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Waas 36?


GeoBobC
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It looks like WAAS will finally be usable in areas that is previously was not.  I have a couple questions for the experts: (1) does receiving two WAAS signals improve accuracy over receiving only one? I think I was  told that Garmin uses only one signal;

That's not how it works.

 

Each WAAS-type (SBAS) station transmits the same information for the same service area. The improved performance comes from the fact that your GPSr now has additional or enhanced information with which to calculate a solution.

 

The advantage of having more than one SV covering a service area is availablity: in some local terrain, your GPS might not see one WAAS satellite, but could still receive the same signal from another.

 

-Wlw

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Just a small contribution that may be perfectly obvious to some, but not others:

ALL WAAS satellites are in geostationary orbit, which is over the equator. The pictorials above sometimes show them elsewhere, but that is just "artistic license". They are on the equator or very, very close to it.

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It looks like WAAS will finally be usable in areas that is previously was not.  I have a couple questions for the experts: (1) does receiving two WAAS signals improve accuracy over receiving only one? I think I was  told that Garmin uses only one signal;

That's not how it works.

 

Each WAAS-type (SBAS) station transmits the same information for the same service area. The improved performance comes from the fact that your GPSr now has additional or enhanced information with which to calculate a solution.

 

The advantage of having more than one SV covering a service area is availablity: in some local terrain, your GPS might not see one WAAS satellite, but could still receive the same signal from another.

 

-Wlw

You can also get a little extra geometry by just ranging off the second WAAS satellite. A fairly minor improvement, but an extra measurement is an extra measurement.

 

Theoretically, you could mix the messages from the two satellites to get your 'D's a little faster since the same messages don't come out at the same time. But that's forbidden by the spec because they don't guarantee that it will be compatible, even if it probably is most of the time.

2) does WAAS really improve accuracy? - I've noticed with my 60c that the EPE does not change when receiving "D" corrections. In fact, I've seen EPE of 6 feet without WAAS. I know that EPE is computed and may not reflect reality.

 

Has anyone run tests with two identical units side-by-side, one with WAAS enabled, and one without? I think a test like that would be insightful. The track logs could be downloaded and compared to see the dispersion around a known location.

First of all, the EPE will be a better estimate with WAAS than it is without. The errors will be better defined. WAAS tends to be very conservative with it's error bounding because it's more concerned about integrity then accuracy (can I really trust this system to make my final landing approach for me?). Your GPSr probably shaves some of the conservativeness off of the EPE, but it can't necessarily be directly compared to the non-WAAS number.

 

Secondly, I don't know how Garmin handles the case with some 'D' and some without. If you don't mix them in the solution, you might have a solution with 5 'D's which might not be as good as 8 no-'D's was. If you do mix them, you'll probably de-weight the no-'D's and have a similar but muted situation where you don't have as many "good" measurements.

 

Lastly, not all 'D's are created equal. There are three corrections in WAAS, the slow orbit correction, the fast clock correction, and the ionosphere grid. (see the secret screen shots above) The 'D' probably shows up as soon as it gets the fast and slow corrections, because that's enough to get started with. It takes a little longer to get the iono grid which is why you might see the EPE go down further after the point when it first gets all 'D's.

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I guess Joe Mehaffey said it best: "WAAS is not guaranteed to work, it is not guaranteed to increase accuracy, it is not really able to tell you when it will improve accuracy, when it will have no effect, or when it will make for less accuracy than the normal GPS signal."

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Lastly, not all 'D's are created equal. There are three corrections in WAAS, the slow orbit correction, the fast clock correction, and the ionosphere grid. (see the secret screen shots above) The 'D' probably shows up as soon as it gets the fast and slow corrections, because that's enough to get started with. It takes a little longer to get the iono grid which is why you might see the EPE go down further after the point when it first gets all 'D's.

Of course, it is important to note that the ionospheric correction either applies to all satellites or to none. It's not transmitted in a way that allows it to be only applied to one or a few satellites.

 

Since the iono correction is generally the largest, I don't think it makes any sense to show a satellite as 'D' unless the iono corrections have been received. Thus, the only situation in which some satellites might have a 'D' and others would not would be when the iono correction has been received but the other corrections have not for some satellites.

 

But that's only how I would do it. Goodness knows what Garmin actually does; we know they have a somewhat spotty record when it comes to proper use of WAAS.

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I guess Joe Mehaffey said it best: "WAAS is not guaranteed to work, it is not guaranteed to increase accuracy, it is not really able to tell you when it will improve accuracy, when it will have no effect, or when it will make for less accuracy than the normal GPS signal."

This quote was written when WAAS was still in its testing phase. I think you could safely say it is guaranteed to work now. There are times when corrections are needed and times when there is little to correct, thus increase in accuracy will vary. Example, at night there is little or no ionospheric corrections to be made.

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Whether you are recieving corrections from one WAAS sat (35) or two (35 and 47) makes no difference as they both transmit the same correction data. It is your GPS that applies those corrections in accordance with where your are. The benefit of two WAAS sats over one is the improved ability to "see" a WAAS sat and collect the data.

 

WAAS corrections are only applied when three or four or more corrected sats are collected.

 

It is my understanding from a Garmin tech that corrected sats and uncorrected sats are mixed and used together for the best solution if he understood my question correctly. It was also my understanding from him that, though a postion fix occurs when you recieve 3 or four sats, you still use all the collected sats being recieved to calculate a position.

 

I know some of that may bring up some new questions.

 

As far as the epe, yes it is just a calculated guessimate, and does some funny things when WAAS is first applied. As far as testing it, yes, I have done my own simple test before and found that when a good WAAS signal is in place my accuracy did improve at a known point, from about 12' to 14' out to about 3' to 6' feet within the target.

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What constitutes a "Differential Position" and a corrected "D" sat? I think Fizzymagic might have it right. Once you get 3 corrected sats "D", that is enough to get you the ionospheric correction to achieve a "Differential Postion" because the ionospheric corrections are based on your GPS's position. The other corrections are errors having to do with the individual sats and their positions.

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Outstanding information. Thank you. Another question: does/can a WAAS satellite serve as one of the three or four standard/non-WAAS satellites to provide position? In other words, if I can lock on two normal satellites plus a WAAS satellite, will my GPS be able to determine its location? I ask because the signal from the new 36 (on my 60c) is often the strongest of all satellite signals.

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I guess it is called the ranging function. So yes, but not right now. Sat 47 still will.

#36 has been sending ranging data quite a bit of the time as well. Since it is still in a testing phase I'd expect the data provided by it to vary. Yesterday I was on a bike ride early and never even saw a signal from 36 although there were no obstructions in that direction. I did get corrections and ranging data intermittently from 47, which is quite low in the west from my location outside San Francisco. But later yesterday 36 was back on the air providing both corrections and ranging.

 

On most Garmins you can tell if a given satellite is being used as part of the position calculation by whether the corresponding signal bar is a solid dark color (or black on b&w models) rather than a light shade, gray, or unfilled rectangle. So if you see a solid bar for a WAAS satellite it shows that it must be sending ranging data. OTOH the satellite might still be sending ranging data but not get a solid bar if your receiver just hasn't gotten the full ephemeris data from it yet.

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On most Garmins you can tell if a given satellite is being used as part of the position calculation by whether the corresponding signal bar is a solid dark color (or black on b&w models) rather than a light shade, gray, or unfilled rectangle.

Rumor has it that the new SiRF-based Garmin units cannot use the ranging info from the WAAS satellites, so the bars never show up as dark, leading people to believe that they don't have a "lock" on the satellite. Something to do with the SiRF chipset. I have seen no confirmation of this, so it could very well be nonsense, but the person who reported it said that he got the information from a Garmin representative.

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I read some comments from new SiRF users about not displaying a solid bar for WAAS. However, I believe those comments were written during the time that no one could get a lock so perhaps it was not related to SiRF, but rather what FAA was doing. If it is a SiRF limitation, that would appear to be a serious problem.

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Rumor has it that the new SiRF-based Garmin units cannot use the ranging info from the WAAS satellites, so the bars never show up as dark, leading people to believe that they don't have a "lock" on the satellite.  Something to do with the SiRF chipset.

Your rumour is probably a misleading technomyth. There WAS an early issue with WAAS functions on the "old" SirfstarII chipset - but this was sorted out by a firmware mod, ages ago.

 

Garmin's own website and literature state clearly that the new models are "WAAS-capable" - a claim they'd hardly dare to make public if it wasn't true. :ph34r:

 

-Wlw

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Garmin's own website and literature state clearly that the new models are "WAAS-capable" - a claim they'd hardly dare to make public if it wasn't true.

It's likely a myth, but you seem to misunderstand what I was saying: the rumor is that the Garmins can't use the ranging data from the WAAS satellites; that is, they can't use the WAAS satellites like regular GPS satellites in generating the position solution.

 

There has never been any doubt that the Garmins can make use of the correction data from the WAAS satellites, which is what the Garmin literature claims.

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Last week, according to a Garmin tech "the FAA changed the sequence of message that they are using on their test signals to include message MT0 which is the do not use message."

 

I asked Garmin because I noticed that 36 would blip and immediately disappear from my satellite page. If you weren't watching, you might conclude it never saw 36.

 

36 must be transmitting the do not use message all the time now. I haven't seen it for a while now.

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Last week, according to a Garmin tech "the FAA changed the sequence of message that they are using on their test signals to include message MT0 which is the do not use message."

 

I asked Garmin because I noticed that 36 would blip and immediately disappear from my satellite page. If you weren't watching, you might conclude it never saw 36.

 

36 must be transmitting the do not use message all the time now. I haven't seen it for a while now.

 

I have not seen number 36 either.

Also I have not seen 35 either for some time on the 60Cx.

 

Hope there is SOME coverage here in the east, I like WAASing through the woods with the 60Cx.

 

-

Geoff

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On the newsgroup sci.geo.satellite-nav today.

 

QuickTime animations showing the westward move of Inmarsat 3-F4 (AOR-W)

from 54°W to 142°W have been created by the Geodetic Research Lab at

UNB. The two animations show the subsatellite point and the 0°, 30°,

and 60° elevation angle contours. The animations can be downloaded

from here:

http://gauss.gge.unb.ca/AOR-W-move.1.mov(cylindrical)

http://gauss.gge.unb.ca/AOR-W-move.2.mov(spherical)

 

Richard B. Langley

Prof. of Geodesy and Precision Navigation

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Here in Seattle, my 60c saw 36 momentarily today and blipped off. Oddly, I experienced the same with 33 today: a quick hit, and then it's gone within seconds. I think the satellite movement and testing are making these WAAS birds unreliable for some period of time.

Huh? This report is bizarre.

 

First, the satellite you see as #36 isn't scheduled to go live until this fall; of course it is unreliable, as it is just being tested. When it goes live, it will not show up as #36, since that PRN is only being used for the testing phase.

 

Second, the satellite you see as #33 is below the horizon in Seattle. I can't understand why your unit would think it saw it, since that is physically impossible.

 

In any case, neither of these satellites is supposed to be usable from your position right now.

Edited by fizzymagic
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Here is what the Satellites are like today 13 Feb:

WAAS-SporTrakColor13Jan2006.jpg

 

This is what they were on the 27th of January:

2005Jan27-WAASsatellites-SporTrakCo.jpg

 

Sorry about the bad pictures, I need a new camera

 

Garmin # 33 is almost on the edge of the horizon at 7 deg above

 

Garmin # 35 has moved from Bearing of 140 degrees and 33 deg elevation, to

a bearing of 158 degrees, and an elevation of 38 degrees

 

Garmin #36 is showing only on the Map60C, but not the SporTrak Color, but it is close to the edge of the sky view on the Map60C, and I won't have much of anything, until the waas satellite at 107 degree west longitude comes online.

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I've recently gotten my Globalsat 338 setup on a Palm TX and absolutely love it. Has anyone had a problem with the 338 obtaining a WAAS lock? Mine keeps flipping between sat 33 (this one is an EGNOS, correct?) and sat 35. I don't think I've gotten differential data from the unit yet. Any suggestions? Interestingly enough, our Etrex Legend has never spotted sat 33. Differences in sensitivity of the two units?

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In the heart of Kent WA today, got twelve birds including two WAAS and the 'ol B+W Garmin Legend was showing an accuracy of 7 feet. Amazing. Never got a WAAS hit before at all out here.

 

I've really been noticing the difference here in the Bay Area as AOR-W is rising in the sky. I am getting a WAAS lock on my Magellan pretty much instantly.

 

Today I turned on a GPS that I hadn't used in a few months, to see how long it would take to update the almanac and show AOR-W in its current position. Took about 15 minutes or so; but it was receiving the WAAS corrections within less than 1 minute.

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The eastern WAAS sat (35) has moved west. It is showing on my eXplorist, Meridian and Legend in its new position. I can now receive it in the house, and get WAAS corrections on the eXplorist and Meridian in my house which I never could previously, here in So Calif. I should add that because of a hill, I can never get the western WAAS (47) here.

Edited by EScout
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My SporTrak Color is showing WAAS 122(35) at bearing 183 deg and elevation of 41 deg, so I missed it yesterday. I tried to get the SporTrak Color to update the waas satellite position yesterday, and it wouldn't so I missed seeing the satellite being at a bearing of 180 deg(due south) and an elevation of 42 deg.

 

OH WELL.

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Yesterday I had a strong signal on 38 on the satellite page of my 60c. Today I had a strong signal on 48. They appear to be in the same location as 36 was previously.

Maybe that's because it's the same satellite.

 

As previously noted in this thread, the PRN numbers (from which Garmin gets its satellite designations) for testing are not the same as those that the satellite will use when it is operational.

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