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Why does eTrex Vista = less accurate WITH WAAS on?


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This is just my experience from using WAAS a handful of times, but I've never known it to make any noticable improvement in my accuracy when I have it on. In fact, a few times I was around 20 foot accuracy, and turning on WAAS made it jump to about 25-28.

 

Just wondering if it's always a good thing to activate WAAS... I've never had it in my other GPS' so I'm afraid I don't know a lot about it. I just always assumed that WAAS always = more accuracy. icon_confused.gif

 

Rubbertoe

 

ToeCam: http://www.bigfoot.com/~rbatina/webcam

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RT, like you I've based my decision to keep WAAS disabled on my GPSmap76 upon some unscientific experiences. Without a certified benchmark I have no way of knowing for sure what the story is. With WAAS disabled, the position fixes I get very closely match those from my GPS-12XL; but with WAAS enabled, there is almost always a greater disparity. I can get a solid WAAS signal in my area about 75-80% of the time, and the nearest ground station is about 200 miles away. icon_confused.gif

 

~Rich in NEPA~

 

1132_1200.jpg

 

=== A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ===

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I'll hazard a guess here. Maybe someone else can back it up. My WAAS GPS can handle 12 satalites. A common good lock is 20 feet of accurace as reported by my GPS. Turning WAAS on, drops two satalites immediatly to 'make room' for the WAAS satalites. If one of those satalites was being used for a position fix, you will immediatly lost some reported accuracy. Also I have not seen any indication that my GPS reports any better accuracy with WAAS enabled. Meaning that if I was at 20' of accuracy before and WAAS gets me to 10' my GPS will still say 20'. Or so it seems.

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quote:
Originally posted by Renegade Knight:

Turning WAAS on, drops two satalites immediatly to 'make room' for the WAAS satalites. If one of those satalites was being used for a position fix, you will immediatly lost some reported accuracy.


 

RK, what you are saying is probably right, but I know in my case I seldom get a lock on 12 satellites at once. I have seen it happen only a handful of times when I was on high, exposed ridges and summits. Also, I believe that most GPS receivers are biased to use only the most effective satellite configuration in their position solutions, and seldom use more than four or five anyway. (I suppose this could depend on which brand/model you are using, of course.)

 

Quite often I can receive the AOR-W signal but in the 3 months that I've had my map76 I have never seen it lock onto 11 or 12 satellites. So, I can't say for sure if having WAAS enabled always drops out two channels regardless if only one WAAS signal (or none) is being received. RK, do you know for sure that this is the case? What receiver are you using or referring to?

 

There doesn't seem to be a lot of useful information on how important WAAS is in recreation-grade GPS receivers. I would think boaters could benefit from extremely high accuracy. As for Geocaching, if everyone had 3-meter precision that would surely take a lot of the challenge out of the sport, no? Just a thought for discussion. icon_smile.gif

 

~Rich in NEPA~

 

1132_1200.jpg

 

=== A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ===

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I've been experimenting with my vista for a few months and have noticed that it takes longer for it to make an accurate lock with WAAS enabled. If I have the time and good sat reception, then I'll turn on WAAS and gain another 5 feet of accuracy. But when operating under tree cover, don't bother with WAAS.

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12 sats means very little and besides the % of time this will occur is rather low. Also the number of Sats doesn't really equate to accuracy (as such) where as the geometry of the Sats do.

 

Actually using all satellites available can be a detriment to accuracy especially when considering low on the horizon sats. However some manufacturers have made this 12 channel thing a selling point but in fact if they configured the receiver settings correctly there'd be no way anybody would (should) see 12 satellites.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Rich. The only time I've actually looked for the WAAS satalites when I swithed on WAAS it actually did knock off a satalite that it was using to make room for the WAAS satalites. As later posters pointed out, it takes a long time to get a WAAS lock. My GPS never did get one while I was messing around with the WAAS function.

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From what the people at Garmin tell me is that the WAAS sats are only broadcasting test signals not any actual differential signals.

 

As to when they will actually start broadcasting I have no idea.

 

When I had me Etrex Venture I found no difference between WAAS & non-WAAS other than battery life.

 

Now with my Meridian I do not have a choice of turning WAAAS off and still get the same accuracy. Go Figure!! icon_wink.gif

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quote:
Originally posted by Alpha6H:

From what the people at Garmin tell me is that the WAAS sats are only broadcasting test signals not any actual differential signals.


 

I think the person you talked to was a bit confused. WAAS signals have been sending valid correction data for some time although they still haven't demonstrated the level of 'integrity' required by the FAA.

The corresponding European system (EGNOS) is also sending correction data, but with a flag indicating that it is still in test mode and not to be used for critical navigation. Garmin receivers are not using the EGNOS data as a result of this test mode flag. Magellan units that are WAAS-capable are using the EGNOS signals.

 

To evaluate whether and to what extent WAAS/EGNOS is working to improve accuracy you really need to take a series of measurements at a known location. 'EPE' and similar measures are not a valid indication.

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quote:
Originally posted by Alpha6H:

 

Now with my Meridian I do not have a choice of turning WAAAS off and still get the same accuracy. Go Figure!! icon_wink.gif


 

Actually you can turn off the WAAS on the Meridian. When you are turning it on you have to hold down the Nav-Goto-Power keys (I think, I don't have my unit in front of me). You then go to screen three and you can turn off WAAS. To turn your unit off, you have to press Menu-goto-esc (I think). For more on how to do this, look for ClayJar's report on the Meridian on the discussion boards (plus he has the correct buttons to push to do this).

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First off, WAAS is NOT broadcasting only "test signals".

 

Here's why it may be better to have WAAS turned off in some cases. If the eTrex can get a WAAS signal from enough satellites to calculate a solution, it will use those signals exclusively. You may have a lock on 9 satellites, but the unit will only be looking at those it has WAAS data for, ignoring all others. So if you've got WAAS data on all 9 sats, then you're better off with WAAS turned on. But if you only have WAAS data on 4 sats, you'll probably get a better fix turning off WAAS and letting the unit use all available sats. This will largely become a moot point when there are more WAAS birds in the sky.

 

PS_sig.gif

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quote:
Originally posted by Prime Suspect:

First off, WAAS is NOT broadcasting only "test signals".


 

I suppose it depends on what one wants to use it for as WAAS is still broadcasting a Type 0 message and that is a Test mode.

 

So in effect while that Type 0 message is being transmitted it effectively means they are not going to guarantee the accuracy, integrity, continuity or availability of the system for other than test mode purposes. The signal as available to the general community I believe still falls under the Type 0 test mode.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Kerry:

The signal as available to the general community I believe still falls under the Type 0 test mode.

 


 

According to Raytheon and the FAA, the Type 0 test mode message is to prevent aircraft from using the signal before the FAA certifies the technology. That's not scheduled to happen till sometime in 2003. The reliability of WAAS should only get better, for the most part, between now and then. Also, some of the satelites up there are not sending good data yet. They should soon though.

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Once I figured out that I needed to take my GPS out of battery saver mode to acquire WAAS information, I use WAAS all the time.

 

In an ideal situation WAAS may not make a lot of difference. If the satellite information is corrupted by weather or other problems and WAAS is able to correct the data you will see a difference.

 

Also as time goes on the WAAS signal will continue to improve.

 

I actually went for months thinking I was using WAAS when I really wasn't.

 

You need to acquire WAAS tables just like the satellite data that you downloaded the first time you turned on your GPS.

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quote:
Originally posted by Cearnach:

The reliability of WAAS should only get better, for the most part, between now and then.


 

So that that does mean the system is not totally reliable now icon_confused.gif hence the need for the type 0 message (test mode)

 

quote:

Also, some of the satelites up there are not sending good data yet.


 

So does that also mean the reliability (based on lack of "good" data) is also a reason for the test mode icon_confused.gif

 

quote:

They should soon though


 

What is about to change soon icon_confused.gif

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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