Garmin 492 GPSmap shows motion when stopped

FourPoints · Aug 23, 2011

I think you aren't very observant

every boat i have watched the gps on while at anchor displays that...

Watch the direction indicated and i bet you see it is perpendicular to the direction the bow is facing, And reverses course when the boat swings.the other way.

Also, .7kts of swing isn't hard to acheive on anchor...

Rick D · Aug 23, 2011

Maybe Signal Drift

...yea, I get .1 to .3 knots commonly when on my mooring or even in my slip I get it. I suspect it's just signal drift. That's on my WAAS Raymarine plotter.

Joe · Aug 23, 2011

Hey Rich...... is that apparent speed or true? heh, heh.

ifmdalvey · Aug 24, 2011

Take your GPS shoreside and let it run for 24 hours. Then replay through your computer where it has been in that period, all over the place usually in figure 8`s and thats normal.

Bill Roosa · Aug 24, 2011

I get it also on my Raynav 300
The GPS signal is only accurate to about 30 meters or 90 ft so each time it calculates the boat position it compares it to the last few positions and does some averaging in a complex math problem to try and dampen this down. If you have lots of sats visible AND being “USED” and not showing "SPAIR" then the accuracy is great for the most part. Unfortunately all the WIFI Nimrods insist on using freqs that are close to the GPS freqs and you only get sats that are almost directly overhead now. I remember in the 90s that getting 12 sats in "USING" mode was pretty common and GPS actually gave you the 30 meter accuracy they say it can.

The problem is one of geometry. As some of you sextant users know you want to get a set of three stars that are not all located directly overhead or in one section of the sky because the math gives you a long thin triangle with lots of error in one direction when you do it that way. Widely spaced stars give you a nice isoclines triangle and a low uncertainty of location. GPS does a very similar math calculation except the clocks are better than your wrist watch and the “stars” move around a lot so you need their ephemeris data to be updated in a real time fashion. That last fact is what takes GPS so long to fire up after you have not used it in a few weeks. The ephemeris data is being down loaded from the sats.

Since we have competing signals and the signals from the low elevation sats (on or near the horizon) are both farther away and have to traverse a lot of air they tend to be lost in the noise. That leaves the ones directly overhead which give you a significantly lower than 30 meter accuracy in some random direction.

They are stealing our precious GPS bandwidth folks so they can watch Youtube videos of monkeys playing with poop.

FourPoints · Aug 24, 2011

Gps systems use the following two frequencies (military uses a few additional also) 1.575Ghz and 1.227Ghz, wifi devices operate on 2.402Ghz - 2.475Ghz, they do not even come close to interfering... I have no idea what you are talking about, so if you have any supporting documentation please post it for all to read.

Also I am aware of the light squared lte cellular proposal, and the problems that will likely cause, but that is not online yet (yes they have done testing, but only in a few areas) so it doesn't count.

alexco38 · Aug 24, 2011

I like that GPS. Make me feel going faster ;-)

zeehag · Aug 24, 2011

Rich Stidger said:
OK, I guess I'll have to accept the fact that I am oblivious to the obvious.

My wife has been telling me that same thing for years, but I never accepted her word as gospel.

Thanks for all your input. Now I have a few more "obvious" observations that I need to query you all on. I guess I will get with those....

wow-are you learning to listen to her yet??? she sounds like a wise lady.....

zeehag · Aug 24, 2011

roflmao.... we had .5 mph movement registering at anchor when i was sailing with someone else 2009-2010......was faster when dragging anchor...