GPS Drift

[segler999]

People have talked in these forums about drift. Firmware, tree cover, reflections, etc., all get discussed.

 

Sometimes I wonder whether the sat constellation itself is the cause. For example, can there be a time period of a few seconds to few minutes when the triangulation gets goofed up until a new constellation fix gets calculated?

 

Yesterday I was hiking along the shoreline of a mountain lake in the Cascades. I had a clear view of the sky down to about 20 degrees above horizon in all directions. I went some 400 yards along the shoreline to take some pictures, then double-back. For about 2 minutes my track showed me hiking about 300' out in the water (to the east of my actual location). My actual was out and back over the same track within a few feet. The track, however, showed this big wow of 300' to the east.

 

My gpsr is a 60Cx with the latest firmware and software versions. By the way, this 60Cx is most excellent under the dense tree cover that you encounter pretty much everywhere on the west side of the Pacific Northwest.

 

Comments?

[+rjmlakota]

Funny you posted this.... I have a 400t and have experienced the same issue you are referring to. I JUST got off the phone with Garmin. That is exactly what I was told. Depending on the canopy cover or buildings around you, you may experience the "drift". I was told this happens because of the high sensitivity receiver that's installed in the 400t.

 

It's annoying though.

 

RJ

[segler999]

However, I would not expect this to occur with a clear view of the sky and no trees or mountains any higher than 20 degrees from the horizon.

[+StarBrand]

Standing next to the Ocean - I would expect a certain amount of multipath error from time - to time. Nothing to do with the "drift" that is often discussed - especially in relation to the Garmin Colorado units.

[segler999]

Let's say you're standing next to the ocean on a flat beach. In all directions around you there is nothing sticking up higher than 3 degrees above the horizon. How can there be any multipath in this instance?

 

I can understand multipath from mountain cliffs, and maybe trees.

[+StarBrand]

A calm ocean is a great relector for GPS frequencies. A Satellite near the horizon and a calm ocean could easily account for multipath errors on a flat beach with you as the tallest object around.

[+twolpert]

Satellites close to the horizon are a major source of multi-path problems. This is why the unit is usally designed to "mask" satellites near the horizon. Seems that I have heard the number "20 degrees" bandied about, but I don't recall for sure.

 

To answer the OP's original question, I would not expect a bad constallation to cause this kind of problem. First, it's not like the satellites jump suddenly from one place to another. Their tracks are continuous, and the unit is continually updating your position based on the current ephemeris data. Second, bad satellite geometry will lead to a higher EPE (meaning more uncertainty in your location). But it should not result in what the forums call "drift" -- which is a long-lasting, gradually accumulating error in your position which does not correct itself quickly even when good reception conditions return.

[segler999]

Good point about the low angle multipath. Thanks.

 

What I didn't check during the wow in my return path along the shoreline was whether the EPE "blue circle" was really big at the time.

 

I'll tell you one thing, tho. This gps technology has completely revolutionized outdoorsmanship (to include, of course, outdoorswomanship). Given the fabulous usefulness of my 60Cx with 24k maps, even under dense tree cover, I'll live happily with the occasional wow in the track.

[+ArcherDragoon]

A calm ocean is a great relector for GPS frequencies. A Satellite near the horizon and a calm ocean could easily account for multipath errors on a flat beach with you as the tallest object around.

Some should try snow!!! Especially the kind-of melted and then refrozen snow that has a nice shiny, slick surface...really causes signal issues...

[MtnHermit]

I'll tell you one thing, tho. This gps technology has completely revolutionized outdoorsmanship (to include, of course, outdoorswomanship). Given the fabulous usefulness of my 60Cx with 24k maps, even under dense tree cover, I'll live happily with the occasional wow in the track.

Totally agree. Where before I could find interesting features on a map, getting me to converge on that point was a guess at best. Now with the 24K maps inside the GPS, it's almost trivial. You didn't mention the plus of having an entire state loaded at 24K, could even get close with paper.

[MtnHermit]

I'll tell you one thing, tho. This gps technology has completely revolutionized outdoorsmanship (to include, of course, outdoorswomanship). Given the fabulous usefulness of my 60Cx with 24k maps, even under dense tree cover, I'll live happily with the occasional wow in the track.

Totally agree. Where before I could find interesting features on a map, getting me to converge on that point was a guess at best. Now with the 24K maps inside the GPS, it's almost trivial. You didn't mention the plus of having an entire state loaded at 24K, could even get close with paper.

[MtnHermit]

I'll tell you one thing, tho. This gps technology has completely revolutionized outdoorsmanship (to include, of course, outdoorswomanship). Given the fabulous usefulness of my 60Cx with 24k maps, even under dense tree cover, I'll live happily with the occasional wow in the track.

Totally agree. Where before I could find interesting features on a map, getting me to converge on that point was a guess at best. Now with the 24K maps inside the GPS, it's almost trivial. You didn't mention the plus of having an entire state loaded at 24K, could even get close with paper.

[+JBnW]

Interesting. Around my home location in Kansas, my MAP76 CSx usually reports an EPE of 12-17 ft. and never better During a recent trip to the Oregon coast, it frequently (almost usually) reported an EPE of around 7-9ft. Seems being near large expanses of flat water worked for me, at least. will have to wait a few months to see if snow has any effect.

[Mag Magician]

Interesting. Around my home location in Kansas, my MAP76 CSx usually reports an EPE of 12-17 ft. and never better During a recent trip to the Oregon coast, it frequently (almost usually) reported an EPE of around 7-9ft. Seems being near large expanses of flat water worked for me, at least. will have to wait a few months to see if snow has any effect.

 

My 60Cx almost constantly give me a reading of accuracy to within 2-5 feet, even in deep forest cover. The drift is something I have learned to live with. You should see the maps I tried to make of a campground hiking trail system. Some real editing was in order, that time.