The Fine Manual in order to use something properly under the highest-stress conditions? I'll save that rant for another time.

And for more than you wanted to know about satellite systems and devices, start here.

-- Rex

David Thomas
(DavidinKenai) - MLife

Locale: North Woods. Far North.
Re: Re: Re: Re: Re: High Latitude and SPOT on 11/05/2013 19:15:24 MST Print View

From wikipedia: (SPOT) has a coverage area that includes a large portion of the planet, with the following exceptions: extreme northern and southern latitudes, south-eastern Africa, India, and adjacent areas immediately north of India.

Like maps of cell-phone coverage, this Globalstar promotional page seems overly optimistic, but clearly isn't a constantly moving constellation providing global coverage:

http://www.globalstar.com/en/index.php?cid=106&sidenav=232

David Thomas
(DavidinKenai) - MLife

Locale: North Woods. Far North.
His map versus mine. on 11/05/2013 19:21:11 MST Print View

Interestingly, the map that Rex linked ("here", the last word in his post, above) is much closer to the experience friends and I have had in Alaska than the Globalstar map I posted a link to.

Rather like the trying to use my cell phone - the map AT&T distributes is WAY too optimistic.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Drama on andromea: Satellite info on 11/05/2013 19:21:54 MST Print View

"Geostationary satellites "hovering over the equator" at 35,786 km (22,236 mi) relay GPS fixes from PLBs to ground stations."

So one might think that reaching a LEO birdcage (by SPOT) would be easier than reaching a GEO (by a typical PLB), and hence More reliable.

And it may be so, but what happens after that point is the crux of the question.

It seem that marketing and economics clobber the science. I concede.

Edited by greg23 on 11/05/2013 19:23:56 MST.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: "Pressing the help button: Drama on andromea" on 11/05/2013 19:27:55 MST Print View

"So is poor GPS coverage in Alaska just a "legacy myth"?"

Greg, as always, the answer is _it_depends_.

As I stated previously, the farther you go in latitude, the worse it gets, but that never gets to a failure point on its own. Terrain will foul you up pretty fast, and that obviously varies from place to place. Trees and obstructions will also foul you up pretty fast, but it varies even more. Part of the problem is when all of these factors start to add up against you. The user is thrown into a least predictable situation. Most of the time, it still works, but how much do you want to risk on "most of the time" today?

Sure, out of ten people in need of rescue, the GPS fix will be sufficient for nine of them. Do you want to volunteer to be number ten?

My earlier point is that if you have the opportunity to think fully about your situation, you will know how to get a good GPS fix in Alaska. You will know to get on the south-facing side of the hill, and not be in the shadow of any peaks or reflectors. You will know to stay away from big radio, television, and microwave emitters. The list goes on and on. Most of the time, you don't need all of those details. However, if your life depended on it...

GPS receivers get very squirrelly when you get down to about four satellites.

--B.G.--

Randy Nelson
(rlnunix) - F - M

Locale: Rockies
Spot accuracy on 11/05/2013 19:31:41 MST Print View

All I know is my SPOT has been very reliable in CO, UT, and WY. And when I go to Google maps and zoom in on the pics, they are pics of my campsites. Maybe not exactly where I left the SPOT when I sent the OK, as I do place it in the most open area close to camp, but close enough that I wouldn't have to raise my voice to be heard by SAR.

Nick, I actually got mine to placate my wife about going solo but have come to value it. It's true that if you leave an itinerary and don't deviate from it at all and leave a call SAR by date/time you will probably not end up like Aron Ralston or Mike Turner. But if you find your self deviating from your route due to unforseen conditions or simply because of "I wonder what's over there?" it's good to have that 911 button. (My Help button says bailing to alternate trailhead. Pick me up there and bring a sandwich and a cold beer. Just kidding. It does say to pick me up at alternate trailhead but my wife already knows the rest. :) ) I've definitely been on large, shifting rocks where those kinds of things can occur. I prefer to be able to summon help to my actual location if the unlikely occurs. I certainly understand why you might not want to carry one. Probably a lot of it depends on where you are going and what the conditions are.

Rex Sanders
(Rex) - M

Locale: Central California Coast
Re: Re: Drama on andromea: Satellite info on 11/05/2013 19:32:38 MST Print View

So one might think that reaching a LEO birdcage (by SPOT) would be easier than reaching a GEO (by a typical PLB), and hence More reliable.

Except it's not. PLBs determine your position and relay an SOS message using two independent satellite systems: GPS positions plus GEO satellite relay, AND LEO satellite Doppler ranging plus LEO satellite relay.

And PLBs use a much lower frequency (406 MHz vs 1610 MHz for Globalstar), that's better at punching through vegetation.

PLBs are much more reliable than any other satellite system, for sending an SOS signal and position under all conditions.

But sometimes you need more than that.

-- Rex

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Re: Re: Drama on andromea: Satellite info on 11/05/2013 19:37:18 MST Print View

"PLBs are much more reliable than any other satellite system, for sending an SOS signal and position under all conditions."

Oh, I'm a believer. I carry and will continue to carry PLB on all ventures, big and small.

And now I'm a much better informed believer.

Rex Sanders
(Rex) - M

Locale: Central California Coast
PLBs: One more feature on 11/05/2013 19:42:05 MST Print View

PLBs also send a 121 MHz homing beacon that helps aircraft and ground crews find you when they get within a few miles.

[Edit} And PLBs send a 5-watt signal at 406 MHz, vs a 0.5 to 1 watt signal from SPOT at 1610 MHz.
5x to 10x more power is good in this case.

-- Rex

Edited by Rex on 11/05/2013 19:51:36 MST.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: PLBs: One more feature on 11/05/2013 19:53:26 MST Print View

"vs a 0.5 to 1 watt signal from SPOT"


Now that's a bad joke ....

Edited by greg23 on 11/05/2013 19:53:57 MST.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: PLBs: One more feature on 11/05/2013 19:57:45 MST Print View

You need to be a little careful when you start comparing two different uplink signals. Different frequencies will propagate differently. The encoding schemes are different. Yes, in general, more power is good, but it is not the entire story.

--B.G.--

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Re: PLBs: One more feature on 11/05/2013 20:05:59 MST Print View

"Different frequencies will propagate differently. The encoding schemes are different."

High frequency fairs worse through trees.

How about through water vapor (atmosphere)?

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: PLBs: One more feature on 11/05/2013 20:16:48 MST Print View

There are many different layers of the atmosphere. For some, water vapor is an issue. For some layers, they are simply sensitive to some frequency bands. That's why the original GPS designers used 1.57GHz for the primary civilian downlink, but then they blast it down 11,000 miles with a lot of watts. Plus, it matters whether you are transmitting the uplink signal or receiving the downlink, or vice-versa.

It is possible to put certain things in a mil-spec satellite that is 11,000 miles up, and it is possible to put certain different things in a commercial satellite that is a small fraction of that altitude.

For one thing, GPS is intended to survive World War III. It might be injured, but the clock will still be ticking.

--B.G.--

Bruce Tolley
(btolley) - F

Locale: San Francisco Bay Area
Press the help Button on 11/05/2013 21:45:37 MST Print View

I have been using one carrier pigeon per trip as recommended by Rex in his very excellent three-part article.

My wife appreciates the update on my progress and so far has not eaten the messenger.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: "Pressing the help button: Drama on andromea" on 11/05/2013 22:44:02 MST Print View

Hi Greg

> I understand failures in the on-the-ground infrastructure of SPOT, but have not
> of "accuracy issues" - at least where good procedure was followed.
First of all, it was a USER failure. They turned the SPOT off before it had a chance to get a valid GPS reading. This has nothing to do with the infrastucture. RTFM indeed!

We have a couple of Product Reviews of the SPOT here at BPL. They were used carefully during testing. In one case mine put me about 1.5 km out of position.

Cheers

Mike W
(skopeo) - F - M

Locale: British Columbia
Pressing the help button: Drama on andromea on 11/06/2013 01:25:31 MST Print View

My field experience with my Spot would suggest that these guys couldn't have done the "right thing" even if they tried. If they had left their Spot turned on while clinging to the side of a mountain, they wouldn't have gotten a GPS fix. Spots GPS receiver is just too poor in difficult terrain or tree cover.

Since these climbers were pressed against the side of a mountain, the Spot device would definitely struggle to get a fix and I'd bet it would have failed regardless of how long it was left on. So the Spot is programmed to send the SOS without GPS coordinates, that is how Spot was designed. Not a bad thing.

The next issue was once they started moving, they should have sent an "I'm OK" message, EXCEPT for the fact that Spot won't send an OK message without a GPS fix!!

Once again, the Spot is programmed to not send OK messages without GPS coordinates. So the climbers may have forgotten to send the OK message, but it wouldn't have worked until they had a good GPS fix, which would mean that they would have to be well away from the mountain before an I'm OK message could be sent.

All GPS receivers will struggle in tough terrain and I saw a good example of this on a backpacking trip this fall. I was hiking along the edge of a lake that was sandwiched between two mountains. My position on my Garmin GPS (the gold standard in recreational receiver technology IMO), showed me walking a perfect track along the east side of the lake... when I was actually walking along the west side of the lake. I've seen wild coordinate jumps before but I have never seen a perfectly formed track misplaced for such a long time. The track moved along the wrong side of the lake for over a mile before jumping a half mile back across the lake to my actual position once I cleared the mountains. The GPS was sitting on top of my pack facing skyward.