Gps

[+Kai Team]

A 14 channel should do better than others under cover. It has 12 regular gps channels and 2 dedicated WAAS channels (as opposed to 10 and 2 switchable). More oportunity to find those sats when walking thru the trees.

I thought the problem under heavy cover is that you can't receive enough satellites, not that you don't have enough channels on which to receive them.

 

For example, if you have a 12 channel receiver and you can only see 8 sats, there are 4 channels available to pick up any sat that shows itself as you're walking through the trees. If you're in the same spot with a 14 channel receiver, you still can only see 8 sats, but will have 6 channels available. I don't see how the extra available channels (without a sat lock) help your accuracy under these conditions.

 

It seems that two dedicated channels for WAAS would only be useful under very good reception conditions, when you're able to receive 12 sats plus 2 WAAS sats (if there is such a place on the face of the earth ). Am I missing something?

 

Edit: Typo

Edited June 23, 2005 by Kai Team

[+sbell111]

Do we really need this type of accuracy?

[+EraSeek]

A 14 channel should do better than others under cover. It has 12 regular gps channels and 2 dedicated WAAS channels (as opposed to 10 and 2 switchable). More oportunity to find those sats when walking thru the trees.

I thought the problem under heavy cover is that you can't receive enough satellites, not that you don't have enough channels on which to receive them.

 

For example, if you have a 12 channel receiver and you can only see 8 sats, there are 4 channels available to pick up any sat that shows itself as you're walking through the trees. If you're in the same spot with a 14 channel receiver, you still can only see 8 sats, but will have 6 channels available. I don't see how the extra available channels (without a sat lock) help your accuracy under these conditions.

 

It seems that two dedicated channels for WAAS would only be useful under very good reception conditions, when you're able to receive 12 sats plus 2 WAAS sats (if there is such a place on the face of the earth ). Am I missing something?

 

Edit: Typo

Well maybe you're right but, as you walk outside and turn on your GPS in an open area, the almanac will look and see what sats are available in the sky where you are at that particular time. You may see 7 channels fill in or 12, or something in between. Each channel is dedicated to a sat. If you only have 7 sats availible in the sky above you 14 channels will not make a difference. if there are 12 sats above you, but you can't get 2 sats because they are blocked, you will have 10 channels locked and 2 empty with the sat numbers below the empty bars. So if you are walking around in the woods with 12 sats above, all 12 channels are looking for those sats. Some may come up solid as you lock on and some may be empty, and as they come and go from view they will fill and go empty. The point is it is a benefit because it offers more of a chance to lock on to enough available sats to get a fix in a tough position. It was one of the saleing points when 12 channel GPS's first came out.

 

As for WAAS, now you don't have to use up those 2 channels for WAAS. Now they have their own. The WAAS sats came also be used for regular position fixes, though they are a bit less reliable for that. WAAS can also be used in obstructed areas such as woods. Though harder to get, once loaded, any opening to a WAAS sat view and it can easily update the corrections quickly. There are plans to launch more WAAS sats that will be a bit higher in the sky over most of us and that will also help reception of them. I think 14 channels will prove to be more of a benefit than you might think.

 

OK, where am I wrong on any of this?

[+Kai Team]

I thought that the GPSr would continue to look for all sats that the almanac says are available and would make an unused channel available for any of those sats that becomes visible, not "dedicate" channels for specfic sats that it can't currently see.

 

The latter wouldn't make much sense, since it has no way of knowing which sats might become visible (it knows from the almanac which ones are overhead, but not which one may pop into view around an environmental obstruction). I could be wrong about that...

 

If the GPSr will make an unused channel available for any additional sat that becomes visible, then as long as there are no more than 12 birds available, there is no need for more than 12 channels. And if your GPSr can see all 12 birds (or 10 plus 2 WAAS), then sbell111 is right - you'll have 5-7 foot accuracy and you don't need more for geocaching!

 

It may be a very different story as more birds are launched, but right now it doesn't matter if you have 100 channels if you can't see more than 12 sats at a time. This is not the same as when we went from 4 channel receivers to 12 channel receivers, because at that time, there were more than 4 sats visible at a time.

[TXcachehunter]

If you can not get WAAS then you might be able to use this. (It a DGPS correction out of CA) It worked on my yellow etrex and gave me about a 7 foot accuracy in Texas.

DGPS over the Internet

[+EraSeek]

I thought that the GPSr would continue to look for all sats that the almanac says are available and would make an unused channel available for any of those sats that becomes visible, not "dedicate" channels for specfic sats that it can't currently see.

Those sats that are available to see, which the almanac tells your GPS about, have dedicated channels. Just because they are behind some local obstruction does not clear that channel. If you have a Garmin (I don't think Magellens show sat id's) you can take it outside and see for yourself. If you don't have 12 sats above you it won't ID those channels. If you have some sats blocked but should be in view you will see the sat # but no bar (hollow,grey, or solid). I'll also try and find the link confirming this.

 

The latter wouldn't make much sense, since it has no way of knowing which sats might become visible (it knows from the almanac which ones are overhead, but not which one may pop into view around an environmental obstruction). I could be wrong about that...

Read the above.

 

If the GPSr will make an unused channel available for any additional sat that becomes visible, then as long as there are no more than 12 birds available, there is no need for more than 12 channels. And if your GPSr can see all 12 birds (or 10 plus 2 WAAS), then sbell111 is right - you'll have 5-7 foot accuracy and you don't need more for geocaching!

If I turn off WAAS I can get up to 12 regular GPS sats, and have before. If I have WAAS on (as Magellans always do) I can only get 10 regular GPS sats and the 2 WAAS sats. Now the WAAS (or GEO) sats can be used as regular GPS sats for a fix but are a bit iffy in that function. If I have 14 channels I have all my bases covered. 12 gps sats. 2 WAAS sats.

 

It may be a very different story as more birds are launched, but right now it doesn't matter if you have 100 channels if you can't see more than 12 sats at a time. This is not the same as when we went from 4 channel receivers to 12 channel receivers, because at that time, there were more than 4 sats visible at a time.

At the time there were not 2 WAAS sats. Now there are. 2 of your available channels have been hijacked for WAAS. But WAAS is good. I think my 45xl which I still have has 8 bars. Yes it is 8. But it is multiplexing as I understand it. It locks on 8 but pops back and forth between them for the position fix. At any rate 12 is more than 8, 14 is more than 12. The use of 14 is better the use of 12 (or 10).

 

My understanding is that the 12 channel qas originally meat to be 11+1. 11 sats channeled plus 1 channel to track any extra sats above that.

 

Hope this makes sense.

[+Glenn]

If I turn off WAAS I can get up to 12 regular GPS sats, and have before. If I have WAAS on (as Magellans always do) I can only get 10 regular GPS sats and the 2 WAAS sats. Now the WAAS (or GEO) sats can be used as regular GPS sats for a fix but are a bit iffy in that function. If I have 14 channels I have all my bases covered. 12 gps sats. 2 WAAS sats.

What is the benefit of receiving two WAAS satellites at the same time? They send the exact same correction information. Is that just a marketing gimic or is there some science behind it?

[+jmstacey]

As far as I am aware each WAAS sattelite is being fed it's correction information from ground stations in the area that the satellite covers.

Unless you are located near the ground station providing the correction data or have similar atmospheric conditions, WAAS will not help your accuracy and may even worsen it with incorrect atmospheric correction data.

Even though the estimated error margin may be lower with data provided by the sattelite, your actual error margin may in fact be greater.

 

The benefit of two WAAS sattelites in theory I would guess is because they would be recieving different correction data depending on their location and coverage.

[+EraSeek]

No that is incorrect. Redundancy. And reciever coverage area. Each WAAS sat sends the exact same data. Your corrections have nothing to do with how close or far away a ground station is. The data from ground stations are sent to a master station where a universal model of corrections for the WAAS coverage area is made. Both sats send this same info to your reciever. Your reciever figures out where you are and applies the correct info for your location.

 

One WAAS sat is in the east and one in the west so that the US is covered. If on the East coast you can just get Garmin#35. If on the West coast and lucky as I am you can get #35 and #47. Thus if one is blocked I may be able to get the other. BIG problem right now is that if one goes down hard, half of the WAAS coverage area is lost to recievers. Thus they are trying to get 1 or 2 more WAAS sats up.

[+EraSeek]

Here, look at this: http://gpsinformation.net/exe/iono-day.gif

It is a moving display of an active ionosphere. But you could also see it much as how the WAAS correction model would look. The points in the net where the strands intersect are like the data points in the corrections. Your GPS finds the closest intersection to where it is and applies that data for corrections.

[+fizzymagic]

What is the benefit of receiving two WAAS satellites at the same time? They send the exact same correction information. Is that just a marketing gimic or is there some science behind it?

Two reasons.

 

First, the WAAS satellites themselves can be used in the position solution, just like the other GPS satellites. They have very different orbits, but the principle is the same.

 

Second, the complete WAAS correction set takes some time to download to your receiver. I don't know the details, but it seems at least plausible that having two WAAS satellites in view at the same time could speed this data download.

[+EraSeek]

 

Let's see if this image works. Here you can see a Garmin sat display page. As you can see some of the channels are solid meaning the sats are locked on. Others are empty, not being recieved yet. If you have gray bars the data for that satellite is being gathered. The "d's" show that corrections are in place for those sats, even though some of the sats signals are not yet recieved or have dropped out. Looks to me like the alamanac has dedicated the availible satellites to the channels even though the sat's signals are not yet recieved.

 

I took my Garmin 12 (a 12 parallel chanell reciever) and my Garmin 45xl ( an 8 chanel multiplexing reciever) out in the yard to see how the dealt with the same sats. Of the 12 possible I recieved about 6 or 7 sats on each. The ones that the 45xl did not list as possible on the display (8 out of 12) were all on the outer edge of the sky view, thus it masked out the lower sats.

 

I assume if you got more that 12 sats on the 12 channel it would do the same. Drop out the lowest sats.

[+PDOP's]

Great discussion here

 

Have you noticed with the latest firmware for Garmins that one of the WAAS channels is released for regular satellites once it has a solid signal from one geo? I'm not sure if this only happens when only one geo is in view (I sometimes can get both 35 and 47 but not often) Below is a screen shot of my GPSMap76C with only one channel being used for WAAS

 

 

For those awaiting the new geos here are coverage maps for the present and new satellites (from

COMMISSIONING THE WIDE AREA AUGMENTATIONSYSTEM on the NAV Canada site)

 

Existing Coverage

 

 

Future Coverage

 

 

The new geostationary are more directly south of us so they will appear further above the horizon too.

[+EraSeek]

Boy that color sure is pretty! I have a Map76. Grey!

I did read somewhere that if one of the WAAS sats are not viewable it reverts to only one channel for them.

 

They are also planning a new generation of GPS sats. Think it is dual band civilian. But problems have delayed the first launch till after July 29th. Enough of them won't be up until 2008 or 10 or 12. In there somewhere in there.

[+JohnnyVegas]

To free up 2 extra channels that thw WAAS sats use. I know they are lower on the horizon and perhaps less likly to get in that situation than some other sats.

Wrong. The WAAS satellites are also used for obtaining 'normal' fix positional information regardless of their WAAS capability.

Yes, I know that. You are not listening to what I said. Being lower on the horizon those sats are more likely to be blocked by the growth and hills in the forested areas around here. Freeing up those two channels, those dedicated to just those two sats, betters the chances of pulling in other sats for a better position.

This is not correct

[+Kai Team]

 

Let's see if this image works. Here you can see a Garmin sat display page. As you can see some of the channels are solid meaning the sats are locked on. Others are empty, not being recieved yet. If you have gray bars the data for that satellite is being gathered. The "d's" show that corrections are in place for those sats, even though some of the sats signals are not yet recieved or have dropped out. Looks to me like the alamanac has dedicated the availible satellites to the channels even though the sat's signals are not yet recieved.

 

I took my Garmin 12 (a 12 parallel chanell reciever) and my Garmin 45xl ( an 8 chanel multiplexing reciever) out in the yard to see how the dealt with the same sats. Of the 12 possible I recieved about 6 or 7 sats on each. The ones that the 45xl did not list as possible on the display (8 out of 12) were all on the outer edge of the sky view, thus it masked out the lower sats.

 

I assume if you got more that 12 sats on the 12 channel it would do the same. Drop out the lowest sats.

OK, but:

 

Present: if there are no more than 12 sats available in any one location (which is usually the case right now), then no sats are being dropped by a 12 channel receiver and the two additional channels are unused, and

 

Future: when more sats are available, a 12 channel receiver will drop the lowest sats when more than 12 are visible. The lowest sats are the ones whose signal has the most atmosphere to pass through and the ones whose angle makes the signal most likely to multiplex, and therefore introduce errors. So you should get better accuracy than today by using the "12 best", and the added accuracy from 14 channels seems minimal at best.

 

For most everyday purposes (e.g. geocaching, hiking and road navigation), there's a rate of diminishing returns once your accuracy is down to 5-7 feet, which it would be if you had 10-11 sats visible plus WAAS. How much better can your accuracy get, for our puposes? Maybe I'm just being dense.

 

PS - Great graphics and links!

[+EraSeek]

Present: if there are no more than 12 sats available in any one location (which is usually the case right now), then no sats are being dropped by a 12 channel receiver and the two additional channels are unused, and

 

Somewhere we 2 are not quite connecting. There are 14 sats (maybe more) possible in the sky right now. 12 GPS sats, and 2 GEO (waas) sats. Those 2 Geo sats are a bit different but they can be used just like GPS sats for a position fix. You can use all 14 channels right now if they are in view. If you have 12 channels you may be missing out on 2 possible sats fot a position fix. How much do the help for accuracy? Maybe not much, but the will help in speed of aquistion and grabbing sats when there are obstructions blocking some of the sats.

 

Also The new generation of sats will give you better accuracy. Benefit for us? As Geocachers not much, but for life saving functions and things like science and agriculture, great benefits.

 

JohnnyVegas, would you like to explain what was wrong?

[+EraSeek]

Great news! I go this reply from the Garmin Techs confirming what Fizzymagic said:

 

Problem:

This is more of a general question. I understand there are 2 methods of

aquireing a position fix. The "useing all or most satellites in the

solution" and the "Use the best 4, and the others stand at the ready". Which

is best, newest, and most in use these days?

 

Also are WAAS and non-WAAS sats mixed in the solution.

Thanks. Just like to know how things work.

 

 

Thank you for contacting Garmin International,

 

I would be happy to help you with this. Garmin produces 12-channel GPS

receivers. This means that it will go out and simultaneously grab all of

the satellites possible within view at the same time. There is no way to

turn this feature on or off. Once you have a satellite lock on 3

satellites, you will have a 2-D fix and when you have a lock on 4 satellites

you will have a 3-D fix. We do not change this designation just because you

might have a lock on more satellites than that.

 

If you set your unit to ENABLE WAAS, the WAAS satellites will be mixed in

the solution. If you set your unit to DISABLE WAAS, they will not.

 

With Best Regards,

 

Garmin International

Senior Product Support Specialist

 

http://www.garmin.com/ <http://www.garmin.com/>

Edited June 25, 2005 by EraSeek

[+fizzymagic]

I wanted to add a couple things to the mix here:

 

The satellites nearest the horizon, which many people think are less likely to give good results, are actually often the best in terms of geometry. The best position calculation comes from satellites as widely separated in the sky as possible.

 

An additional advantage to the overdetermined solution I have been describing, in which you use all visible satellites to calculate the position, is that the method lets you simultaneously calculate the position and test the uncertainty in the position. The solutions you get from taking the satellites 4 at a time should all be consistent with each other; if they are not, then something is wrong.

[+EraSeek]

Fizzymagic, I'm not sure you are right on the low sats. Yes, usually better, but my understanding is that if they are below 5 degrees above the horizon there is a problem. I know they "Mask" them out. Truely I don't know the reasons behind that though.

[+EraSeek]

Here is a bit of a clip from the GPS support center (air force I think):

 

Mask Angle

The angle above the receiver's local horizon below which satellites are not used to perform the calculations. 5.0 degrees

 

Satellite Selection

The TTS satellite selection algorithm can use the Best-4 SVs or All-In-View criteria. All-In-View solutions tend to be more precise than Best-4 solutions, however, Best-4 solutions are more common in military receivers. A Best-4 solution is generated by selecting the 4 currently visible satellites that will minimize the Dilution of Precision for a given site. An All-In-View solution uses all available satellites in view to minimize the Dilution of Precision for a given site. All satellites used, regardless of algorithm, must be above the receiver's mask angle. Best-4

[+fizzymagic]

Fizzymagic, I'm not sure you are right on the low sats. Yes, usually better, but my understanding is that if they are below 5 degrees above the horizon there is a problem. I know they "Mask" them out. Truely I don't know the reasons behind that though.

Your point is well-taken: the satellites close to the horizon, though geometrically optimal, may not be usable for other reasons.

 

I think I understand at least some of those reasons. Those satellites are low enough on the horizon that you could get:

• 
   
  
• Significant errors from propagation in both the troposphere and the ionosphere (the signal goes through a lot of atmosphere!)
   
  
• Potential multipath distortion from interaction with the surface of the Earth.
   
  
• Signal refraction (could change the path length!).
   
  
• Diffraction around objects in the line of sight.
   
  

Hmm... I wrote this list, and then did a Google search. From this page:

 

Elevation Mask Angle

    Usually a user definable angle (in degrees) below which a satellite will be tracked, but not used as a data source for position calculations. This value is set to avoid interference problems caused by ionospheric refraction, multipath errors , obstructions such as buildings and trees, etc.

 

So I guess I was (gasp) right!

 

Good catch.

Edited June 25, 2005 by fizzymagic

[+EraSeek]

Way to go. Good to have Mr. Science around.

Edited June 25, 2005 by EraSeek

[+JohnnyVegas]

Present: if there are no more than 12 sats available in any one location (which is usually the case right now), then no sats are being dropped by a 12 channel receiver and the two additional channels are unused, and

 

Somewhere we 2 are not quite connecting. There are 14 sats (maybe more) possible in the sky right now. 12 GPS sats, and 2 GEO (waas) sats. Those 2 Geo sats are a bit different but they can be used just like GPS sats for a position fix. You can use all 14 channels right now if they are in view. If you have 12 channels you may be missing out on 2 possible sats fot a position fix. How much do the help for accuracy? Maybe not much, but the will help in speed of aquistion and grabbing sats when there are obstructions blocking some of the sats.

 

Also The new generation of sats will give you better accuracy. Benefit for us? As Geocachers not much, but for life saving functions and things like science and agriculture, great benefits.

 

JohnnyVegas, would you like to explain what was wrong?

Simple, most every GPSr made in the past years has at least a 12 dhannel reciever, some have 14. I most areas you are not going to able to lock onto more the 7 or 8 birds. This means if you are locked onto a WAAS sattelite you are still going to have plenty of unused channels, there is nothing to be gained by turnong off WAAS in a GPSr, those who think turning off WAAS is going be helpfull just do not understand what the WAAS system is or how it works. the idea that turning off WAAS to improve GPS function is internet junk science.

[+EraSeek]

Simple, most every GPSr made in the past years has at least a 12 dhannel reciever, some have 14. I most areas you are not going to able to lock onto more the 7 or 8 birds. This means if you are locked onto a WAAS sattelite you are still going to have plenty of unused channels, there is nothing to be gained by turnong off WAAS in a GPSr, those who think turning off WAAS is going be helpfull just do not understand what the WAAS system is or how it works. the idea that turning off WAAS to improve GPS function is internet junk science.

I have gotten 12 birds up before, and actually many times. Besides, refer to the above discusion. You have 12 dedicated channels to 12 possible sats via the almanac. If WAAS is switched on, they take up 2 of those 12. If switched off, you have 2 more open to regular GPS sats. It is simply opening up the geometry a bit, in hopes of grabbing a better positioned sat as you walk through the trees.

 

Yes I agree that having WAAS on AND OPTAINABLE is better. Whenever there is a chance of getting those corrections you have a better position due to those correction.

 

There are times, due to terrain, when the WAAS sat here are not viewable. May as well try and grab 2 other sats from the almanac. If there are only 7 sats overhead in the almanac then there is no need to switch off the WAAS other than to save processing.

 

By the way, sometimes corrections are greater (more needed) than other times. (Active ionosphere vs non-active ionosphere. Day vs Night.)

[+Kai Team]

...Somewhere we 2 are not quite connecting...

I agree - I think the disconnect between us relates to the pracitical significance of two additional channels (14 vs. 12). Let's cut to the chase: I'll concede all of your technical points and your conclusion that 14 channels will provide more precise positions now and in the future.

 

The two additional channels are still of no value on a handheld GPSr because a 12 channel unit receiving 10-12 satellites and WAAS corrections is precise enough (about 5 feet) for handheld GPSr purposes. The increased precision of two more channels is insignificant - i.e. if your accuracy goes from 5 feet to 3 feet (a 40% improvement) with 14 channels, it's still not going to help you find that geocache, navigate a road or trail, or perform search and rescue! This is particularly true in geocaching, given the unknown error in the cache coordinates. 14 channels are technically more precise, but it's a false sense of precision.

 

So, I'm not really arguing if 14 channels lead to more accuracy - I'm arguing that 14 channels do not help find geocaches. It's the start of another urban myth, and if cachers come to believe it, they will limit their choice of GPSr's (when other features may be more important) and spend more money than necessary, for no good reason!

 

If someone tells you their 14 channel GPSr is helping them find geocaches, realize that the number of satellites in the sky and their configuration, atmospheric conditions, the availability of WAAS, the accuracy of the hider's coordinates (and the cleverness of the hide), the terrain where the cache is hidden, the antenna and a dozen other things are all much more important to the ease of finding geocaches than 14 channels!

[+EraSeek]

Yes, I agree with all that. My point is not precision for us but rather availibilty. When I walk outside I usually get all twelve channels dedicated to 12 sats. Not that I am pulling in 12 sats, but my almanac says there is often 12 somewhere in view (sometimes less). So Actually there are 14 in veiw if I add in the WAAS sats.

 

Availibily becomes helpfull in situations such as walking through the woods where you may see this sat for a bit , then that sat, they come and go from veiw via the local obstructions. If I have the availiblity of seeing 14 instead of 12 in this situation I am better off. I think much better off.

 

No, for what we do the accuracy gained by 14 or 12 is no great help for us. Yes, choose other features first.

 

Edit> I will say in that situation, (in the woods,) I do think not only are you able to keep a position lock better, but you will improve accuracy, in this case it will not be 5' or 10', but rather maybe 50' as compared to 100', due to the added satellite positon. Better chance of keeping a poistion lock and better chance of bettr geometry. So it is a feature to consider during purchase. Just my humble opinion.

Edited June 26, 2005 by EraSeek

[+Team Cotati]

...Somewhere we 2 are not quite connecting...

I agree - I think the disconnect between us relates to the pracitical significance of two additional channels (14 vs. 12). Let's cut to the chase: I'll concede all of your technical points and your conclusion that 14 channels will provide more precise positions now and in the future.

 

The two additional channels are still of no value on a handheld GPSr because a 12 channel unit receiving 10-12 satellites and WAAS corrections is precise enough (about 5 feet) for handheld GPSr purposes. The increased precision of two more channels is insignificant - i.e. if your accuracy goes from 5 feet to 3 feet (a 40% improvement) with 14 channels, it's still not going to help you find that geocache, navigate a road or trail, or perform search and rescue! This is particularly true in geocaching, given the unknown error in the cache coordinates. 14 channels are technically more precise, but it's a false sense of precision.

 

So, I'm not really arguing if 14 channels lead to more accuracy - I'm arguing that 14 channels do not help find geocaches. It's the start of another urban myth, and if cachers come to believe it, they will limit their choice of GPSr's (when other features may be more important) and spend more money than necessary, for no good reason!

 

If someone tells you their 14 channel GPSr is helping them find geocaches, realize that the number of satellites in the sky and their configuration, atmospheric conditions, the availability of WAAS, the accuracy of the hider's coordinates (and the cleverness of the hide), the terrain where the cache is hidden, the antenna and a dozen other things are all much more important to the ease of finding geocaches than 14 channels!

I was not aware that this was how you get such accuracy. As a matter of fact, I am quite surprised that you can get such accuracy from a GPSr under any circumstances. What would be the approximate accuracy when you are only getting 5 satellites including WAAS?

Edited June 26, 2005 by Team Cotati

[+EraSeek]

Here is something of interest I found on a Dale DePriest site:

" On 12 channel parallel units Garmin has developed a technique that permits them to calculate an overdetermined solution by using more than the standard 4 satellites. This can lead to more accuracy, reported to be on the order of 15%, and permits a smoother transition when a satellite drops out of view for any reason. ."

I also don't think 14 channels is all that great a help in accuracy over 12. I just don't see it.. except in availablity as I have mentioned.

 

However to answer your question, as the quote mentions, using many sats instead of just 4 gains you about 15% according to Dale DePriest.

 

Having 5 sats with WAAS, assuming good satellite geomerty should get you about 3 meters or less.

[+Kai Team]

...My point is not precision for us but rather availibilty...I will say in that situation, (in the woods,) I do think not only are you able to keep a position lock better, but you will improve accuracy, in this case it will not be 5' or 10', but rather maybe 50' as compared to 100', due to the added satellite positon. Better chance of keeping a poistion lock and better chance of bettr geometry. So it is a feature to consider during purchase. Just my humble opinion.[/i]

EraSeek,

 

Yes, I agree availability might be a little better with 14 channels vs. 12. However, if the 12 channel GPSr is programmed to pick the 12 sats that give the best solution (best geometry, WAAS, position overhead) and drop those (over 12) that will give the worst solution, then adding in those "least helpful" sats is not going to gain much accuracy. Also just my humble opinion...

[+EraSeek]

But with the 14 channel you are not adding anything, other than 2 channels for WAAS. My old Garmin 12 pulls in 12. My Map76 pulls in 12 if I have WAAS turned off. If turned on, then 10. Someone with a Magellan always only pulls in 10 GPS sats, because their unit is robbing 2 channels for WAAS all the time. The GEO sats do GPS, but are a bit inconsistant because their orbits wobbles.

 

A 12 channel reciever is a 12 channel reciever because, as I understand it, that is all the sats you are going to be able to get usually. (other than the 2 extra WAAS sats now.) What are there? I forget. 24..27 GPS sats worldwide. The rest of them are on the otherside of the globe. We see about 12 at best at any given time. It's not the 12 best. It is THE 12. All there is that we can see. Maybe an extra every now and then. As we have discussed, those sats beyond the 4 or 5 best do add to the position solution, but as I understand it, more as a confirmation and eliminators of error, thus improving the solution. Some of those in view may be not used because they are too low and thus masked out. Any thing above the masking may be used up to 12.

 

Yes 14, or 12, or 10, sats probably won't make much or "any" difference in accuracy. You are correct.

 

Now... take my 8 channel multitrack Garmin 45xl out into the woods, along with my old Garmin 12. Tell me there is no difference! There absolutly is! The Garmin 12 will get you to the cache with a bit of trouble. You would have to sit and burn sacrifices at the foot of my Garmin 45xl for hours before you saw any hint of life. Those extra channels make a dfference. Granted, those 2 extra channels are the low waas sats. That is why I turn them off now in when I'm in the woods, to get 2 others up as possibles.

 

All in all, we are to the point of splitting hairs. Are 14 channels better than 12? Probably. By much? Doubtful. Probably not.

 

So why sell them? Marketing. Increased technology. 2 extra chances to get a signal. But truely I think we agree. I just happen to think it will help in the woods is all.

[zzumwalt]

More from Garmin:

 

"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."

I just started geocaching but I've heard about WAAS for quite some time, and I've always had the same question. Does WAAS do me a bit of good while living here in Hawaii, or am I too far away from the master stations on the mainland?

[+sbell111]

Here we go again.

[zzumwalt]

Hey Thanks! I'll be sure to read up!

[+sbell111]

Feel free to quit after the first few posts.

 

To simply answer your question, WAAS will give you improved coords, even in Hawaii.

[+EraSeek]

Someone aways back said gettin 12 sats was rare. Someone back there said you can't get it up north well. WELL, I'm here to tell ya'...

 

[zzumwalt]

I've been playing around with the WAAS feature in my Garmin Etrex Vista on Oahu, Hawaii and so far I haven't seen any difference in reception compared to when it's off. I pick up another satellite or 2, but there's no change in accuracy. I read farther up this discussion that it may take an hour or two for it to start improving when turning WAAS on for the first time, but I sat out in an open field for close to 2 hours just yesterday and nothing changed. The best EPE I've ever seen on this handheld is 16ft and I've gotten that with WAAS on and off. I get around 8 or 9 "d"s on the satellite bars indicating that WAAS in working and I'm receiving differential GPS, but still no 3-4ft EPE like some of you have seen.

[+sbell111]

Don't fret. 3-4 ft is pretty rare. You probably just have crappy sat geometry, that will be better or worse depending on how the birds are positioned. In the mean time, 16 feet isn't horrible.