Gaia: Multipath error

[Jon Fong / Flat Cat Gear]

I was out backpacking in SEKI up to Hamilton Lake.  We were playing with Gaia and bread crumbing and got this cool error.  Apparently the satellite signals were bouncing off some of the granite there.  Weird stuff.

 

[Kevin Babione]

What error?  It looks like you pulled out your packraft a couple of times…

[Jerry Adams]

I’ve seen errors like that with my Garmin GPS.

Once, I got to a road and walked one way down it.  The GPS track showed I was going the other way.  It was obvious I was going the right way so I ignored it, but thought it rather curious.  Eventually, it snapped back to the correct location.

[William Chilton]

I’ve often seen this with Garmin in narrow areas with high cliffs. The satellite signals reflect off the cliff and confuse the device.

[Geoff Caplan]

Yup – GPS is an aid to map reading, but only the naive would use it as a stand-alone alternative. The old skills still count.

[Roger Caffin]

There are a lot of naive around.

Cheers

[Chris R]

When surveying one of the reasons we use a 2 metre pole is to reduce the effect of gps signals bouncing off the user’s head. Mind you, we are aiming for sub 20mm accuracy.  When we take a measurement our equipment takes the best of 5 readings for each point recorded, unfortunately it relies upon a cell signal to QC the gps signal.

[Jerry Adams]

or, just keep in mind that GPSs have errors like this

if a GPS tells you to walk into a lake or over a cliff or drive up a snowy pass, don’t do it

and, GPS can stop working so always be looking around and keeping in mind where you have to go to get back to the car, like “my car is down that particular canyon on the other side of that peak”

[NoCO-Jim]

Of course, the classic episode…

https://vimeo.com/15390422

[PaulW]

Thanks for pointing this out Jon.
I experienced this all the time with Gaia here on the Colorado Plateau, but it never occurred to me it was a multi-path error, mainly because the problem was much worse with Gaia than with other GPS apps I was using. When I first encountered the error, I reported it to Gaia, but got nowhere with their tech support, but some searching on my own led to an article (on the Gaia website) entitled “Android – Track recording inaccurate or has spikes”. I find that if I run another GPS app along with Gaia, the track recording is much more accurate and I see few spikes even while deep inside narrow canyons and washes.
I’m currently using GPS Locker, but I’ve tested the Caltopo app too and they both help. The downside is that battery usage goes up, but for day hikes this is rarely an issue.

[Jon Fong / Flat Cat Gear]

I knew the trail up to Hamilton Lake, we were using the GPS for fun.  By recording the path, you can track speed, elevation change and track progress.  One of the things thta I found interesting was the difference between our route and a downloaded gpx track.  The other thing that was interesting was the repeatability of the reading (see below).  The gpx track is in blue and our path is in red/orange.

 

[Eric Blumensaadt]

Hey, I always carry a map and compass B/C I know the compass needle always points to the right direction to walk. Right? ;o)

Seriously though, I recently found an old SILVA Huntsman compass, Silva’s smallest rotating bezel compass back in the day. So I’ll be carrying that instead of my larger SILVA Ranger.

The Huntsman is made with an aluminum base and “lid” with a brass safety pin/hinge so you can pin the compass to your shirt for hands-free bearing use. Neat. Wish SILVA still made it.

[Jerry Adams]

I carry a zipper pull compass

It would enable me to keep going in a straight line walking through a forest

[Geoff Caplan]

As a crusty old-time navigator, I’ve resisted the use of GPS. But I’ve recently moved to the Lake District, and the terrain here is so rugged and complex that it can be really helpful to be able to pinpoint your position when visibility is bad. So I’m carrying it regularly these days.

I was careful to select a phone with a good GPS chip that picks up all the satellite networks.

I’ve been very impressed by the accuracy, even when surrounded by relatively big cliffs. Acquisition takes under 2 secs, and it’s been pinpoint to within a meter or two every time. I’m using ViewRanger, which is a professional level app used by dozens of SAR teams around the world.

I wonder how much the software and the chip factor into multi-path errors? Does anyone have any actual data on this?

[Roger Caffin]

I wonder how much the software and the chip factor into multi-path errors?
No data, but I suspect that any GPS chip will find it very hard to distinguish between a good signal and a reflected signal. The receivers are just not that smart, and probably cannot be that smart.

Whether the SW could flag the fact that the users seems to have suddenly moved 2 km up a mountain side in the space of the last 10 minutes – difficult. You might need an embedded topo, a larger processor, and a bigger battery and a lot more SW to handle that.

Cheers

[Jerry Adams]

That is a time when you can just use your brain

Everything doesnt need a technological solution

[Pedestrian]

“That is a time when you can just use your brain

Everything doesnt need a technological solution”

A technological solution is nothing but the application of distilled knowledge (“software”/”firmware”/”hardware”) from possibly many smart brains (“algorithms”). You’re simply leveraging many other brains by using “technological solutions”.

I know it’s easier to be lazy and say something trite like “Everything doesnt need a technological solution”.

 

“If I have seen further than others, it is by standing upon the shoulders of giants.”

Isaac Newton

 

 

[Pedestrian]

And this…..

Which can predict one’s reaction to any technology.

 

 

[Jerry Adams]

Yeah, maybe that came off wrong : )

My GPS seldomly tells me to drive into a lake, and the rare time it does I just drive around, so I can live with that just fine.

Phone with GPS is great.  I could see not bringing a paper map at all.

[Rex Sanders]

Illustration of GPS multipath, from Wikimedia

Mitigating GPS/GNSS multipath errors has been a research topic for decades. Proposed and sometimes implemented techniques include checking for frequency, phase, and code differences; better antenna design; and other complex techniques.

However:

“Currently, the vast majority of smartphones in the market are configured for a single-frequency GNSS … it is not resistant to multipath errors which can surface from time to time.”
https://www.geospatialworld.net/blogs/advantages-of-dual-frequency-gnss-in-smartphones/

Could be worse. In 2003 GPS signals in Moss Landing Harbor, California were effectively jammed by three poorly designed off-the-shelf amplified TV antennas!

— Rex

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