Pressing the help button: Drama on andromea

[David Thomas]

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]

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]

"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.

[Bob Gross]

"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]

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]

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]

"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]

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

[Greg Mihalik]

"vs a 0.5 to 1 watt signal from SPOT"

Now that's a bad joke ….

[Bob Gross]

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]

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

High frequency fairs worse through trees.

How about through water vapor (atmosphere)?

[Bob Gross]

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]

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]

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]

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.

[Bob Gross]

"All GPS receivers will struggle in tough terrain"

Part of the problem here is that there is no definition of what constitutes tough.

In many terrain situations like you describe, the problem is not one of acquiring sufficient satellites. The problem is multipath interference caused by signals reflecting off one vertical mountain and then over to the receiver antenna in an indirect shot from space. The patch antenna being cheap and tiny makes that worse. An external antenna that is powered is much better for rejecting multipath. However, that would make a new reliability problem and draw more battery power. Some of the more sophisticated receivers can detect that this is going on, but a choke ring antenna is the only way to stop it dead, and they are heavy and bulky.

When I was going to climb a cold peak on the East Coast, I knew of the potential problems such as these, so I used an external GPS antenna and stuck it in my hats, between the inner and outer layers. The cable ran down my neck and into a warm inside pocket where the receiver was. I ran it on lithium primary batteries to make sure that I had plenty of juice. The antenna was oriented upward on top of my head, and it gave me a perfect tracklog despite the steep terrain and vegetation.

–B.G.–

[Hikin’ Jim]

The one think I'm not sure that's been highlighted enough is that a PLB will work even if it gets no GPS signal at all. A rough position can be calculated with one pass of either a LEO (low earth orbit) or a GEO (geosynchronous earth orbit) COSPAS-SARSAT satellite using Doppler shift calculations on the signal. Each subsequent pass gives a more accurate position. A SAR team can then find the distressed party by homing in on the 121.5 MHz secondary signal.

With a SPOT, you must get an accurate GPS location, period. There is no secondary way to calculate the position, and there is no homing beacon.

HJ
Adventures In Stoving

[Roger Caffin]

> A rough position can be calculated with one pass of either a LEO (low earth orbit)
> or a GEO (geosynchronous earth orbit) COSPAS-SARSAT satellite using Doppler shift
> calculations on the signal.
It sticks in my memory that large commercial jets may also pick up the PLB signal?

Cheers

[Mike In Socal]

Didn't Cospas/Sarsat stop processing 121.5 MHz signals in 2009?

http://www.sarsat.noaa.gov/phaseout.html

Mike

[Roger Caffin]

> Didn't Cospas/Sarsat stop processing 121.5 MHz signals in 2009?
My understanding, which may be wrong, is that they stopped processing the 121.5 MHz signal, but most search groups are still using it as a beacon for directional antennae. My understanding is that it will continue that way for some time too.

Mind you, I would welcome an update.

Cheers

[Hikin’ Jim]

> Didn't Cospas/Sarsat stop processing 121.5 MHz signals in 2009?

As a primary signal, yes. In other words there aren't satellites that are going to pick up a 121.5 MHz signal and create an alert.

The 121.5 MHz frequency is now used as a homing beacon by the Civil Air Patrol (CAP), Search and Rescue (SAR), etc.

HJ
Adventures In Stoving

[Rex Sanders]

121.5 MHz is also the International Air Distress frequency for use by non-military aircraft. According to Wikipedia:
121.5 MHz is monitored by most air traffic control towers, FSS services, national air traffic control centers, and other flight and emergency services, as well as by many commercial aircraft.

NOAA says:
PLBs also have a built-in, low-power homing beacon that transmits on 121.5 MHz. This allows rescue forces to home in on a beacon once the 406 MHz satellite system has gotten them "in the ballpark" (about 2-3 miles).

Also, a small correction:
The GEO satellites are not able to use Doppler location processing since they have no relative motion between them and the emergency beacons.

GEO satellites relay your PLB's serial number and GPS position, if it can acquire one.

— Rex

[David Thomas]

>"most search groups are still using it as a beacon . . . I would welcome an update."

All the pilots I know in Alaska who participate in S&R are listening to 121.5 as they fly their search patterns. The most conscientious ones leave one of their radios to 121.5 whenever they are flying, especially on cross-country legs and in remote terrain.

[Hikin’ Jim]

Thanks for the correction, Rex.

HJ
Adventures In Stoving

[Stuart R]

"The problem is multipath interference caused by signals reflecting off one vertical mountain and then over to the receiver antenna in an indirect shot from space."

Here is a real life example of multipath interference. I was walking along a trail part way up the south side of the valley and at one point the GPS thought it was a mile to the north on the opposide of the valley. It doesn't happen often, but when it does it's way out.

Add: This track was recorded using a Garmin GPS in the top of my pack, and this location is at latitude 43N

[Author]

Posts