Waas On Or Off?

[+gerper]

I have a Garmin Vista, and I am wondering if I should turn the WAAS on or off for geocaching? Does WAAS allow the unit to update sooner and with more or less accuracy?

 

Are there any other adjustments or changes that I could make to the settings of my Vista to make it more accurate for caching?

[+Prime Suspect]

Until there are more WAAS birds in the sky, you're usually better off leaving it turned off. Your GPS won't mix WAAS adjusted and non-WAAS adjusted data when calculating a position. If you have a lock on 8 satellites, but have current WAAS data for only 4 of them, the GPS will only use the WAAS adjusted data, ignoring the other 4 satellites. This means you may end up with a less accurate position that if you had WAAS turned off. And "current WAAS data" means within the last 90 seconds or so.

 

"Don't mess with a geocacher. We know all the best places to hide a body."

[+Renegade Knight]

I leave WAAS off even though it gets me a better location. The reason is that on my GPS V it noticably slows down my position updates. When you get close to a cache and are trying to find ground zero (normally this is when whoever I'm caching with finds the cache) the delay is annoying.

[4x4van]

Is there a way to turn WAAS off on my SporTrak Pro? A secret menu or something? With or without WAAS, I seldom see any better than 25' EPE (Estimated Position Error). I'd like to try it with WAAS off and see if I notice any difference.

[+Cornix]

Is there a way to turn WAAS off on my SporTrak Pro?

It's in the secret menu #3:

• Press the Menu key, then Right, Left, Right, Left on the large key.
   
  
• Now a box with "00" should appear. Change that to "03" with the large key and press Enter.
   
  
• Press Enter again to deactivate/activate WAAS (the selected WAAS mode is shown as On or Off).
   
  
• Press Escape, Escape and Power Off.
  

Cornix

[+Prime Suspect]

Is there a way to turn WAAS off on my SporTrak Pro?

It's in the secret menu #3:

• Press the Menu key, then Right, Left, Right, Left on the large key.
   
  
• Now a box with "00" should appear. Change that to "03" with the large key and press Enter.
   
  
• Press Enter again to deactivate/activate WAAS (the selected WAAS mode is shown as On or Off).
   
  
• Press Escape, Escape and Power Off.
  

Cornix

Is this for real? I'll chalk that up to another reason why I like Garmin.

[+Cornix]

Is this for real?

No kidding. These menus are hidden to protect casual users from shooting themselves in the foot.

 

Here's a list (pdf file).

 

Cornix

[Karl360]

will this work for the sprotrak basic yellow model? I wont mess up the unit will I? Its easy to turn the WAAS back on?

[+EraSeek]

Until there are more WAAS birds in the sky, you're usually better off leaving it turned off. Your GPS won't mix WAAS adjusted and non-WAAS adjusted data when calculating a position. If you have a lock on 8 satellites, but have current WAAS data for only 4 of them, the GPS will only use the WAAS adjusted data, ignoring the other 4 satellites. This means you may end up with a less accurate position that if you had WAAS turned off. And "current WAAS data" means within the last 90 seconds or so.

 

 

You are better leaving WAAS on, unless you know you can not pick up the WAAS sats, then you may as well use the slots for other sats. They did not spend major bucks developing the WAAS system for nothing! WAAS resolves a number of error producing factors and will often improve your postion from 12'-18' down to a few feet. Don't think so? Enable your waas, find a GPS grade benchmark, stand on it and see what happens with WAAS on and off. WAAS, when enabled, and signal achieved, will begin adjusting all the sats in your window, not just one or two or four. It is just a matter of allowing the the almanac to download its data. Patience is the key. 12 mins may be required to acquire the complete almanac, but improvements in position will usually take less. Improved position data will remain in effect for 2 mins after losing the WAAS signal.

 

Now the downside. WAAS uses more power, but only a little. Waas slows processing time, but only really noticable if you are using other intense apps such as auto-routing. WAAS uses up 2 sat slots when turned on, so if you know you can't get them turn it off (yeah Garmin!). If you got losey geometry, you got losey geometry WAAS or not. WAAS improves your position significantly, but in terms of Geocaching it is not a big deal. 15' is almost as good as 5' for us.

[+GeckoGeek]

For those of you who have had negative WAAS experience, you might check and see if there's a firmware update. I noticed in the past year Garmin has come out with new firmware that prevented units from using WAAS if the unit was outside the WAAS service area. Apparently the old firmware was doing things wrong in some situations.

[+Cornix]

will this work for the sprotrak basic yellow model?  ...  Its easy to turn the WAAS back on?

It should work for all Meridian and SporTrak models. When you are in menu 03 you can toggle WAAS on/off with the Enter key.

 

But as EraSeek said: "You are better leaving WAAS on, unless you know you can not pick up the WAAS sats ..."

 

Cornix

[+Prime Suspect]

Sorry, but I have to disagree. WAAS was designed for the aeronautics industry. If tree cover is concern during a flight, then you've got much bigger problems than your WAAS setting. However, it is a concern when geocaching. WAAS works best when you can get a continuous, uninterrupted signal. Not too hard for an airplane in flight, but not too easy when walking around in the woods. When you only have partial WAAS data, you're actually reducing the number of satellites that are used to calculate your position, which can result in a less accurate calculation than if you had WAAS turned off. Now, if you're in a situation where you will always have nothing but open sky overhead, (and have the time to wait for the data transfer), then yes, you may want to turn WAAS on. Otherwise, you're probably better off without it.

[+GeckoGeek]

When you only have partial WAAS data, you're actually reducing the number of satellites that are used to calculate your position, which can result in a less accurate calculation than if you had WAAS turned off.

Assuming you can see more then 10 sats.

[+blindleader]

When you only have partial WAAS data, you're actually reducing the number of satellites that are used to calculate your position

I just have to ask if you have a source for that. I've never seen any manufacturer of consumer GPSRs share the detailed operation of their software with the public. If you know a place on the web I can get this information I'd be obliged.

 

If I were programming a GPSR I would never do as you describe and ignore satellites for which it did not receive WAAS data if that were to decrease the accuracy of the fix.

[+Cornix]

I'm using a SporTrak Pro in Europe (N 49°) with WAAS (EGNOS) enabled. The unit has no problems if there are no EGNOS birds visible. But when EGNOS signals are available, the accuracy is great (EPE = 3m).

 

Cornix

[+TeamCNJC]

Assuming you can see more then 10 sats.

 

Actually, if you're only receiving corrections for 4 sats, you'll only be calculating a position based on those 4 sats, no matter how many sats are visible. That implemention of WAAS seems to be particular to Garmin units, and may have been corrected in later firmware versions. There's also a concern that Magellan units may continue to use "stale" WAAS corrections, so it's unclear if any manufacturer has figured out WAAS completely.

 

I just have to ask if you have a source for that

 

There's a lot of good WAAS information at gpsinformation.net. If you don't agree with them, please don't flame the messenger.

[+wallygrundle]

Dang. Good thread. This explains a lot about why I get funky readings in the trees sometimes despite the fact that I've got multiple birds acquired in my Legend. I'm going to re-visit a few caches I did last weekend, but I'll do it with WAAS off and see if I get better results.

 

Is it fair to say (now that I've read the whole thread) that WAAS works better when you have better line of sight, but it should be turned off if you have bad line of sight?

 

Thx, all.

[Kerry.]

If I were programming a GPSR I would never do as you describe and ignore satellites for which it did not receive WAAS data if that were to decrease the accuracy of the fix.

That's exactly what differential type systems must do to maintain integrity and that is they simply can't use satellites for which there is no transmitted corrections.

 

If receivers didn't ignore satellites for which it did not receive data then the accuracy & integrity of the position is simply compromised and uncertain.

 

Satellites MUST be common for dGPS corrections as it is range corrections that are transmitted for each satellite (max 9) not simply a position correction. The receiver then applies what common satellite range corrections it receives to the satellite ranging data that is common.

 

WAAS is slightly different but very similar as there must be a certain number of common satellites and these satellites must be in view from several different ground stations. Data that is not common must be ignored.

 

Cheers, Kerry.

[+blindleader]

There's a lot of good WAAS information at gpsinformation.net. If you don't agree with them, please don't flame the messenger

Without the link to the source, I don't know what I'm disagreeing with or even if I am disagreeing with anyone. I'm just looking for information other than the hearsay of this board.

 

Still waiting...

 

That's exactly what differential type systems must do

As always, Kerry, your post is largely tangential to the discussion at hand (though this one is closer than most). The subject is consumer hand held units. You may be right (I don't know) that the WAAS specification prohibits using a mix of signals from WAAS corrected and non-WAAS corrected satellites, but there is no inherent technical or mathematical reason it can't or even shouldn't be done.

 

More to the point in the discussion at hand is whether a particular consumer grade GPSR would be programmed (for example) to use four WAAS corrected satellites that are bunched together in a 20 degree range of azimuths (giving DOP in double or even triple digits) when eight other satellites are nicely spaced for a sub 2.0 GDOP. This is the kind of information I've never seen published by any manufacturer. I doubt it is to be found among the fifty plus articles mentioning WAAS at gpsinformation.net.

 

Still waiting for that source...

[+nincehelser]

I've never been real impressed with WAAS, so I just leave it off.

 

I just don't need that level of accuracy for the things I do, so why sweat it? It just doesn't seem worth the worry of if I'm getting the WAAS signals or not.

 

I'm not a pilot, so I don't quite understand why WAAS was developed. It's more accurate, sure, but from what I understand it still isn't good enough to land a plane.

 

I guess what I'm wondering is who really benefits from WAAS? Is it improving airline safety or something like that?

 

George

[+EraSeek]

You may be right (I don't know) that the WAAS specification prohibits using a mix of signals from WAAS corrected and non-WAAS corrected satellites, but there is no inherent technical or mathematical reason it can't or even shouldn't be done.

 

Let me take a guess at this one. Say you have a mix of WAAS corrected and non corrected sats. Basically you are talking about timing errors. Delays of signals. Corrected sats are going to be using an improved time, where the other sats use a delayed signal complete with an incompatible clock compared with the improved sats. Thus you would have a much greater error when triangulating than if you used all unimproved, or all improved signals. The timing errors are different, thus throwing you position off even further.

[+EraSeek]

I've never been real impressed with WAAS, so I just leave it off.

 

I just don't need that level of accuracy for the things I do, so why sweat it? It just doesn't seem worth the worry of if I'm getting the WAAS signals or not.

 

I'm not a pilot, so I don't quite understand why WAAS was developed. It's more accurate, sure, but from what I understand it still isn't good enough to land a plane.

 

I guess what I'm wondering is who really benefits from WAAS? Is it improving airline safety or something like that?

 

George

Anytime you have improved accuracy, you have improved safety. No, its not vital to us, but it can be vital to others. WAAS is discribed as Bouy-bumping accuracy. Say you are in a thick fog. You need to bump that bouy. No Waas and you miss it by 20' maybe 50'. Big deal. You missed the bouy. In Alaska I've been in situations like that. Only it is not a bouy you are trying to hit, but a very narrow channel between shoals. Without WAAS your postion can jump around quite a bit. A charged ionosphere contributes to much of this. WAAS not only corrects this errors but does it often enough to give a much more stable postion. I've pulled bodies out of the water that have missed some of these channels.

[+PDOP's]

I'm not a pilot, so I don't quite understand why WAAS was developed.  It's more accurate, sure, but from what I understand it still isn't good enough to land a plane. 

 

I guess what I'm wondering is who really benefits from WAAS?  Is it improving airline safety or something like that?

 

George

Part of of what WAAS does is to monitor the health of the GPS system and immediately recognize if there is a problem that could affect the accuracy of the system. If a satellite has problems the WAAS system recognizes it and does not use that satellite for position fixes.

 

Once the system is complete including L(local)AAS it will be able to accurate enough to land a plane.

 

Free flight is one of potential benefits. This would allow individual aircraft to navigate their own direct route instead of following the "highways in the sky" thus saving time and money while increasing the capacity of the traffic system..

Edited December 5, 2003 by PDOP's

[+GeckoGeek]

Actually, if you're only receiving corrections for 4 sats, you'll only be calculating a position based on those 4 sats, no matter how many sats are visible.

Ah, that's a major help in understanding the Garmin's sat display. If all the sats have have the "D" then no point turning WAAS off, but if only a few have the "D"....

[Kerry.]

.... but there is no inherent technical or mathematical reason it can't or even shouldn't be done.

 

No, I suppose there no technical reason why it couldn't be done, why one would even want to do that is beyond me? Unless of course one wants a position solution based on partly corrected data and uncorrected data, just produces part of an unknown position solution that would basically be totally suspect and I mean totally suspect.

 

Well we could go into all this in rather quite some depth but maybe that just might be to tangential for some to follow. In depth now just imagine the "tangential" comment then

 

 

Cheers, Kerry.

[+blindleader]

I'm not a pilot, so I don't quite understand why WAAS was developed. It's more accurate, sure, but from what I understand it still isn't good enough to land a plane.

Good question.

 

There is currently no operational navigation system that's good enough to land a plane... Yet. Hopefully someday, I think LAAS will provide that capability.

 

Warning: The following is probably more than you want to know.

 

What WAAS will do is extend precision approach capability from the current two hundred or so airports that have ILS (Instrument Landing System) to several thousand smaller airports (yeah, the same ones that NIMBYs are always trying to close down) without having to build and man expensive ILS facilities at all those airports. The key to a precision approach is vertical navigation and WAAS provides enough vertical and horizontal accuracy to put a plane on a proper glide-slope aligned with a runway at three or four hundred feet AGL (above ground level). From there on down it's visual, just as with ILS. The only practical disadvantage of ILS is that it is typically good enough to bring a plane to two hundred feet AGL, versus three hundred feet plus for WAAS. And yes, that means what you think it means. If the pilot cannot see the runway at the minimum descent height, then the plane is not going to land. Non-precision approaches common at those smaller airports generally have a minimum descent height of more than six hundred feet, which means it doesn?t take much weather to close them down.

 

No, I suppose there no technical reason why it couldn't be done, why one would even want to do that is beyond me? Unless of course one wants a position solution based on partly corrected data and uncorrected data, just produces part of an unknown position solution that would basically be totally suspect and I mean totally suspect.

Isn't that the same as saying that a fix using NO corrections (WAAS off) is totally suspect? C'mon reread my post (or maybe read it with understanding for the first time) for the why. A fix requires multiple measurements. All of those measurements have degrees of accuracy that are different from each other. You can't get around that. If you have an opportunity to improve the accuracy of just one of those measurements, no matter what the cause (WAAS, better angle, or just virtuous living) of that improvement is, then that improves the accuracy of the fix. As I said before, we don't know what approach any particular consumer GPSR uses. Garmin and Magellan are not likely to tell us. So when My Merigreen says it's reading a WAAS fix I don't know which satellites it has correction data for. I also don't know if it is using only WAAS corrected satellites for that fix (I doubt it). Likewise, when it is giving Non-WAAS fixes, I don't know if it is using WAAS correction on those for which it is available. There's just no basis for saying it has to be one approach or another.

[Kerry.]

.... Isn't that the same as saying that a fix using NO corrections (WAAS off) is totally suspect?....

 

Would that be the same as saying a fix with WAAS ON and "some" corrections is not suspect, cause this is what receiving invalid WAAS corrections does to the position solution.

 

Affect of WAAS "corrections" on accuracy outside the coverage area

 

Realize this is a long way outside the coverage area but similar things could be expected around the edges of WAAS limits.

 

Probably one of the first things in understanding this issue is to understand exactly what an AUGMENTATION is, what it does, why and how it works to achieve the purpose of an augmentation system.

 

Don't want to get too tangential (do we ) but in reality WAAS corrections have basically a different approach than LAAS or Differential corrections. WAAS puts errors into categories and creates models for these categories, which is applied to the GPS observed ranges at the user end as a vector correction. WAAS in this regard is a little different to differential that actually transmits a scalar correction (applied to the pseudo-range) for each and every individual satellite in view (max of 9 in the case of differential).

 

WAAS modelling of errors (basically 4 different error models) is one of the reasons that accuracy is maintained over large areas, where as the further one gets away from a LAAS reference station or USCG type differential site the accuracy can tend to degrade.

 

So this talk about minumum common satellites and multiple reference stations is not really the same thing as conventional differential requiring exact common satellites.

 

Now to improve the accuracy of a fix one must have some basis for that to occur. The thing is one has to KNOW that whatever information is being used is correct and without additional verification then that information is useless to be incorporated into a system solution like WAAS.

 

I also don't know if it is using only WAAS corrected satellites for that fix (I doubt it). Likewise, when it is giving Non-WAAS fixes, I don't know if it is using WAAS correction on those for which it is available. There's just no basis for saying it has to be one approach or another.

 

The whole purpose of WAAS is for airbourne navigation with a CERTIFIED level of integrity in REAL-TIME. This simply can't be achieved from a base system for which is was never designed or intended. To assume recreational receivers are somehow going to be different and implement what amounts to unknown error sources is simply a fallacy.

 

 

Cheers, Kerry.

[georapper]

lets do a poll. i only trust the opinion for geocaching specifically of those geocachers who have been caching for 5 years and have over 500 finds. of those cachers, do they have waas enabled or disabled.

 

my opinion is if a person has been geocaching for over a few years and has over 500 finds, they don't use waas. for geocaching waas is not useful but only those with alot of experience really know that. you have to find out for yourself. eventually you all will come to that conclusion.

[+EraSeek]

By your own standards I should and do find your conclusion suspect.

I use WAAS most of the time and have created a cache where waas is useful. True, to us it is not critcal but then you can also find many caches without a GPS. But why not use the technology that holds our interest?

[+Prime Suspect]

lets do a poll. i only trust the opinion for geocaching specifically of those geocachers who have been caching for 5 years and have over 500 finds.  of those cachers, do they have waas enabled or disabled.

I can say positively that of everyone who fits your criteria, absolutely none of them have WAAS turned on.

 

Of course, that's because Geocaching has only been around a little less than four years.

[georapper]

you know what i mean. let's get some opinions from the old timers, who have been doing caching since the beginning and who have more than 500 finds. those are the cachers who would be the most reliable. geocaching is realatively easy, getting within 50 feet then gettig on your hands and knees to make the final discovery on 5/5 caches, waas would not change anything....... when i first started caching, i had the same opinion as many on this topic, waas was the latest greatest techi thing for caching and it was great, i didn't want to cache without it, but as i started finding some very difficult caches and getting skunked on some other 5/5 caches, i started to realize that waas would not have changed anything. that is why i would only trust cachers opinions who have over 500 finds, they are the ones who would know weather waas is useful for caching.....

[+NightPilot]

I've been flying GPS approaches for years. Down to 300' AGL over land, and to 200' MSL over water, using weather radar in conjunction with the GPS, landing to offshore oil rigs. GPS without WAAS provides more than adequate horizontal accuracy, but the altitude information is poor. Thus precision approaches (those that provide vertical guidance via a 'glideslope' in addition to horizontal guidance) are not allowed using GPS. WAAS may someday provide that, but it's doubtful. LAAS is more likely to give precision approach capability, but either one is years away. Normal ILS systems don't land the plane, either - the pilot does that. But GPS approaches can put the aircraft into a position from which the pilot can land it. Category II and III ILS approaches, available only to some airline operations, can permit autolanding, provided the aircraft and the crew are properly certified, but that won't be available to general aviation pilots, in all likelihood.

 

Right now, WAAS means nothing to me as a pilot, but it might in several years when enough WAAS satellites are in orbit to make a difference, and the equipment in the aircraft is able to use it. In the meantime, normal GPS approaches are good enough in most cases.

[+GeckoGeek]

when i first started caching, i had the same opinion as many on this topic, waas was the latest greatest techi thing for caching and it was great, i didn't want to cache without it, but as i started finding some very difficult caches and getting skunked on some other 5/5 caches, i started to realize that waas would not have changed anything.

Keep in mind that it appears that Garmin botched WAAS implementation for awhile. A look at the firmware revisions of Geko, eTrex, Rino and V firmware shows the following:

 

"Improved WAAS/EGNOS satellite selection algorithm to select the satellite with the most beneficial corrections given the unit's current position. A unit will not use a WAAS/EGNOS satellite if the unit's current position is outside of a given WAAS/EGNOS satellite's service volume."

 

Sound pretty important to getting good results with WAAS to me. Perhaps the early experience with botched firmware soured people on WAAS. I think Garmin got this fixed only in the last 3 months or so.

 

Also keep in mind that your coordinates are only as accurate as the GPS of the person who placed the cache. Depending on if he has WAAS on or off and how how careful he was with his GPS, the stated coordinates could be leading you astray. But for us benchmarkers, we have a much more accurate coordinates to deal with

[+Baptist Deacon]

INteresting thread. Just thought I would throw my 2 cents in the fire.... I have an Etrex Vista and have been using it for the last 2 months with waas on. After reading this thread a couple of days ago, I turned waas off and have noticed a much better performance. I maintain signal more when in trees and it seems that the screen does not take as long to update. Before, it seemed like the scren would lock up for 5-10 seconds when updating. Maybe it's just me or my unit, but it appears to work much better with waas off. Yesterday when in the woods it was showing accuracy to 10 feet. Carried me to right on top of the cache. Pretty nice. If you have waas on, I would suggest turning it off and giving it a try for a week or two. Then you can decide for yourself if it helps or hinders your unit.

[+Prime Suspect]

"Improved WAAS/EGNOS satellite selection algorithm to select the satellite with the most beneficial corrections given the unit's current position. A unit will not use a WAAS/EGNOS satellite if the unit's current position is outside of a given WAAS/EGNOS satellite's service volume."

I think you're misreading this a bit. This apparently has to do with people who are located in an area outside of the WAAS reporting area but who can still receive a WAAS signal. If you're in an area that's served by WAAS, which is pretty much everywhere in the US, this will have no effect on how your unit operates. If you're outside that area, you shouldn't have WAAS turned on in the first place. The same goes for EGNOS areas as well.

 

Basically, it's "stupid people" protection, probably put in place at the request of Garmin's help-desk staff.

Edited December 7, 2003 by Prime Suspect

[+EraSeek]

>I have over 500 finds and almost 50 hides.

>I have been using a GPS for about 5 years or so

>In terms of Geocaching, I am an oldtimer

>I use WAAS as often as I can on hides, and finds

>I turn it off when I know I can not use it. Tree cover, canyons, the like.

>Waas will improve elevation as well as horizontal coords.

>""Improved WAAS/EGNOS satellite selection algorithm to select the satellite with the most beneficial corrections given the unit's current position. A unit will not use a WAAS/EGNOS satellite if the unit's current position is outside of a given WAAS/EGNOS satellite's service volume." -- Can be read both ways, but yes, it keeps the WAAS service area from being corrupted by the EGNOS data, and vis-versa.

>Don't like it, no need to use it. Find it of interest, use it. I use it.

>Just for fun, go stand on a benchmark sometime (known coords) with WAAS enabled, then turn it of. See what happens. Have your unit set at GOTO the benchmark coords. Notice the difference between "distance to target" and your accuracy reading.

[+GeckoGeek]

If you're in an area that's served by WAAS, which is pretty much everywhere in the US, this will have no effect on how your unit operates. If you're outside that area, you shouldn't have WAAS turned on in the first place. The same goes for EGNOS areas as well.

 

I'll have to do some more research on that, but I think it's more complex then that. WAAS uses a series of observation stations. In some areas you may be too far away from nearest 3 (or whatever it is) to use 3.

 

Bottom line, Garmin made a change, it could affect one's experience.

[+EraSeek]

No. This applies more to DGPS than to WAAS. Yes WAAS uses stations, but the data from those stations are used to make a model for the whole area (US and a bit more). As long as you are in the service area your GPS figures out what data to use depending on where you are in the model's grid.

[+blindleader]

Thus precision approaches (those that provide vertical guidance via a 'glideslope' in addition to horizontal guidance) are not allowed using GPS. WAAS may someday provide that, but it's doubtful.

I'm afraid your information is out of date in the first case and just plain wrong in the second. It isn't 'doubtful' that WAAS will provide precision approaches someday because that is it's one and only intended purpose. A search on "precision approach" and WAAS will turn up many pages on the subject at the FAAs web site.

 

It didn't instantly open up thousands of precision approaches when it went operational on 10 July, but it did open the door. I believe a few have been published already, though I don't have them at my fingertips. Here's one that isn't complete yet but illustrates how it works RNAV at RGK. Manufacturers are just now releasing IFR certifiable WAAS capable GPS systems.

 

I've been flying GPS approaches for years.

With all due respect, you haven't been flying published gps approaches to airports with 200' minimums, because they don't exist (as you said yourself). You've been 'sneaking' into oil rigs in weather, with a helicopter, which isn't even similar to a published approach at an airport with a fixed wing aircraft. More power to you, as long as the FAA doesn't find out, and decide that you're a bad person for doing it.

 

Edited to add reference.

Edited December 7, 2003 by blindleader

[+EraSeek]

You may be right (I don't know) that the WAAS specification prohibits using a mix of signals from WAAS corrected and non-WAAS corrected satellites, but there is no inherent technical or mathematical reason it can't or even shouldn't be done.

 

I also am not sure on this (mixing sats). I just took my unit outside and turned it on and this is the progression.

Got a few grey bars up and one WAAS. grey bars fill in and I get a fix, and the unit says I have a 3d fix. as soon as I have three black bars with "W's" it tells me I have a 2D differencial fix (running on WAAS), and this is within a minute of the first fix. Never the less I have another black bar that is not "W'd" and I also have an elevation displayed, which tells me that I have 3D but just not corrected 3D. I now get 4,5,6 black bars with "W" and with the fourth one it goes to 3D Diferencial (how do you spell that?) with still one black bar without the "W" and a couple grey guys. Anyway, it appears to me that you can mix, but it will tell you when it is corrected.

[Kerry.]

Many of the changes to firmware has to do with interference between different WADGPS operations, especially now that EGNOS has stepped up testing and simply because one shouldn't be using it outside the coverage area. It was even more disturbing that some manufacturers were advertising WAAS "with pin point accuracy" in countries it never had a hope in hell of working.

 

WAAS's correction principles are a bit more complex and involved than stating there has to be X reference stations with Y satellites in view that are common to user Z. Spatial de-correlation principles are basically the difference between WAAS and dGPS corrections, dGPS corrections are affected by distance, WAAS basically aren't.

 

As for what the satellite display is showing, I believe there needs to be some differentation with what the display is actually trying to tell the user based on how the user has the receiver configured.

 

In affect NMEA data is a very good indicator of what is going on with respect type of fix, validity of fix, satellites in fix, satellites in view, fix quality, satellites being tracked etc. As far as Fix quality there's no such thing as "half" a fix (GPS/dGPS/WAAS etc), it's either one or the other, there is nothing in between.

 

Configurations differ between receivers but most have this generic automatic line that basically always have a lowest common denominator. 2D v 3D fix can be dependent on how the unit is set. Where "3D only" is an option then that's exactly the conditions one must have to have a valid fix.

 

Similar with differential settings, Differential mode can be say Automatic or Differential exclusively as well as DGPS modes of say Automatic, WAAS Only, RTCM Only or None etc. How a user applies these options is specific to a users purpose. With all auto then one will get a 2D fix if insufficient satellites, with other conditions in hierachical order depending on user settings.

 

If configured to "WAAS only" then no WAAS corrected position No position/navigation, period, regardless of what is available as far as straight GPS is concerened.

 

Cheers, Kerry.

[+nincehelser]

I was under the impression that the Garmin's mixed the two types, at least at one time.

 

One of the firmware updates seemed to indicate there were times when the GPS wouldn't pick up the best scenario...i.e. is a WAAS-corrected sat in a poor geometry better or worse than a non-WAAS sat in good geometry?

 

I've no idea how the unit goes about it's decision making. It sounds pretty complex.

 

George

[Z15]

WAAS

 

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

 

Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.

 

100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program. 

15 meters: Typical GPS position accuracy without SA.

3-5 meters: Typical differential GPS (DGPS) position accuracy.

< 3 meters: Typical WAAS position accuracy.

 

 

btw

Mine 2 Garmins (Vista and GPS V) are WAAS enabled all the time since day 1. I use mine for Survey work

 

Latitude: 46°-10'-20" North

 

Longitude: 088°-25'-44" West

 

Elevation 605 ft

Edited December 7, 2003 by elcamino

[+NightPilot]

Blindleader, I didn't say I've flown down to 200' over land, but offshore. Read the post carefully. There are many GPS approaches to airports with 300' minimum descent altitudes. And offshore, we fly down to 200' under Part 135, legally, not sneaking. The FAA knows about it and has approved it. I do nothing that's illegal. And again, there are currently zero approved WAAS approaches in the U.S. The FAA has waffled for years between LAAS, WAAS, and whatever else was popular that week. At the speed the FAA moves, I don't believe any precision GPS approaches will be approved in the near future. The FAA has backed off on using GPS as a sole-source navigation means, and likely the ILS will be the only precision approach for some time.

Edited December 15, 2003 by NightPilot

[+Halden]

My legend averages an accuracy of 6 metres with WAAS off and 4 metres with WAAS on. Not a giant difference but it can be helpful. I find the WAAS does slow my receive lock down and does fade fast under Tree cover so I just adjust as the situation dictates. Sometimes off but mostly on.

[+blindleader]

Blindleader, I didn't say I've flown down to 200' over land, but offshore. Read the post carefully.

I stand corrected and apologize. I didn't mean to seriously suggest you were violating the FARs (regulations for you non-aviators). But that's all a sideshow to the main argument.

 

I'm not going to let you talk past me on the issue of when and where "precision" GPS approaches will be or have been approved. ILS is, indeed the only precision approach available with MDA (minimum descent altitude) of 200 feet, but that is not the definition of precision approach. A precision approach is one with both lateral and vertical guidance, whatever the MDA. As such there are approximately six hundred such non-ILS precision approaches published. They are listed as LNAV/VNAV on the RNAV approach plates and are open to use with GPS. Quibble if you like about whether those require WAAS or not. I won't argue that here, and you can maintain your position, wrong as it is, if you like.

 

But I must shoot you down in ignominious flames (figuratively of course You've got enough problems keeping those ugly whirly things from killing you) on the following statement:

I don't believe any precision GPS approaches will be approved in the near future...

I refer you to Leesburg VA Executive airport. RNAV approach for Runway 17. There are currently seven of these LPV approaches published, and about three hundred per year are expected. Identifying the other six LPVs is left as an exercise.

[+NightPilot]

A couple of quibbles:

 

Precision approaches do not have an MDA, they have a DA, or Decision Altitude, which is not a minimum altitude, it's the altitude at which the pilot must make a decision to continue to landing or execute a missed approach.

 

The FAA has issued approaches with VNAV minimums for some time, but none I know of can actually be flown to VNAV minimums. Your link was broken. Is there actually any WAAS-enabled avionics available? I haven't heard of any, and the operator has to get approval to use them to VNAV minimums. I would love to see WAAS-enabled and certified GPS units in our aircraft, because when they are available they will permit us to fly without all the other stuff we're required to have, but don't use very often, like VOR and ADF receivers. For now, we have to have 2 operational VOR receivers to fly IFR offshore, where there are exactly zero VOR transmitters. The FAA often makes no logical sense in its regulations. But the VORs do come in handy on those dark, stormy nights when the weather is down to 100' indefinite obscured and 1/4 mile vis, for flying the ILS. And yes, we can do that legally. Helicopters can reduce the published visibility by half, down to 1/4 mile, provided we fly the final approach at less than 90 kts airspeed.

[Kerry.]

I do recall seeing the first GPS/WAAS certified avionics receiver announced but this was only in the last few months. Not sure if that was both horizontal or vertical certification but more likely only horizontal as vertical didn't have certification the last time I checked.

 

The very first WAAS completed (proving) commercial flight was in March 2003 and done using a specially programmed and modified receiver that ignored the type 0 (zero) message. One of those just making sure they could do it exercises.

 

Really until there is at least a third WAAS Geo then without the availability and reliability one would be putting an awfull lot of confidence on basically a single transmission source as certainly the luxury of receiving both the existing Geo's is minimal.

 

Cheers, Kerry.