WAAS myths

[+EraSeek]

I know some of this has been hashed over before, but I am interested in real answers to these myths.

 

First myth: Using WAAS enabled eats batteries. Here is what Raytheon has to say about that:

 

''As to battery consumption, you will need to contact the manufacturer of the GPS receiver. We are not aware of any reason why the battery life should be shortened by the reception of the WAAS signal.''

I am currently E-Mailing the tech crew at Garmin for their answer.

 

Second Myth: You have to have WAAS enabled for 'insert long period of time' before it is functioning.

 

I tried mine again the other day at a great site and it immediately droped my accuracy reading from around 20' to less than 9' after a minute's time.

 

Third myth: You have to be near a WAAS station to get a good drop in your readings

 

I've heard before that this is not true because the corrections work by zone or area not distance from a station.

Anyone know the facts? ''Just the facts Ma'am.''

 

[+Kodak's4]

One possibility for why a unit might draw greater power with WAAS enabled than with it disabled might be computational burden. In the manual for my GPS V, Garmin state that enabling WAAS may make computation intensive tasks (such as auto-routing) run considerably slower. That suggests that with WAAS disabled, a unit might have considerable more compute power than needed, and might have an opportunity to save power by idling the CPU when no computation needs be done. With WAAS enabled, the computations may suck up this otherwise idle time, so that the processor is drawing more power.

 

Is this the real reason? I have no idea, this is all conjecture.

 

Re: must have WAAS enabled for a long period of time. I just went outside, turned on my GPS V, and let it get a fix. Then I enabled WAAS. It spent the next ten minutes unsuccessfully searching for satellites 33-51, which I presume are the WAAS satellites. It took a considerable amount of time to run through all of them. It appears to me that at least in that location (the end of my driveway) the time from enabling WAAS to getting a WAAS augmented fix might be quite long.

[+EraSeek]

The Waas sats are 35 and 47. One east and one to the west. I just did the same thing as you, went in the driveway turned on the gps, got a lock and then switched to WAAS and got a lock on 35 within a minute with a few ''D's'' on the other sats, and I don't have real great exposure to the sky here (a bit to the east). I think this has more to do with positioning and time (exposure to the WAAS sats) then anything else. Sometimes I can't get them.

 

My Accuracy did not drop though. I know one thing though, WAAS does not always help. Bad geometery is bad geometery, and sometimes the ionoshephere cannot be improved upon so corrections may be useless.

 

[+EraSeek]

Here is what I've learned while waiting for the big boys to respond.

 

You do not have to be near a WAAS station to get workable data. These stations send the data to a set of master stations which gets the whole picture and sets up an averaged grid system. For some of the corrections needed, your GPS computes were you are on that grid and applies the proper info.

 

How long you have to be locked onto a WAAS sat to get useable data depends on what the WAAS sat is sending at the time. You need to recieve a lot of data but to get what you need may take 10 mins, 5 mins, or 12 seconds. Best to watch your accuracy reading for the clue.

[+EraSeek]

Once you receive the usable data from a WAAS satellite it is locked into your GPS and will continue to make good corrections for a period of time (2 minutes, I think) even if you lose the satellite. This is why you will see "d's" up with no sat.

 

On the power issue, I tend to think Kodak's 4 is correct about idle time. Let's see what Garmin says.

[+EraSeek]

One way or another I am going to educate myself and everyone else here.

 

Here is a cool .gif image link that shows how the ionosphere dances about throughout 24 hours causing fluctuating signal delays. http://www.gpsinformation.net/exe/iono-day.gif

 

This is what the WAAS satellites are correcting.

 

Notice how as night passes by it settles down. There is less ionosphere interference at night, therefore any WAAS corrections will be less. Less correction is needed.

 

[This message was edited by EraSeek on July 21, 2002 at 11:28 PM.]

[+apersson850]

With Garmin units (all I know about, but certainly for the eTrex Vista), WAAS is impossible if you use the battery saver mode. Hence, WAAS requires more power than the minimum possible, but I couldn't measure any higher consumption using Normal mode and WAAS, compared to Normal mode and no WAAS.

 

It takes a while to download the required data from a WAAS satellite. Now, if you happen to get a reception from that satellite just after it began sending that data, you have to wait for the entire data stream to pass twice, before you get it all. But if you start just at the end of the data sending, you only have to wait for the next broadcast.

 

Anders

[+TeamCNJC]

quote:

---

Originally posted by EraSeek:

Third myth: You have to be near a WAAS station to get a good drop in your readings

 

I've heard before that this is not true because the corrections work by zone or area not distance from a station.

---

 

I think this may be a "partial" myth. WAAS corrects for errors due to timing (clock), ionospheric interference, and satellite positioning (ephemeris). While you may be able to get a large area of correction for clock and ephemeris errors, the graphic that you mention above shows that ionospheric errors are localized and rapidly changing, so there has to be some correlation with distance from a ground station. The WAAS FAQs on Joe Mehaffey and Jack Yeazel's GPS Information Website seems to support this, if not the other two "myths."

 

As far as myth #2, with my Garmin Venture I did not see an appreciable drop in EPE, though I did get differential signals if I was lined up just right after about 3-4 minutes. I've left the unit on for over an hour, stationary, receiving WAAS corrections, and the EPE stayed the same. OTOH, my Magellan Meridian Gold will show a drop in EPE to about 13 or 10 feet after receiving WAAS signals for 1 or 2 minutes.

 

-Craig/TeamCNJC

 

... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

[Rubbertoe]

I assume that WAAS mode eats up more juice, because why would they include an option to disable it?

 

There must be at least one negative to running WAAS, otherwise it'd just be something that was on all of the time.

 

*shrug*

 

Rubbertoe - Webcam - Image Archives

--== http://www.bigfoot.com/~rbatina ==--

[+apersson850]

WAAS certainly eats more juice, but I think it's more like CPU cycles than milliamperes. I've found that computationally intensive tasks, like updating the map screen when rescaling and rotation is necessary, takes longer time when WAAS is enabled.

So I usually keep it off, especially since there is no WAAS (or rather EGNOS) operational here yet!

 

Anders

[+TeamCNJC]

quote:

---

Originally posted by Rubbertoe:

There must be at least one negative to running WAAS, otherwise it'd just be something that was on all of the time.

 

*shrug*

---

 

In the latest Magellan units, WAAS is on all the time. The "option" to disable it is buried in a hidden menu.

 

-Craig/TeamCNJC

 

... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

[+EraSeek]

quote:

---

Originally posted by TeamCNJC:

 

I think this may be a "partial" myth. WAAS corrects for errors due to timing (clock), ionospheric interference, and satellite positioning (ephemeris). While you may be able to get a large area of correction for clock and ephemeris errors, the http://www.gpsinformation.net/exe/iono-day.gif that you mention above shows that ionospheric errors are localized and rapidly changing, so there has to be some correlation with distance from a ground station. The http://www.gpsinformation.net/waasgps.htm on Joe Mehaffey and Jack Yeazel's GPS Information Website seems to support this, if not the other two "myths."

..

---

 

I know you will find some conflicting info from the experts but the following, if I am reading it correctly, clearly states (or so I think) that proximity to ground stations WILL matter with DGPS but not WAAS as long as you are in the reference area (the US). The stations send their data to master stations. Master stations make a model of the ionoshere as a whole, and sets up a tight series of points. Your GPS figures out where you are in reference to those points which are not based any longer on the reference stations. If you are between these points, your GPS triangulates between the three nearest points and averages the results. The key is once the general model is made, the reference stations are no longer part of the picture. (the basic gps iono model spoken of below is the program your GPS uses without WAAS to figure out Ionosphere delays)

 

''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.''

[+TeamCNJC]

Interesting. Can you supply the reference for those statements (for my information)?

 

I found a reference that seemed to contradict, but on closer examination, it was talking about decreasing accuracy once you left the WAAS service area, not within the WAAS service area. I copied the reference, tried pasting in here, lost it, couldn't find it again, lost interest in finding it again...

 

I found another reference that supports no-relation-to-ground-station-distance theory here. The space-based correction systems (i.e., WAAS) are constant, while the ground-based correction systems are range dependent.

 

I guess this debunks myth 3, but the last line in your quote seems to support myth 2.

 

This has been a good discussion. Thanks.

 

-Craig/TeamCNJC

 

... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

[+EraSeek]

It is here, http://www.gpsinformation.net/exe/waas.html

 

Below the second map near the bottom of the page.

[+TeamCNJC]

It's the same site that offers all of the opinions expressed here so far. It's no wonder that we're confused.

 

-Craig/TeamCNJC

 

... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

[+TeamCNJC]

It's the same site that offers all of the opinions expressed here so far. It's no wonder that we're confused.

 

-Craig/TeamCNJC

 

... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

[+EraSeek]

Yeah, their statements conflict sometimes. I think he was using someone elses info here. Anyway here are some more facts that seem to answer some of the questions. Again, on the last statement it seems to suggest aggumentation cacn occur between 10 seconds and 10 minutes depending on the data stream and your needs.

 

 

My GPS has WAAS but it will not work when I operate the GPS in "power save" mode. Why is that?

Answer: WAAS requires a lot of digital data be received by your GPS. To do this, the RF receiver section of your GPS must be operating full time. Power save mode turns off power to your RF receiver for about 4 seconds out of 5 when GPS signals are stable and WAAS cannot function with this limited receiver ON time.

-------------------------------

1. Q: What are the sources of this error?

A: They are well understood and the dominant contributors are listed in this table along with the likely amount that they contribute (assuming a good sky view and reasonable satellite geometry). Note that in real life the errors may be higher than those listed in the table.

Error Value

Ionosphere 4 meters

Clock 2.1 meters

Ephemeris 2.1 meters

Troposphere 0.7 meters

Receiver 0.5 meters

Multipath 1 meter

Total 10.4

 

----------------------------------------

Clock errors can change rapidly so this data is update every minute if required, ephemeris errors and ionosphere errors don't change nearly so fast so they are only updated every 2 minutes and can be generally be considered valid for up to 3 times that period of time. Even this time is very conservative in practice

 

------------------------------------------------

It is possible to leave the WAAS enabled all the time. The only negative is that the unit will expend some energy attempting to lock onto the WAAS satellites and the two channels will not be available for standard gps devices. Even a momentary break in terrain will cause the gps to lock onto a GEO sv if it can see one. WAAS data is sent in packets that are one second long (250 bits) and a lock can occur at any one second interval. Once locked it will take some period for the gps receiver to download enough data to be useful. How long depends on what data the GEO is sending at that moment. Often differential corrections can begin in 10 to 12 seconds from lock based on a need to download correction data for some of the SV's. At this point, even if the GEO is lost again, differential corrections will continue to be applied for about 2 minutes

[+EraSeek]

I think the seeming conflicts on the Joe Mehaffey and Jack Yeazel's GPS Information Website come from applying the DGPS info to the newer WAAS system. Proximity to stations do matter with DGPS but not WAAS, that is the beauty of the system, plus it uses your GPS to do much of the calculations.

 

On Joe's site they also state that if you are too far away, say another country, that the WAAS can actually degrade your accuracy. Raytheon's answer man denies this and says it cannot degrade, simply provides less or no augumentation. I'm not sure where to side on that.

[+Kodak's4]

quote:

---

Originally posted by EraSeek:

My Accuracy did not drop though. I know one thing though, WAAS does not always help. Bad geometery is bad geometery, and sometimes the ionoshephere cannot be improved upon so corrections may be useless.

---

 

Are you saying that the accuracy didn't improve, or are you saying that the GPSR estimate of the position error didn't decrease?

 

The two are not the same thing. It's likely that they're correlated but based on experiments I did at a NGS Benchmark with my GPS III, it is not perfect. I have certainly seen times when the error estimate was low but the position was several meters farther off the correct cooridinates than other times when the error estimate was quite large.

 

Most of the 'experiments' that folks have done to test whether WAAS improves accuracy seem to be poorly designed. NGS benchmarks provide a good *known* location to perform experiments, and it would seem to me that repeated observations at different times of day, different conditions, etc. would pretty conclusively tell us whether WAAS helps or not.

 

I'd have done such an experiment myself but didn't have a WAAS capable receiver until Friday. But it's on my list, now.

[+EraSeek]

My Garmin 12 has an EPE reading, my map76 has an accuracy reading. Yes they are not the same, but I can not recall what the differance is.

 

My Accuracy reading on my map76 did not change. Probably for a couple of reasons. First, bad geometery. I look east from my driveway, so do not get north, south, well, or east at all. Not good for triangulation. I also did not have a lock for that long, so less info incoming.

 

Yes, I agree, benchmarks seem to be a good way of checking it. I've done several down in Redmond and with WAAS in and out I had it within a meter

or two of the benchmark.

 

If you have not used your WAAS yet, you need to go to somewhere with good exposure and enable WAAS and let it download the almanac for a bit before you will be able to use it.

 

]

[+Lil Devil]

The following is an excerpt from here. Highlighting is mine.

 

How much improvement in accuracy can I expect when WAAS corrections are working properly?

Answer: This is difficult to say with any certainty. We have observed that MOST OF THE TIME WAAS appears to improve averaged accuracy at an OPEN fixed location with 10 minutes averaging from about 7 meters to perhaps 3 meters. Sometimes there has been no improvement and a few times (when WAAS signals were not of good quality or when we were under moderate tree cover) the measurements have actually had more error with WAAS than without. These errors were almost four times the average EPE (Accuracy) displayed, therefor EPE can't be used to detect invalid WAAS corrections.

The upshot of this is: WAAS is a tool for casual GPS users. It 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.

[END QUOTING]

 

On my GPS V, I have never observed any difference in the estimated Accuracy displayed by the unit, so I performed a test:

 

I found a nearby benchmark that itself was a reference point for several other benchmarks. It used to be a beacon of some sort, but all that is left is a concrete foundation with a survey marker embedded. I travelled to this marker from considerable distance with WAAS turned on. After leaving my truck, I walked directly to the center of the foundation, and after letting my GPSr stabilize, it said I was 1.2 feet NE of the mark. The displayed EPE was 15 feet.

 

I then drove a mile away, turned off WAAS, and power-cycled the GPSr. I drove back, walked to the center point, and got a reading of 8.2 feet away, with an EPE of 14 feet.

 

About a week later, I repeated the test with the following results:

WAAS on: 2.1 feet from mark, EPE 21 feet.

WAAS off: 6.7 feet from mark, EPE 20 feet.

 

So my opinion is that EPE cannot be used to determine WAAS accuracy, but it does appear to have an effect on absolute accuracy.

 

I hope this helps to dispel some myths.

 

Lil Devil

[DARC]

A previous thread reported that there was a measureable battery drain with WAAS enabled on a Garmin Etrex Vista. I haven't noticed it being enough to be concerned about.

 

I have observed that there is a noticable delay in getting the Estimated Accuracy lower with WAAS enabled vrs faster with it disabled. My vista is noticabbly laggey with WAAS enabled and trying to get within the last 40 feet under any kind of tree cover with WAAS seems to add more error to justify turning it off in the woods.

 

I haven't even found a good position benchmark yet in my area to try out Lil Devil's test. The observation of an increase in Absolute Accuracy is something that I'll persue.

 

Currently my degree of confidence in a reading is by using two different GPS units and noting the sat positions used to resolve the location. Typically the WAAS, Non-WAAS, and Other GPS readings are only off by under 6 feet with good readings.

[+EraSeek]

Be careful when they are speaking about averaging. There is some disscussion that since SA was turned off averaging can hurt as much as help.

 

This is what I know about the accuracy reading. I read one study on it and it matches my experience, and that is that something like 60 or 80 percent of the time your actual accuracy will be much better than your accuracy or EPE reading.

 

This I also know, if you have a good WAAS lock and it is giving you aggmentation it will drop. I just did it. I was at maybe 18', turned on WAAS, first it went way up, then as the locks came in it went down to 12 then 10. Then I had all 12 sats with D's across the board and my accuracy reading went to 8.2'. Not bad.

 

The best I've had is 7' the first time I ran WAAS.

[+EraSeek]

So far my conclusion is that;

 

WAAS enabled will use more of your GPSR's resources, but whether it acually eats batteries quicker is uncertain.

 

It will take anywhere from 10 seconds to 10 minutes to get aggumentation from WAAS. This depends on how much and what part of the data stream your unit has downloaded from the WAAS Sats.

 

It does not matter how close you are to a WAAS reference station to get proper aggumentation, as long as you are within the reference coverage area (the US). This is because the data from the reference stations are are turned into a useable mathematical model for the entire covage area by the Master stations and your unit assists in the calculations of your position.

[+Kodak's4]

I had to go to a friend's house on Lake Washington for a meeting. His house has a totally unobstructed view to the east, south,and west. I took the GPS V out onto the end of his dock and let it get a lock, then turned on WAAS. It cycled through the WAAS satellites, finally getting a lock on BOTH. After about 5 minutes, I had 'd's in all the signal strength bars, although I didn't see the estimated error drop at all. Before turning on WAAS and getting the WAAS fix, I watched the coordinates, which were drifting around in the usual way. With the WAAS enabled and d's in the bars, the position did not seem to drift at all.

 

Getting a WAAS fix is much faster now. I presume that once the unit got a lock on the two WAAS sats it downloads some ephemeris that lets it know which ones to look for; it no longer seems to cycle through searching.

 

I would like to perform the 'take measurements at a NGS marker' experiments but I don't understand the different classes of benchmarks. Some are labelled GPS; I presume these were assigned lat and long using GPS observations. Some were clearly surveyed. How do I locate a nearby benchmark that was surveyed and is known to have a

small error, and which has a good view of the sky?

 

-Paul

[+apersson850]

quote:

---

Originally posted by EraSeek:

So far my conclusion is that;

 

WAAS enabled will use more of your GPSR's resources, but whether it acually eats batteries quicker is uncertain.

 

---

 

I would say that having WAAS enabled does not consume any noticeable amount of extra power. However, since you can't use Battery Save mode, having to run in Normal mode will of course use more power.

 

Now, to avoid confusion, this is based on my measurements of my own Vista. Other models or brands may handle satellite based augmentation differently. Still, I've seen in various Garmin manuals, that all Garmin models seem to require that you use Normal mode for WAAS to work.

 

I usually run my Vista in Battery Save mode, with WAAS enabled. WAAS will not work then, of course, but as soon as I connect an external power supply to the unit, it automatically runs in Normal mode. That means that WAAS kicks in, too.

Now, I can't use WAAS here (N57 E14), but I can see if the EGNOS test bed is awake or not.

 

According to the almanac data in the unit, the AOR-E and IOR satellites are visible on the sky. Now and then, I get a signal from AOR-E, but I haven't as of today seen anything from IOR yet.

 

Anders

[+infosponge]

One thing I often see reported is that having WAAS enabled eats up two channels of your receiver that could otherwise be used to get "regular" GPS information and improve accuracy.

A few weeks ago I read that the WAAS birds also transmit regular GPS clock data, so there's no sacrifice of available channels by using WAAS. I can't find this reference now.

I wonder if this is an implementation issue and varies between GPSR manufacturers?

[+apersson850]

At least the Garmin Vista does use the augmentation satellite itself for positioning. That's easy to see, since sats that are received, but not used for positioning, are light gray, but those used for positioning are dark gray. You can get "D" indicators in the sat strength bars, even if the WAAS sat itself isn't used in the position calculation.

 

This would be more of an issue, if it always stole two ordinary channels. Then, when few satellites are visible, very few would remain for ordinary positioning. But it will only use the last two channels, so there are always up to ten channels available for the other sats.

 

In Europe, it will spoil two channels so far, since EGNOS isn't operational, and has the "don't use" status bit on.

 

Anders

[+headmj]

If you want to use a benchmark for a test, look for a first or second order horizontal control. First order controls are accurate to 1:100000 and second order controls are accurate to 1:50000. You can search for benchmarks on the NGS website and specify what kind of control you are looking for NGS search engine. If you are looking on the benchmark pages here triangulation stations are always good horizontal markers. I mention horizontal as opposed to vertical controls because the accuracy on vertical controls is not as good.

[+Kodak's4]

quote:

---

Originally posted by headmj:

If you want to use a benchmark for a test, look for a first or second order horizontal control. First order controls are accurate to 1:100000 and second order controls are accurate to 1:50000. You can search for benchmarks on the NGS website and specify what kind of control you are looking for http://www.ngs.noaa.gov/datasheet.html. If you are looking on the benchmark pages here triangulation stations are always good horizontal markers. I mention horizontal as opposed to vertical controls because the accuracy on vertical controls is not as good.

---

 

Thanks, this was the key I needed. I've identified several nearby horizontal controls and I'll go scope them out today to select one that has a good view of the sky (fortunately the one that is closest to my home seems most promising!)

 

-Paul

[+EraSeek]

Augmentation, spelling it wrong, aren't I?

 

''Power save mode turns off power to your RF receiver for about 4 seconds out of 5 when GPS signals are stable and WAAS cannot function with this limited receiver ON time.''

 

I have read in a report that over here that when one of the WAAS sats went haywire the GPS units would recognize the "Do Not Use" and kick it out and make the channel available for another Sat.

 

Also, for everyones info, they are now working on getting a third bird over the US. This one will be higher in the sky so everyone should be able to get at least one or two at all times.

[+EraSeek]

I just got back from doing a test on top of a 'First Order Horizontal Control' NSG marker. I took a while to get a stable lock on the WAAS satellites because of the area, but when I did this is what I got:

 

With WAAS: accuracy reading of 7.7' and actual distance to mark 3.35'

 

I then switched WAAS off and got: Accuracy 18' and actual 7.3'

 

In each case about 40 pecent difference between the Accuracy reading and the actual.

 

And about a 54% reduction in actual error with WAAS.

 

[+fizzymagic]

quote:

---

Originally posted by EraSeek:

I just got back from doing a test on top of a 'First Order Horizontal Control' NSG marker.

 

In each case about 40 pecent difference between the Accuracy reading and the actual.

---

 

This has got to be the biggest myth of them all: that the accuracy reading reflects that actual accuracy of the GPSr.

 

First of all, the accuracy reading is an estimated accuracy that is based almost entirely on the geometry of the satellites. It is also a probabilitistic estimate, in that it estimates that 60% of the time you'll be closer than the EPE to the true coordinates.

 

People seem to take the EPE number as some sort of Gospel Truth. It's not; it is merely a guideline to give you some rough idea of how the receiver is doing.

 

I can't count the number of times people have claimed that WAAS is no good because it doesn't make the EPE smaller. Well, it probably won't affect the EPE much, because the EPE mainly reflects statistical errors, while WAAS corrects for systematic errors.

 

In other words:

 

Don't trust the EPE number on your GPSr! It doesn't mean what you think it does!

 

[This message was edited by fizzymagic on July 23, 2002 at 06:46 PM.]

[+infosponge]

Good point aboue EPE, fizzymagic.

After the latest Meridian firmware upgrade, I started displaying EPE on my compass screen. I've been using it as a general indication of how good the GPSR is working, to clue me in to reception issues due to terrain, etc...not as a definitive number to say "I'm going to wind up within +/- 23 feet of the cache".

[+EraSeek]

Good point Fizzymagic!

 

Here is a link to what will occur when initializing WAAS for the first time.

http://www.gpsinformation.net/waas/acquire-waas-a.html

 

He also make the comment that with Metroguide Map installed, you will not get an accuracy reading under 20'. Hmmm.

[+fizzymagic]

quote:

---

Originally posted by EraSeek:

 

He also make the comment that with Metroguide Map installed, you will not get an accuracy reading under 20'. Hmmm.

---

 

I have a Magellan unit, so I am not as familiar with the Garmin screens as I could be. But I was amazed to see the EPE showing up on the screen as Accuracy! That is a major problem, in my mind, since it represents EPE as something it isn't!

 

At any rate, EraSeek and Antagonist both did exactly the right thing to determine if WAAS helps or not: they actually measured their accuracy against a known position. Everybody should bookmark their posts for future reference when the subject comes up again, because it will!

[+apersson850]

quote:

---

Originally posted by EraSeek:

Good point Fizzymagic!

 

He also make the comment that with Metroguide Map installed, you will not get an accuracy reading under 20'. Hmmm.

---

 

Oh yes. On top of Omberg, a mountain in southern Sweden, with a very clear view of the sky, and a lot of birds overhead, I had an EPE of 4 meters on my Vista. That's 13 feet, if you use feet for thinking (pun ).

 

No WAAS (or rather EGNOS) available, either.

But sure, EPE is a statistical measure. What they actually show seems to differ from one unit to another, but as far as I've understood, most consumer GPS units go with the 50% figure (we are 50% sure that the position error is no bigger than this). In that case, double the EPE, and you get the 95% figure.

 

No, there is no 100% figure...

 

Regarding the MetroGuide, perhaps you are referring to the uncertainity circle, which will be shown on the map (provided that the unit hasn't "locked on road"). But that circle isn't just the positioning uncertainity, but a combination of that and the assumed position error of the map objects. So, if the map is correct to +/- 10 meters, the uncertainity of your position on that map can never be smaller than that. Then your position uncertainity, relative to the earth, not relative to the map, adds on.

 

Anders

[+apersson850]

quote:

---

Originally posted by EraSeek:

Good point Fizzymagic!

 

He also make the comment that with Metroguide Map installed, you will not get an accuracy reading under 20'. Hmmm.

---

 

Oh yes. On top of Omberg, a mountain in southern Sweden, with a very clear view of the sky, and a lot of birds overhead, I had an EPE of 4 meters on my Vista. That's 13 feet, if you use feet for thinking (pun ).

 

No WAAS (or rather EGNOS) available, either.

But sure, EPE is a statistical measure. What they actually show seems to differ from one unit to another, but as far as I've understood, most consumer GPS units go with the 50% figure (we are 50% sure that the position error is no bigger than this). In that case, double the EPE, and you get the 95% figure.

 

No, there is no 100% figure...

 

Regarding the MetroGuide, perhaps you are referring to the uncertainity circle, which will be shown on the map (provided that the unit hasn't "locked on road"). But that circle isn't just the positioning uncertainity, but a combination of that and the assumed position error of the map objects. So, if the map is correct to +/- 10 meters, the uncertainity of your position on that map can never be smaller than that. Then your position uncertainity, relative to the earth, not relative to the map, adds on.

 

Anders

[+Kodak's4]

Encouraged by this thread I went out yesterday to do some checking at three different NGS horizontal first order control benchmarks close to my home. I was using a Garmin GPS V.

 

Rather to my surprise, I could get a WAAS lock at all three locations. Two of the locations involved standing either in the middle of the roadway or else on a very narrow shoulder to position the GPSR over the benchmark.

 

Despite this, I managed to compare WAAS and no WAAS fixes at all three benchmarks.

 

1. The big surprise was that, in all three locations, the errors for ALL fixes, both WAAS and non-WAAS, was much smaller than I expected. In all cases the difference between the listed coordinates for the benchmark and the indicated position for the GPS was less than 12 feet.

 

2. WAAS seems to help. In no case was the WAAS fix worse than the non-WAAS fix. In one instance, the WAAS fix showed a difference of less than two feet when the previous non-WAAS fix showed a distance of some 12 feet.

 

3. The non-WAAS fix seemed to wander around quite a lot; the WAAS fix seemed to be more stable. That is, I was standing on the benchmark with the unit trying to navigate to the benchmark. In non-WAAS mode, the spot the unit was indicating was 'ground zero' would wander around me, first north of me, then west, etc. In WAAS mode, it seemed that the readings would show ground zero in one spot, and that spot would be in one place and move around relatively little.

 

One observation I made was that the points I picked to make these observations all have a clear view of the sky, and I had a good constellation available. The result is that even the non-WAAS fixes were generated using from seven to 11 satellites.

 

It would be very interesting to know how WAAS affects the results when you're down to just four

satellites.

 

One thing that surprised me was that the unit seemed to be indicating that it was using the WAAS satellite in generating the fix. I was under the impression that the geosynch WAAS sats provided only correction data but perhaps that's not so. Anyone know for sure?

[+apersson850]

The WAAS satellite itself can be used for positioning (dark signal strength bar), but it doesn't have to, in order to get differential data for the other satellites. In such a case, you get the "D" symbols in various bars, but the WAAS bar is still light gray.

 

Anders

[Kerry.]

quote:

---

Originally posted by Kodak's4:

One thing that surprised me was that the unit seemed to be indicating that it was using the WAAS satellite in generating the fix. I was under the impression that the geosynch WAAS sats provided only correction data but perhaps that's not so. Anyone know for sure?

---

 

The WAAS geo's can/are used directly in the position solution if the navigation message allows it.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

quote:

---

Originally posted by Kodak's4:

One thing that surprised me was that the unit seemed to be indicating that it was using the WAAS satellite in generating the fix. I was under the impression that the geosynch WAAS sats provided only correction data but perhaps that's not so. Anyone know for sure?

---

 

The WAAS geo's can/are used directly in the position solution if the navigation message allows it.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

Something that's not a myth is the amount of $$'s so far expended on WAAS with an awful amount of $$'s still to be spent and really one has to wonder what this amount of $$'s have really done

 

US$11 billion for GPS and provides a service to the whole world, US$5-6 billion so far for WAAS and only covers a very limited part of the world.

 

There just doesn't appear to be a six billion dollar improvement (yet) in many respects.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

Something that's not a myth is the amount of $$'s so far expended on WAAS with an awful amount of $$'s still to be spent and really one has to wonder what this amount of $$'s have really done

 

US$11 billion for GPS and provides a service to the whole world, US$5-6 billion so far for WAAS and only covers a very limited part of the world.

 

There just doesn't appear to be a six billion dollar improvement (yet) in many respects.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+ZingerHead]

I'm not privy to Garmin's firmware, but in many low power applications (PDAs and the like) the microprocessor is put to "sleep" whenever it is not busy processing. In this mode the processor consumes considerably less power from the batteries. The duration of each "nap" is on the order of microseconds (for example, between screen refreshes), but over the life of a battery charge this can add up to considerable energy savings. Since WAAS is computationally intensive, there will necessarily be fewer opportunities for the chip to enter sleep mode, so you use more power over time. How much is entirely dependent on usage.

[+EraSeek]

Myth #1 is true. WAAS does eat batteries. Here is Garmin's reply:

 

"Dear Sir,

 

To bad Raytheon can't be a little more honest. Using WAAS causes the unit to have to pick up on a second satellite system. Then once that has been done, the additional information has to be processed, then added to the GPS information. this adds to the load the processor is under causing a use of more power. Once you sit back and think about it, it has to use more power. Reception of a second system and additional processing.

 

If you have further questions, please do not hesitate to contact us.

 

Thank you and best Regards,

 

Product Support Specialist

* Garmin International

1200 E. 151st Street"

 

 

[This message was edited by EraSeek on July 28, 2002 at 08:34 PM.]

[+Gliderguy]

quote:

---

Originally posted by Kerry:

Something that's not a myth is the amount of $$'s so far expended on WAAS with an awful amount of $$'s still to be spent and really one has to wonder what this amount of $$'s have really done

 

US$11 billion for GPS and provides a service to the whole world, US$5-6 billion so far for WAAS and only covers a very limited part of the world.

 

There just doesn't appear to be a six billion dollar improvement (yet) in many respects.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

---

 

Kerry, of course you do know the original intent of the WAAS was so aircraft can shoot precision approaches in low visibility conditions without any equipment being required on the ground at that airport.

 

That would actually save millions every year on required maintenance on all the approach equipment around the nation. Plus now you could shoot a precision approach to some podunk airport that never had any approach equipment installed.

 

Of course, the primary problem is the system has to be able to report any errors in position IMMEDIATELY so an approach could be aborted before a hazardous situation occured. This is why the system has not been implemented in aviation yet.

 

Being airborne takes care of a lot of the reception problems that ground based users are reporting in using and maintaining WAAS.

[+Gliderguy]

quote:

---

Originally posted by Kerry:

Something that's not a myth is the amount of $$'s so far expended on WAAS with an awful amount of $$'s still to be spent and really one has to wonder what this amount of $$'s have really done

 

US$11 billion for GPS and provides a service to the whole world, US$5-6 billion so far for WAAS and only covers a very limited part of the world.

 

There just doesn't appear to be a six billion dollar improvement (yet) in many respects.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

---

 

Kerry, of course you do know the original intent of the WAAS was so aircraft can shoot precision approaches in low visibility conditions without any equipment being required on the ground at that airport.

 

That would actually save millions every year on required maintenance on all the approach equipment around the nation. Plus now you could shoot a precision approach to some podunk airport that never had any approach equipment installed.

 

Of course, the primary problem is the system has to be able to report any errors in position IMMEDIATELY so an approach could be aborted before a hazardous situation occured. This is why the system has not been implemented in aviation yet.

 

Being airborne takes care of a lot of the reception problems that ground based users are reporting in using and maintaining WAAS.

[+apersson850]

While I don't know, it seems reasonable that Garmin has choosen a design, where the CPU or a Vista, for example, can go into IDLE mode, and that this IDLE mode means a substantial reduction in power consumption. Or at least that it can slow down, when no computationally intensive tasks are running, and save power that way. Most low power designs eat more when running at higher speed.

 

Still, I would assume that running the twelve channel receiver all the time accounts for a great deal of the power consumption, so I think the fact that you can't use battery save mode is the biggest criminal in this case.

 

Anders

[Kerry.]

Gliderguy, I think what sort of slowed WAAS down a little was the setting to zero of SA and the $$$'s that were steadily and steadily building.

 

At this point in time most countries simply couldn't afford or justify a WADGPS (WAAS) type system but still not a wheels on the ground type landing system in any weather as that where LAAS sort of fits in.

 

The reporting of those errors considering the time frame they need to be reported in is a critical component with WAAS (still to be solved in some instances). I suppose WAAS is WAAS but as it's implemented in most handheld type receivers is rather a little different to the full capability of the system as far as error reporting and capability is concerned.

 

For recreational type handhelds it's rather not an ideal type of system to even be bothering with at this point in time but some have seen the need to promote it as "the thing" (sometimes the only thing), which really it's not but as you say for aircraft that's a totally different story but for some reason developement has appeared to have slowed remarkedly in recent times?

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go