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Old 11-09-2013, 11:13 PM   #16
mcnut
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I would think the pos solution refresh rate available is dependent on intended use. Battery life is often an issue with handhelds so Garmin offers a Battery Saver mode which reduces the sampling rate resulting in reduced power draw by reducing the load on the processor and display redraws.

For hiking a fix every 5 sec. is probably overkill and works fine for my MC use and I would think marine use. That said a pos fix every 5 sec. or even every second might seem an eternity at 550 mph for aircraft navigation. Guessing the EchoMap 50 (a marine unit) platform, fix rate of 10/sec, may be shared with some of Garmin's aviation products.

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Old 11-10-2013, 12:16 PM   #17
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I would think the pos solution refresh rate available is dependent on intended use. Battery life is often an issue with handhelds so Garmin offers a Battery Saver mode which reduces the sampling rate resulting in reduced power draw by reducing the load on the processor and display redraws. Bruce
Very good point. I wonder what really uses power? It seems to be associated with pos calculation, I use 76 (non-x) at computer and batteries last forever if you "use with GPS off" on satellites page.
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Old 11-12-2013, 12:41 AM   #18
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An obvious question

Has anybody considered that the satellites are not "actually" stationary? Their "floating" motion can explain the randomness and directionality of the track points.
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Old 11-12-2013, 08:29 AM   #19
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Yup, and they go from being at the horizon to overhead which probably causes all sorts of variables.
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Old 11-12-2013, 08:47 AM   #20
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I believe these satellites are in Low Earth Orbits as opposed to high stationary orbits. As such they are zipping right along and not exactly "floating".
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Old 11-12-2013, 09:38 AM   #21
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Quote:
Originally Posted by UrsaMobili View Post
Has anybody considered that the satellites are not "actually" stationary? Their "floating" motion can explain the randomness and directionality of the track points.
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Originally Posted by 250senuf View Post
Yup, and they go from being at the horizon to overhead which probably causes all sorts of variables.
Do you not understand how GPS works? All GPS/GLONASS satellites are in motion, while WAAS/EGNOS satellites are geo-stationary.

This is why your GPSr is constantly downloading new ephemeris data, which informs the GPSr when and where each satellite will be, and is required before any position can be calculated.

The WAAS/EGNOS satellites transmit correction data that allow the GPSr to calculate current position more precisely.

When calculating your position on Earth, the only unknown is your position. The GPSr knows precisely where each satellite is.
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Old 11-12-2013, 09:52 AM   #22
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Has anybody considered that the satellites are not "actually" stationary? Their "floating" motion can explain the randomness and directionality of the track points.
Yes since all 8-12 satellites sending data are in motion, they probably are the cause of an error term that varies. I think the calculation uses data from all satellites with a signal at the receiver this could very well appear as a random error that changes in any direction for each calculation resulting the the "jitter" observed. I would think other errors like temperature or atmosphere would not change at such a fast rate.
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Countdown screwed with this post 11-12-2013 at 12:45 PM
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Old 11-12-2013, 09:58 AM   #23
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Old 11-12-2013, 12:49 PM   #24
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Has anybody considered that the satellites are not "actually" stationary? Their "floating" motion can explain the randomness and directionality of the track points.
Well I would hope so, I certainly have. The sat constellation was designed to be dynamic (ever changing) so that given poor geometry at a specific location, in time it will improve, and conversely good become less good over time.

That said with a good view of the sky the unit should perform to predicted and published standards. Also a stationary unit (again with a good view of the sky) should not be reporting to the user that is has moved 10s of meters at the same time it is reporting an error +- several meters.

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Old 11-12-2013, 01:04 PM   #25
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Also a stationary unit (again with a good view of the sky) should not be reporting to the user that is has moved 10s of meters at the same time it is reporting an error +- several meters. Bruce
Does any one know what the units dynamic reported accuracy is based on?

In my example (in most cases) it was only reporting moving less than 10 ft but usually once while stopped it would jump about 20-40 feet. However I was in tall pines with high horizions. I think most false reported movement was within the accuracy but the big one were not.
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Countdown screwed with this post 11-12-2013 at 01:17 PM
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Old 11-12-2013, 02:08 PM   #26
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Does any one know what the units dynamic reported accuracy is based on?

Sorry, you asked;

http://en.wikipedia.org/wiki/Error_a...tioning_System

From the wiki:

"However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally."

I like to keep it simple: If I know the GPS is stationary, I would deem it less then desirable if the unit is telling me I have moved. Worse yet would be if the GPS is showing a small error (DOP? of less then say 5 meters) at the same time it is showing a change in position of 15 meters (and one likely cause of maps spinning) which I believe some on the Montana thread have reported.

Jerry, I know you are thinking software and SteveAZ is thinking hardware so it would be nice to be able zero in on this.

As for your questions regarding track handling I would suggest you look at post #9981 in the "New Garmin Montana" thread. Might be a cross platform issue, another reason I dislike mega-threads.

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Old 11-12-2013, 06:54 PM   #27
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Sorry, you asked;

http://en.wikipedia.org/wiki/Error_a...tioning_System

This is great, I will study it. After a quick look, it looks like the accuracy displayed is difined as PDOP and is calculated from the receiver and satellite positions.

I like to keep it simple: If I know the GPS is stationary, I would deem it less then desirable if the unit is telling me I have moved.

This is true but very difficult to achieve without a giant reduction in resolution. When the unit makes a new pos calculation that is different from the previous one, it has no idea if it has moved or the calculation error has changed. The only fool proof way to achieve this is to make the resolution enough worse than any possible calculated pos error short time change. The other option is to use an algorithum to determine if the unit probably moved based on speed/direction of acceptable moves before reporting a new position to user.

Worse yet would be if the GPS is showing a small error (DOP? of less then say 5 meters) at the same time it is showing a change in position of 15 meters (and one likely cause of maps spinning) which I believe some on the Montana thread have reported.

At first glance, this would be impossible. However the unit shows error due to PDOP which it can calculate, however the total error is due to 1) Incorrect Pseudorange, 2) Ephemoris, 3) PDOP, and 4) Numerical error. Thus the actual error can be greater than the displayed maximum error.

Jerry, I know you are thinking software and SteveAZ is thinking hardware so it would be nice to be able zero in on this.

Your statement is not clear. I am quite sure the difference in observed "Jitter" is due to software differences between old and new generation units. However I am almost 100% sure the actual pos error has nothing to do with software.

As for your questions regarding track handling I would suggest you look at post #9981 in the "New Garmin Montana" thread. Might be a cross platform issue,

Will do.

another reason I dislike mega-threads.

Yes VERY true, but also true with more that one topic in a post.

Bruce
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Old 11-13-2013, 08:42 PM   #28
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Completed and in depth review of the wikipedia error analysis

http://en.wikipedia.org/wiki/Error_a...tioning_System

As suspected this analysis was for static error and did not address much about short term dynamic change in error.

The error terms that could reasonably have a dynamic aspect are:

Atmospheric effects both ionosphere and troposphere. Changes due to solar rediation could be rather fast but the paper stated the obviouis; "Humidity also causes a variable delay ---- this effect both is more localized and changes more quickly than ionospheric effects. In high wind conditions, the amount of water in the path could change quite fast.

Multipath effects. The paper stated; "multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge." This could be the key to quick changes in the pos error. Although the receiver is stationary, the angle to each satellite does change in 2-5 seconds and could cause a reflected signal to come and go.

Interference. Some motorcycles do have a lot of EMI.

In summary, I believe that the displayed accuracy on the Satellite page of the receiver is only the PDOP calculatad by the receiver from known positions. The actual error can be much higher and can be dynamic.

Given this possible "Jitter" in each pos calculation of a stationary antenna, the question is, does the receiver try to determine if a move actually occured? If so, is there a difference between last generation and current generation units?
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Old 11-13-2013, 08:50 PM   #29
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Effects of space weather:

http://www.spaceweather.ca/tech/se-eng.php


And the space weather forecast:

http://www.spaceweather.ca/current-actuelle/sf-eng.php
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Old 11-14-2013, 08:49 AM   #30
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On the one hand I don't see how the GPS can be expected to really know if it is stationary or if somebody on foot is looking in a small area for a geocache or something.

On the other hand the Montana has profiles and maybe it could be smart enough to know that somebody using a motorcycle or automobile profile is not driving around in 20 ft circles.
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