Wonky Erratic Screen Gyrations

[+Ima Freeman]

Anyone else seem to think their machine is acting more "unstable" as of late ?

 

I am still finding the booty alright, but while getting to where I am going my location on the map seems to be bouncing all over the place.

 

When I first started a couple years ago with my Montana everything on the screen seemed so much more "steady". now the screen dances around constantly.

 

Is this cause for concern ? Any ideas ?

 

Thanks

 

KT

[7rxc]

Don't know the Montana series, but...

I'd look at the settings and see if the map is set for 'North UP' or something else.

 

IF it is set to 'Track Up' and you are zoomed in the gps itself may move your 'position' and the map would swing around... especially if there is a multi-path signal and/or some obscuring objects overhead, walls or trees for example.

 

Not sure if that helps or not, but it's a good place to start.

Otherwise, how about a better description of what's happening.

 

Doug 7rxc

[+Ima Freeman]

Don't know the Montana series, but...

I'd look at the settings and see if the map is set for 'North UP' or something else.

 

IF it is set to 'Track Up' and you are zoomed in the gps itself may move your 'position' and the map would swing around... especially if there is a multi-path signal and/or some obscuring objects overhead, walls or trees for example.

 

Not sure if that helps or not, but it's a good place to start.

Otherwise, how about a better description of what's happening.

 

Doug 7rxc

 

Hi Doug, thanks for the come back. I'll play with the tracking settings you mentioned and see if things improve.

 

What is happening, especially while I am standing still trying to figure my next move is that my little blue triangle dances and hunts the screen like a beagle sniffing for a hare.

 

Lets say I am navigating to a cache and it appears on my screen that my prize is 90 degrees to my left, no wait behind me at 7 o'clock, no just up ahead, oooh no, out further at 2 o'clock, nope here to my left..... ggggrrrrr.....

 

Maybe I flummoxed a setting to change things along the way (?) but I remember a more steady and precise heading a while back and less frustration.

 

Any and all ideas are welcomed.

 

KT

 

PS: Should I not be in "track up" mode ? Seems like I want to see what's coming up...

Edited April 10, 2014 by Ima Freeman

[+Mineral2]

Try recalibrating your compass.

[+Ima Freeman]

Try recalibrating your compass.

 

Oh yeahhhh..... an everyday occurrence. Sometimes 2 or 3 !

Good place to start though, thanks for checking in.

 

KT

[7rxc]

Hi Doug, thanks for the come back. I'll play with the tracking settings you mentioned and see if things improve.

 

What is happening, especially while I am standing still trying to figure my next move is that my little blue triangle dances and hunts the screen like a beagle sniffing for a hare.

 

Lets say I am navigating to a cache and it appears on my screen that my prize is 90 degrees to my left, no wait behind me at 7 o'clock, no just up ahead, oooh no, out further at 2 o'clock, nope here to my left..... ggggrrrrr.....

 

Maybe I flummoxed a setting to change things along the way (?) but I remember a more steady and precise heading a while back and less frustration.

 

Any and all ideas are welcomed.

 

KT

 

PS: Should I not be in "track up" mode ? Seems like I want to see what's coming up...

 

I will think some more on this, but from the other thread and this one, it's getting to sound like it is being defined better. As for the 'mode'... That is a personal preferance or perhaps an occasional one. I see at least one other agrees with me in the other thread. I prefer to use 'map up' to avoid this problem (I don't use a montana as I said, and I don't use an electronic compass for the same reason).

 

Fact is that all GPS units are not absolutely accurate all the time due to satellite movement and constant recalculation of your position... as I stated, the closer in you are zoomed, the more GZ will move around to match. 'track up' results in you device constantlly turning the map to face the way it thinks you have to move.

I get dizzy easily so I don't. Older devices you can see this as the coordinates constantly changing value in the end decimal points.

 

To be honest, I use maps for coarse navigation, not close in follow the arrow type. I only use a display with a bearing indicator, and field boxes for Distance to Target, Bearing to Target (GPS style not E.compass), EPE or Accuracy ( for me in metres) and the Time. However that is just MY prefered mode of hunting. You can and should do what works for you. My first GPS didn't have much other than the indicator and distance/bearing, no maps etc.. I got used to doing it that way... not to mention the coordinates only went to two decimal places not three. GZ wandering was often hundreds of feet and most of the time accuracy level was 5 metres or more on top of that..

 

Just test carefully, listen and think it out. Best advice would be to find a cacher local to you with the same or similar model of Montana and get hands on advice from them. Let them see what they think is up.

 

Doug 7rxc

[+Walts Hunting]

If you are inside the EPE radius that is not uncommon. Remember it recalculated your and the caches location each time and if your EPE (50% chance of each point being within it) is 20' then each of them can move that far on each calculation. When you get that close put it away and look.

[+coshida]

Below is a screen grab from a commercial GNSS. The large circle is a 5 meter radius, the small a 1 metre radius. Every blue dot is a sampled position. The green circle is average position calculated from the samples. Note the 10 metre plus total drift North-East and South-West.

 

The averaged position error in the pic is only 0.23 metres, but it's taken 880 samples to get that. (15 GPS sats in view, & 1 Egnos.) If you randomly sample one position from that scatter, you could be ten metres out north-east or south-west. The sample rate is one per second.

 

The receiver will calculate your travel direction (COG) and speed-over-ground (SOG) between any two consecutive points. More scatter between points means the receiver's indicated position 'fluctuates'. It can do this every second: ie every time it calculates a 'fix'.

 

This is why receivers often indicate you're doing 2-3 MPH even when you're standing still. Every 1 second 'fix' produces a different set of numbers. 1 MPH is 0.44704 metres per second. If the receiver 'fix' is 10 metres from the last, one second later, (see pic) you've 'travelled' at 4.4 metres per sec, (SOG) for that time. That's 2.24 MPH.

 

Receivers mostly have SOG filters in them to mitigate this.

 

From this you can infer that a spot-sampled GPS position is often no better than 10 meters or more, and that averaging is needed for better position. 15 minutes should do it.

 

Regarding satellite movement; the receiver knows this so it's not an issue. There are small timing errors in the receiver which add to varying indicated positions but they are very small at 0.000786 parts per million or so in a modern receiver. (Tested in a U-Blox 622 GNSS) Clock drift is also very small at 0.5 us/s (micro-seconds per second) on an average receiver, but all of it adds up to 'spider tracks' on map displays and swinging needles on handsets, made worse by multi-path signals and moisture in the air.

 

As an example, the screen shot below was taken in clear sky. An hour later cloud cover had arrived, and the averaged position worsened to 5.67 metres, twice as 'bad'. Water vapour attenuates radio signals and refracts them so you might see more spider tracks in cloudy weather or fog.

 

You can see from the above that averaging a position, (available on most Garmins) is the best way of getting a close to absolute position fix if you are placing a geocache. If you are searching, remember the 10 metre (or worse) differences between 1 second position 'fixes'.

 

Also remember that the quoted accuracy for a GPS handset is an 'averaged' accuracy, not an instantaneous and repeatable one. If you work to an accuracy of ten metres, that's pretty good on a sunny day on flat plain.

 

The 'any instantaneous fix can be >10 metres out' is a reason why two receivers placed in the same spot will rarely exactly agree.

 

For flight purposes the accuracy of non-corrected GPS (USA system) is quoted at 17 metres. Averaging and /or differential corrections improve that: EGNOS-corrected flight GPS quotes 1 metre instantaneous.

 

That's how it is going to stay until we get the ionosphere repaired and add atomic clocks to the insides of our handhelds :-)

 

Finally, if you have nothing better to do here's an early report on GPS Averaging. No one need ever ask the question again after this post. :-)

 

http://i58.tinypic.com/ettug4.jpg

Edited April 12, 2014 by coshida

[+ecanderson]

Finally, if you have nothing better to do here's an early report on GPS Averaging. No one need ever ask the question again after this post. :-)

Not about the theory, no. But look at the timeframe where that study was conducted, and the device used. We're talking prior to opening up selective to the public, and use of a GPS38, a unit with less sensitivity and a whole lot less processor horsepower for working a fix than what we're all using today. One cannot take the actual numbers demonstrated there as relevant to the current time. Still, it makes an important point. That said, a study like that provides more relevant numbers when done with today's gear and today's constellation.

[+Ima Freeman]

Thanks for the theory link and clarification of said link... looks like fine winter reading !

 

Special thanks to Doug, Walt & Min for their advise also. I'll keep working at this and see if I can zero in better as to what is happening when and why and report back.

 

In the meantime, here is an interesting link for you other Garmin Montana users: http://garminmontanagpsr.wikispaces.com/Common+Issues?responseToken=09cfdf7f9734dc91c894d35ad3a6d700f

 

KT

[+Psychaesthetic]

Thanks for the theory link and clarification of said link... looks like fine winter reading !

 

Special thanks to Doug, Walt & Min for their advise also. I'll keep working at this and see if I can zero in better as to what is happening when and why and report back.

 

In the meantime, here is an interesting link for you other Garmin Montana users: http://garminmontanagpsr.wikispaces.com/Common+Issues?responseToken=09cfdf7f9734dc91c894d35ad3a6d700f

 

KT

 

OMFG WHAT A CUTE DOG!!

[+Ima Freeman]

OMFG WHAT A CUTE DOG!!

 

heh, heh, heh.... That's Thea, she never fails to bring a smile to my face !

 

Here's a link to another funny rascal:

 

Edited April 17, 2014 by Ima Freeman

[+Psychaesthetic]

I got myself a mynxy shewolf who does the same to me!

 

[+Psychaesthetic]

OMFG WHAT A CUTE DOG!!

 

heh, heh, heh.... That's Thea, she never fails to bring a smile to my face !

 

Here's a link to another funny rascal:

 

 

Dog's are just the best animals on the planet, I reakon.

 

Nothin funny about a cat! But dogs, ah yes.. Cutest of the cute