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WAAS in Western Canada- Does it work well?


tirediron
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I have seen a number of posts querying how well WAAS works outside of the US. I am wondering if anyone can tell me (preferably from experience) how well it works in Canada, especially western BC? Also, what is the difference between WAAS and differential GPS? Are there any websites that can illustrate it's coverage? Thanks all

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Indeed it does. You may need to find an unobstructed view to the Southwest however, to lock onto Satellite 47 (WAAS) or Southeast for 35. A high hill or mountaintop is good for this first aquisition of the WAAS satellites. My eTrex gets two meter resolution once it settles, and I live in Victoria. It may take 5 to 10 minutes to aquire and lock in to 35 or 47, so you may need a little patience. I can not say how long a Magellan takes, but locals such as eroyd and Dagg could tell you.

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I was on a cache hunt here in Alberta with my Rino 120. I was getting poor satellite reception and was quite a few feet from the actual cache but it said I was right on top of it I turned on my Waas and the Cache location jumped away from where I was standing, I then easily found the location

 

More to see, More to do

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The fact that you can receive a WAAS signal, does NOT in any way mean that you are receiving valid differentially corrected data. In most cases, the corrections received in most parts of Canada will make the reported position much worse than if you do not use WAAS.

 

The key is your proximity to a ground station. The ground stations broadcast the corrections for their local area. However, those corrections can be received (and incorrectly applied) thousands of kilometers away.

 

Here are the current (WAAS) ionosphere correction ground station locations.

 

http://waas.stanford.edu/tmslive/graphics/screen1.gif

 

The total accumulated corrections from valid differentially corrected WAAS data is 7m. The majority of the errors are ionospheric-delay errors. The balance are clock and orbit.

 

The problem is you will receive ALL the corrections, regardless of where you are. Orbit and clock corrections will be valid everywhere, as they relate to the SV's themselves. The ionospheric-delay errors will NOT be corrected "correctly" if you are outside the correction area.

 

Also, a big factor is how the GPSr interprets the received corrections. Garmin does not apply any correction adjustment for positions outside the correction area as many other manufacturers do. Also, Garmin receivers do not consider non-DGPS signals with ones that are receiving corrections. This will inevitably make your reported position solution worse.

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quote:
Originally posted by MrGigabyte:

The key is your proximity to a ground station. The ground stations broadcast the corrections for their local area. However, those corrections can be received (and incorrectly applied) thousands of kilometers away.

 


The key is not really proximity to ground stations per se, but rather if you are inside the modeled coverage area. Whether you are close or less close to a ground station will not matter with WAAS. Here is the coverage area:

waas-coverage.gif .

The corrections are not 'just' dependant on whether you are recieving a signal from the satellite, but on your own unit. This is because part of the system relies on your unit to compute your unit's postion and how the corrections apply to that postion.

 

''However, when using these satellites outside the coverage area of the ground station network, the WAAS corrections do little to improve the accuracy, because there is no IONO delay information available. Now that S/A is gone, IONO is probably the most significant source of positioning error.''

 

''Ionospheric Corrections:

The IONO information transmitted by the WAAS system is much more accurate than the basic GPS IONO model. Also, the WAAS system will generally be more accurate than beacon based DGPS because of the way the corrections are rendered by the WAAS system and applied by the GPS receiver. The primary factor is spatial decorrelation, which is the degradation of corrections due to separation from the reference station. RTCM based DGPS corrections suffer from spatial decorrelation, but WAAS corrections do not.

 

This Iono data (and other corrections) are constantly uploaded to the Geo Sats for re-transmission to GPS navigation receivers. There is no interpolation between ground stations by the receiver. This is because the WAAS master system computes a "grid of Iono corrections" which are location dependent based on the user's position. There is an interpolation/extrapolation process to determine the iono correction, but it is not specifically related to the location of ground stations that collect the information. The Iono-corrections grid offered by WAAS are interpolated and applied by the receiver.

 

GPS receivers must then apply the data for corrections appropriate at their location. This may take five or ten minutes to complete in a typical receiver.''

 

4497_300.jpg

 

[This message was edited by EraSeek on February 18, 2003 at 09:20 PM.]

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quote:
Originally posted by MrGigabyte:

 

Also, a big factor is how the GPSr interprets the received corrections. Garmin does not apply any correction adjustment for positions outside the correction area as many other manufacturers do. Also, Garmin receivers do not consider non-DGPS signals with ones that are receiving corrections. This will inevitably make your reported position solution worse.


 

I'm not sure where this info is taken from, or for sure what it means, but it may be refering to the fact that some sats (also the new european beta versions of WAAS sats) broadcast a ''DO NOT USE'' signal. Garmin honors this and does not use the info, but Magellen still tries to use it anyway. This is a good thing?? Unless I'm missing something (which is possible) I'll stick with Garmin's methods. Seems to me that when you use bogus data and apply it to a postion that it is not meant to be applied to, well, that is where your solution is going to get worse.

 

[This message was edited by EraSeek on February 18, 2003 at 10:38 PM.]

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I don't mean to hog your board here, but let me try an answer the orginal question as best I can. Yes, you may be able to use WAAS in some parts of Canada. It depends on whether or not you fall within the 'Modeled' correction area which falls just outside the US borders.

 

Most of the needed correction is going to fall further south, because that is where most of the ionospheric distrubances are going to be (which is the cause of the majority of the signal delay) as I understand it. At least that is what it looks like on the ionosphere models I've seen.

 

The differance between WAAS and deferential DGPS is that WAAS is modeled and does not depend on how close to a station you are, where as the beacon DGPS does depend on how close to a station you are and accuracy will drop the further away you get.

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On reasonably clear days in areas unobstructed to the SW my Magellan Sporttrak does indeed claim to be getting WAAS. When at cache sites I often find myself within less than 3 meters after sitting for a few minutes. Does that mean anything? I don't know. I'm pretty unexperienced and sceptical of the WAAS thing.

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Referring to EraSeek's first post and the info on the GPSinfo.net. The information on WAAS is confusing at best. If you look at a map of the wide area ground reference station coverage areas at the gpsinformation.net

the northern boundary in Canada varies a great deal. Even the radius of coverage of individual reference stations seems to be inconsistent.

 

Yet the grid of Iono corrections seems uniform extending to the mouth of James Bay at about 55º latitude. This would be over 400 miles from the nearest reference station below the 49th parallel.

 

Then there's the question if your GPSr can make use of just the clock and orbit corrections which are not dependent on proximity to a reference station or is it an all or nothing situation

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As Mr Gigabyte asked for some clarification, I thought I'd share with all.

 

I had some professional surveying completed during a project I was managing. The surveyor was using a Trimble XR-Pro with a differential signal (from the Comox Coast Guard Station Transmitter, I believe) correction. I also had a similar survey completed with a Trimble Pathfinder Pro XRS12/TSCI with differential correction. Both accuracies were sub 1 meter, actually around 10 cm, IIRC. I waymarked each position both with WAAS on and WAAS off. It did take time to "lock on" the WAAS bird and apply corrections (shown by a little "D" in the sat signal strength meters). The UTM Northings and Eastings (zone 10) in WGS-84 converted to my NAD-83 waymarks exactly with WAAS on, or at least to 3 decimal minutes. With WAAS off I occasionaly noticed +/-.001' error. I checked both in real-time and after post-processing the datum.

 

I haven't actually calculated (nor do I know) the inherent accuracy of a fix in decimal minutes, but as I said they corrolated to the survey results. Should be good enough for geocaching... icon_wink.gif

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It is good to hear you got the D's up. I have heard 3 senerios for what happens when getting WAAS sats outside of the coverage area (I'm not sure if Powell River is or not), but those 3 are:

 

1) You collect all the timing errors and such, but not the Ionosphere corrections.

1a)I have also heard that in these cases your programed in ionosphere corrections(into the GPS itself for non-WAAS times) are zeroed out, so basically you have no correction whatsoever (not even the base program).

 

2)Kerry's data shows that WAAS reception outside the area destroys your position accruacy.

 

3)A Ratheon spokesman said you can still use the data and it may help some (he also said it doesn't eat batteries, but Garmin says it does and Ratheon is full of it.

 

I would love to know the real answers to some of this stuff, as well as a good map of the coverage area. I think the best one can do with conflicting info is see what makes sense and keep looking to fit the pieces of the puzzle together.

 

4497_300.jpg

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Goodness, NO, they don't clash! I get them together all the time. They are providing an extra source for the data, and can both be used as regular navigation satellites as well. The trick is that they provide the data, and your very own GPS (which knows where you are) applies the proper corrections for where you are in the coverage grid. Your GPS is a big part of what is going own with this system. That is really the beauty of it.

 

Boy! Here is a really great site that explains a lot, even about outside coverage area usage.

Look mostly toward the middle and bottom of the page:

http://www.gpsinformation.org/dale/dgps.htm

 

[This message was edited by EraSeek on February 21, 2003 at 06:39 PM.]

 

[This message was edited by EraSeek on February 21, 2003 at 06:40 PM.]

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The gpsinformation.net web page on WAAS has been updated to show the correct map of ground reference stations.

 

The exact distance from the ground refernce stations that WAAS ionospheric corrections are valid is still unknown. The subject was discussed on the sci.geo.satellite-nav newsgroup and no one was able to point to a definitive answer. The figure of 200 nautical miles now shown on the gpsinformation.net WAAS page is a best guess

 

http://members.shaw.ca/pdops/

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