Got A Question About Waas

[+Thot]

Attacking the sources as "boooooooooooooogus!"  and "useless outdated data" is not productive and certainly serves no purpose.  Yelling the word "FACT" before each statement.... 

 

Talking, intelligently about the details, . . . is productive.

Sometimes you're faced with fighting fire with fire. When somebody makes an unsubstantiated statement that such and such is so without giving any evidence or logic to support his statement and you know it’s wrong, about all you can do is say, “That’s bogus.” Unless I missed it, nobody in this discussion has given a reason/a basis for their belief. Probably because I suspect he’s correct, it seems to me EraSeek came the closest to giving a reason for his belief.

 

I admit I don’t know the answer to this question, but I can give the reason for my guess.

 

How far the WAAS satellite is away from your location has nothing to do with how good the correction data is that the satellite has/sends, because it’s data comes from ground stations near you. That is, ground stations near you determine what corrections need to be made to signals received in your area. These corrections are then sent by secure channels to the satellite. So, the data being sent from the satellite is good, and is not degraded by how far away it is.

 

WAAS is a very modern/recent system. Therefore, it’s almost certain the data/signals it sends contain error detection and/or correction methods (Sort of like CRC checks on computer transmissions). All modern data transmission systems have error handing. So, your receiver knows whether it got good data from the transmitter or not, and very possibly is able to correct any errors encountered in transmission/receipt. I think that’s what EraSeek means when he says

 

If you you recieve the WAAS signal weak or not you get the data. Distance from ground stastion is NOT a factor with WAAS

 

That is, if your GPSr receives the WAAS signal it can verify if it is correct data or not, and if not it won’t be used. 

So, your GPSr is either using good/valid correction data, or it’s not using WAAS correction data at all. In either case it cannot make your position error worse.

 

Maybe I missed it by I didn’t see where NightPilot said WAAS ever degraded results. He just explained why the GPSrs on his aircraft don’t use it.

 

There’s one way I can imagine using WAAS might make things worse but I don’t know how GPSrs work well enough to know if they do this. A GPSr can only receive so many satellites at one time. Let’s call the possible satellite capacity channels. If it could use all these channels for GPS satellites, but must devote one of two of these channels to WAAS instead of GPS satellites. And, it can’t switch over to GPS when WAAS is not available, then it’s conceivable that the loss of these otherwise available GPS channels could reduce your position slightly. As I said, I have no idea whether satellite channel assignments work like this at all, and even if they do, it would surely drop the least valuable satellites. And, most receivers have something like 12 channel capability anyway. So, it seems unlikely the loss of accuracy would be meaningful. If GPSrs devote channels to WAAS whether using WAAS or not, then all this last hypothetical/conjectural paragraph is BS.

Edited March 27, 2005 by Thot

[+VirginiaJim]

Not agreeing with a post or information can be part of any discussion. Intelligent mature people can exchange information and ideas without insults. Those who result to them often have other agendas than the exchange of information. Eraseek, you have been a gentleman and have no need to be sorry.

 

I now this is taking a risk, but here is a quote about WAAS and DGPS degradation.

 

http://gpsinformation.net/exe/waas.html

 

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."

 

It appears that this explains why distance is not a factor in WAAS corrections in the first paragraph.

 

What do you think?

 

"Unless I missed it, nobody in this discussion has given a reason/a basis for their belief. Probably because I suspect he’s correct, it seems to me EraSeek came the closest to giving a reason for his belief."

 

Thank you for this concept Thot, giving a reason or basis for something. Not just labeling input as bogus or attacking the author with a false sense of superiority.

[+EraSeek]

The link wouldn't work for me, but YES this is exactly correct. The above explains it very well. With WAAS the reference stations collect data from their locations, they send it to the master station which make a nation-wide model (grid) where they take the info from the individual stations and fill in the blanks, createing the grid model. This info is tranmitted through the WAAS sats to your GPS. Your GPS knows where it is in the modeled grid and applies the proper correction data for your location.

 

Now with regular DGPS, the collection stations simply transmit corrections for their locations and the further away you are the more in-correct it will be. With regular station tranmited DGPS you also need special equipment and a subscription as I understand it.

 

Actually both system work faily well. It seems WAAS just is a better system, and cheaper.

 

I must say that most of this discussion is somewhat academic, because although you have greater acurracy with DGPS/WAAS it is not a huge difference for what we are doing.

 

Also I have no special training in this area, just an interest. I can certainly be wrong about things. My only interest is in learning. If anyone correctly points out where I am wrong I welcome it. I'd rather be correct than "right".

[+EraSeek]

Oh, one other thing. The GPS signal is by its nature very weak, but sufficiant to do the job. It transmits on a frequency that is chosen to be able to penetrate rain, snow, sleet, clouds, fog, and the like with out noticable hinderance. The global positioning system also has a built in failsafe where if there is unusable data it is flagged immediatly for safty reasons. However, I understand that this feature is only in very high end units. But remember, the almanac is re-transmited every 12minutes in the regular gps signal, so for our purposes (not flying aircraft) we would hardly notice any occasional glitch.

Edited March 28, 2005 by EraSeek

[+EraSeek]

Gees, I keep remembering things here. One of the great benefits of WAAS that we do not see (beyond the corrections) is this integrety of the system issue. That is one of it's main purposes. To insure the integrety of the system. How that is accomplished by WAAS I'm not sure, but I know it is a big thing.

 

Another thing is that the WAAS almanac is much like a windows update download. If the signal is broken during the transmission by trees or whatever, when the signal is picked up again it resumes where it left off, so you have not lost what you have gained. Only after you have aquired the fundamental parts of the almanac will it begin to apply it and correct the satilittes. Only after a certain number of sats have been corrected only then will the corrected position solution be applied to your GPS. So no, niether WAAS nor a weak or broken signal can degrade your fix, only lousy firmware can.

[+NightPilot]

WAAS doesn't insure integrity. Certified aviation units have Receiver Autonomous Integrity Monitoring, or RAIM, and even units without WAAS have it. It predicts reception in the future, from satellite orbits, and warns if integrity will be lost, in addition to any realtime loss of signal. We get RAIM warnings now and then, and have to abort the GPS approach if we're not past the final approach fix, or notify ATC if a warning doesn't go away within 5 minutes enroute, if we're using GPS as sole source of navigation. RAIM warnings typically last for a few seconds. All this is entirely without WAAS capability. All WAAS does for integrity is give slightly more reliable signals.

 

The question of whether WAAS can degrade position accuracy depends on the implementation of software in the GPS receiver. From what I've read, accuracy could be degraded on some older units if it was turned on and the satellites weren't received, but newer units should not have that problem. My opinion is "maybe". As I said previously, I keep it turned off on my Legend, because in the thick woods down here in SE TX, you can't receive the WAAS satellites anyway, so even if it doesn't hurt, it certainly can't help. And if I need WAAS to find a lamp pole in a parking lot, then I'll think about quitting altogether. YMMV.

[+Thot]

As I said previously, I keep it turned off on my Legend, because in the thick woods down here in SE TX, you can't receive the WAAS satellites anyway, so even if it doesn't hurt, it certainly can't help. 

Again I’m no expert on WAAS and may have misconceptions about it but as I understand how it works, once your GPSr has received the correction grid (shows WAAS obtained after about 5 minutes) it can interpolate the corrections for your location over a large area (hundred or more miles?). It doesn’t throw this grid and the correction data away when it loses the signal, it continues to use the latest grid it has. It will change gradually with time but not with distances of less than 10 miles. So, when you walk a thousand yards into the woods, even if you lose WAAS signal along the way the correction data is, for all practical purposes, as good as it was when your unit acquired the correction grid data while you were receiving signal.

 

In short, once you have obtained WAAS correction data it is as effective in the woods and anywhere else. And, as flat as Houston is and a far south as it is, I would think you have much better WAAS satellite acquisition, even under marginal conditions, than most people in the US.

Edited March 29, 2005 by Thot

[reidster]

Not agreeing with a post or information can be part of any discussion. Intelligent mature people can exchange information and ideas without insults. Those who result to them often have other agendas than the exchange of information. ...

 

It appears that this explains why distance is not a factor in WAAS corrections in the first paragraph.

 

What do you think?

 

"Unless I missed it, nobody in this discussion has given a reason/a basis for their belief. Probably because I suspect he’s correct, it seems to me EraSeek came the closest to giving a reason for his belief."

 

Thank you for this concept Thot, giving a reason or basis for something. Not just labeling input as bogus or attacking the author with a false sense of superiority.

What do I think? That my statement was correct all along! Thanks for your support.

 

My original post was based on hours of research and facts.

Yes, I did have an agenda and that is why I said what I said.

I am a mature intelligent person who sees the benefit, and often facilitates, the exchange of factual information.

I've seen the statement I and others referred to as bogus here and elsewhere and felt it was time for a thorough discussion of what WAAS is and isn't. I want to thank Thot, EraSeek, JohnnyVegas, and others for their contributions.

 

VJ,

Did you even read my post?

My post did more than just label the author's statement as bogus.

It contains relevant statements of facts to support my position!

Facts that were endorsed by others and proven true.

"false sense of superiority"? ROTFL

[+EraSeek]

Nightpilot I thought there was an additional more fundemental integrety issue with WAAS. Maybe not. I'll see if I can fine it.

 

I also turn off WAAS in the woods. WAAS takes up 2 channels, and if you can't get it anyway may as well free up the channels.

 

Your unit retains much of the almanac till the next data cycle is recieved, but after loosing the WAAS sat signal, yes, there correction are still good for a good period of time, ...however with Garmin units the corrections are only apllied for 2 minutes after loosing the WAAS signal. Don't know why.

[+Thot]

with Garmin units the corrections are only apllied for 2 minutes after loosing the WAAS signal. Don't know why.

Once my SporTrack Pro acquires the WAAS grid data, it essentially never stops indicating that it’s using it. [unless it loses GPS lock entirely.]

 

As far as I know, the SporTrack has no way to turn WAAS off.

Edited March 29, 2005 by Thot

[+JohnnyVegas]

Oh, one other thing. The GPS signal is by its nature very weak, but sufficiant to do the job. It transmits on a frequency that is chosen to be able to penetrate rain, snow, sleet, clouds, fog, and the like with out noticable hinderance.

It is also transmitting data which does not require a very strong signal.

[+VirginiaJim]

 

Did you even read my post?

I will not belabor the point that you can discuss things without screaming FACT, name calling, and a condesending attitude. Manners and maturity here and elsewhere speak for themselves. They are also qualities that will encourage others to listen to your "FACTS."

 

From the Garmin Etrex Manual

 

Becasue the Side Area Augmentation System is currently under development and is not full supported by ge-stationary satellites, effctive use of the WAAS feature may be limited by your geographic lication in relation to those satelliltes now in service.

 

Currently, WAAS satellite signal reception requires an absolute clear view of the sky and works best when there are no nearby obstructions such as buildings, mountains, etc. 

 

Not enabling WAAS when you cannot receive a signal will speed unit map drawing and conserve battery power.

 

What do you think? Garmin is speaking about the limitations of WAAS and the negative effect on their GPS units. A degradation of the units ability to draw maps. Therefore, a reason not to use WAAS in these situations. So, even the makers of these units at Garmin believe there can be a degradation of the performance of their units with WAAS under certain conditions. Exactly what my original post and quotes suggested.

 

Don't get upset with me for posting a quote from Garmin. I'm just asking you to consider that they may be right. That your "FACTS" might be incorrect.

[+VirginiaJim]

So we agree that distance from the ground stations is not a factor?

 

Most seem to agree that WAAS, like any technology has limitations?

 

How about that there can be a degradation of the gps units performance using WAAS under certain conditions which you and I are often in while geocaching according to Garmin?

[+Thot]

Most seem to agree that WAAS, like any technology has limitations?

I’m not sure what you’re trying to say. Everything has “limitations” – you can’t drive a toaster to town. You can’t watch Jay Leno on your shoes. There’s no doubt that sometimes your GPSr will not be able to acquire a WAAS signal.

 

How about that there can be a degradation of the gps units performance using WAAS under certain conditions which you and I are often in while geocaching according to Garmin?

It seems like this discussion is shifting. You originally quoted this statement:

 

Under certrain conditions-say, when weak WAAS satellite signals are being received or the GPS receiver is a long way from a ground station-accuracy can actually worsen when WASS is enabled.

Later you said

You will find in multiple discussions about WAAS that poor reception can in some cases reduce the accuracy of your GPS.

These statements regarding accuracy began this somewhat contentious exchange. I think the consensus among this less than expert group is that poor reception cannot reduce accuracy.

 

Now you seem to be considering things like causing a slowdown in calculating position as “a limitation” of WAAS technology causing “degraded performance”. I suspect when you say limitation and degradation many readers think you are still talking about degraded accuracy. I think it will be more productive if you ask about specific issues rather than vague terms like “limitations” and “degrade”

 

By the way, this topic is probably more appropriate for the GPS forum.

Edited March 29, 2005 by Thot

[reidster]

 

Did you even read my post?

I will not belabor the point that you can discuss things without screaming FACT, name calling, and a condesending attitude. Manners and maturity here and elsewhere speak for themselves. They are also qualities that will encourage others to listen to your "FACTS."

 

From the Garmin Etrex Manual

 

Becasue the Side Area Augmentation System is currently under development and is not full supported by ge-stationary satellites, effctive use of the WAAS feature may be limited by your geographic lication in relation to those satelliltes now in service.

 

Currently, WAAS satellite signal reception requires an absolute clear view of the sky and works best when there are no nearby obstructions such as buildings, mountains, etc. 

 

Not enabling WAAS when you cannot receive a signal will speed unit map drawing and conserve battery power.

 

What do you think? Garmin is speaking about the limitations of WAAS and the negative effect on their GPS units. A degradation of the units ability to draw maps. Therefore, a reason not to use WAAS in these situations. So, even the makers of these units at Garmin believe there can be a degradation of the performance of their units with WAAS under certain conditions. Exactly what my original post and quotes suggested.

 

Don't get upset with me for posting a quote from Garmin. I'm just asking you to consider that they may be right. That your "FACTS" might be incorrect.

Upset? Not in the least. In fact it just goes to prove my point even better.

The source (copyright October 2002) you quoted is out-of-date and inaccurate. WAAS is developed to the point that it has been FAA certified for aviation navigation, July 10, 2003. If that's not good enough, God help us all. The remainder of what they say, although true, does not support what your original post suggested. The author's quote you posted only addressed accuracy, which is the only thing I ever disputed. To broaden the scope to include issues (mapping speed & power consumption) that have nothing to do with my original premise (accuracy) in order to discredit my integrity is truly immature.

So stop attacking me with petty nonsense, but feel free to address "exactly" what I stated as fact with facts.

Edited March 29, 2005 by reidster

[+StarBrand]

Let me chime in to say, as an a college instructor for technology including GPS, WAAS can only serve to enhance the position reported by your GPS and in no way is going to make it less accurate. Given that most GPS error is influenced by atmoshpherics (sp) and/or satellite geometry in relation to the reciever - the corrections recieved by WAAS do nothing to help geometry and can only serve to make atmosheric errors predictable. How could it possibly be worse error?? WAAS eats batteries faster and slows the unit down (because of computational requirements) but doesn't induce postional error. The further away you are from the ground stations - it won't give as good a correction but it will be something of a correction. At any rate, in the United states, you would be hard pressed to find many spots more than 200 miles from a ground station.

 

The only real issues are: 1 - can you get the WAAS signal?? 2 - Do you care that your batteries will die sooner?? 3 - Is it really going to help you find the Geocache since you don't know how bad the error was at placement???

[+DaveA]

So we agree that distance from the ground stations is not a factor?

 

Yes, with WAAS, distance from a ground station is not a factor. Whether or not one is in the coverage area of the ground stations is.

 

Most seem to agree that WAAS, like any technology has limitations?

 

Of course.

 

How about that there can be a degradation of the gps units performance using WAAS under certain conditions which you and I are often in while geocaching according to Garmin?

 

No.

 

Maybe.

 

Depends what you mean by "performance".

 

In this thread I have seen several arguments. One argument is 'weak' reception causes positional errors. This is false. The satellites transmit time stamp pings the hardware uses to compute position. Either this data is recieved or it isn't. There are things that can slow this ping down and this does degrade accuaracy, but this is one of the things WAAS corrects for.

 

Another argument is that the map screen will refresh faster and batteries last longer with WAAS off. I suspect this is more dependant upon the hardware than anything else, but I am sure it's a possibility. If an owner's manual says this is true of the unit then I would assume it is true.

 

Another argument is that WAAS accuracy might be degraded under tree canopy, around water, rocks or some other area. This is false. The regular satellites can be affected by a number of things. It slows down the speed at which the ping reaches your unit. This degrades accuracy. WAAS corrects for this (although not for geographically isolated land based degredations). The data WAAS sends is different from the data the other sats send. The WAAS data, within reason, is not time sensitive. If the GPSr is in an area where the pings from the regular satellites is slowed down, the WAAS signal will not make the unit less accurate. WAAS will not be able to correct for the isolated land based degredation of the satellites, but the use of WAAS will not result in decreased positional accuracy.

 

Long story short is unless your unit has bad firmware your positional accuracy improves with WAAS, it never degrades or at least I can't think of any scenario where it would cause a degredation in accuracy, but some of that has to do with the unit's firware, i.e. what it does with the data so I concede that there may be some units on the market that will do goofy things. This goes back to the bad firmware thing though, it isn't a fault in WAAS itself. If you can recieve the data, you will get best accuracy with WAAS enabled with the exception of lousy firmware which I would hope isn't an issue on any second or later generation WAAS enabled units from any manufacturer. Some of the first generation units were actually made before WAAS 'went live'. It was a rush to get to market first kind of thing. I don't know, but wouldn't be surprised if those units behave in a less than optimal manner with WAAS, but I would think a firmware update would address any issues.

[+Thot]

The data WAAS sends is different from the data the other sats send.  The WAAS data, within reason, is not time sensitive. . .

 

. . unless your unit has bad firmware your positional accuracy improves with WAAS, it never degrades

 

If you can recieve the data, you will get best accuracy with WAAS enabled . . .

You speak with authority on this subject. What is the basis for this authoritative knowledge. I am not being critical or making a putdown – almost everything you said agrees with my limited understanding, I just wondered where/how you came to know so much about WAAS.

 

I may have one small quibble with what you said. You say:

with WAAS, distance from a ground station is not a factor

I believe the farther one is from a ground station the less accurate the correction data are. If you are near a ground station the correction is the most accurate. As you move away from ground stations mathematical calculations are used to interpolate/project/estimate the correction factors using the closest stations. So, as you get farther away from a ground station these "calculated" correction factors become less accurate, But, even if less than ideal they’re always more accurate than no correction factors at all -- at least that's how I think it is.

Edited March 29, 2005 by Thot

[+VirginiaJim]

I have made several post regarding accuracy. In fact, I made a post quoting a source that was one of the first to explain why distance from the ground station was not a issue with WAAS accuracy.

 

"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."

 

During the course of the discussion I additionally suggested and talked about limitations of WAAS with our handheld GPS units.

 

In each case I have quoted sources including Garmin to discuss this issues with WAAS and our units.

 

" To broaden the scope to include issues (mapping speed & power consumption) that have nothing to do with my original premise (accuracy) in order to discredit my integrity is truly immature.

So stop attacking me with petty nonsense, but feel free to address "exactly" what I stated as fact with facts."

 

I'm not sure why it is taboo to broaden the scope of the discussion. We already agree on the subject of accuracy. When I quote a source it is to have it considered and discussed.

 

The orginal premise was not even yours. It was:

 

"Do you recomend using WAAS or Not using it?"

 

Discussing the effect of WAAS on mapping speed is a factor to consider in using or not using it.

 

As for your integrity, we proabably shouldn't discuss that here. Let's you and I concentrate on the discussion of WAAS.

Edited March 29, 2005 by VirginiaJim

[+DaveA]

You speak with authority on this subject.  What is the basis for this authoritative knowledge.  I am not being critical or making a putdown – almost everything you said agrees with my limited understanding, I just wondered where/how you came to know so much about WAAS. 

 

Nowhere special, just reading here and there. I am a researcher at heart.

 

I may have one small quibble with what you said.  You say:

with WAAS, distance from a ground station is not a factor

 

I agree with your quibble, but I am not 100% certain on this point. There may be a small degree of lowered accuracy with distance from ground stations or there may not be. I don't understand the method used to determine the corrections needed inbetween stations well enough to say for sure. In any event my understanding is it isn't anything to worry over when speaking in practical terms.

Edited March 29, 2005 by DaveA

[+EraSeek]

The ground stations send their data to the master station. The master station makes a modeled grid based on the data from all the stations and a scientific understanding of the nature of the ionosphere. Model is produced, gound stations disappear. No longer a part of the system. Now all you have is a modeled grid. Like a big net with waves in it. GPS knows where it is in that net. Think of it this way. Ground stations help you understand where the top and trough of the ionospheric wave is, and its form or rate of drop. That's all. The ground station has no relationship to your GPS or transmissions. ONLY data for the forming of the model! Now it is true that the models grid will be most precise at the point of where the ground stations are, but in relation to your GPS corrections it would be miniscule.

 

Same with clouds and rain. It does have some effect on your GPS signal, but never enough to make a difference.

 

Same with the difference between WGS84 and NAD83. Yes there is a difference, but so small your GPS does not have the percision to measure it.

 

Here is what does affect your position beyond WAAS corrections: Satillite geometry and multipathing. WAAS does not, can not fix these.

[+Thot]

Now it is true that the models grid will be most precise at the point of where the ground stations are . . .

That's my point. (Pun intended)

[+EraSeek]

Yes indeed. We are to the point of splitting hairs