Q About Waas (wide Area Augmentation System)

[+PastaPowered]

I've noticed that several product comparisons on the web mention that a unit should be able to "signal average." Is signal averaging the layman's equivalent of WAAS (Wide Area Augmentation System)?

[+PDOP's]

Short answer is no. Signal averaging is basically taking multiple fixes to determine an average position for a waypoint.

Edited May 19, 2004 by PDOP's

[Kerry.]

No

 

Cheers, Kerry.

[CenTexDodger]

WAAS is satellite based differential correction. The WAAS is geostationary. It receives correction signals from ground stations around the US and rebroadcasts them to your GPSr--It is a satellite based differential GPS.

[+Renegade Knight]

Another angle.

 

Survey grade GPS equipment uses two parts. One part sits over a known point and uses that information along with the GPS satalite signal to caculate a correction factor. The roving GPS then uses that correction to increase it's accuracey to approximatly a centimeter of error.

 

WAAS does something similar in that WAAS gives your GPS those corrections via satalite and you don't need a base station for the accurace improvment. The trade off is that you are accurate to within a few feet as opposed to a centimeter.

[+Marky]

Survey grade GPS equipment uses two parts. One part sits over a known point and uses that information along with the GPS satalite signal to caculate a correction factor. The roving GPS then uses that correction to increase it's accuracey to approximatly a centimeter of error.

This was just explained to us at a recent event. One of the geocachers at the event is a surveyor and has a Trimble system (and no he doesn't take it out on cache hunts). I found it very interesting. I had always wondered how they got down to that level of accuracy.

 

--Marky

[+PastaPowered]

Thanks everyone for your help. I was pretty certain that WAAS was indeed a signal correction process, but now I know exactly what it does. I ended up ordering the eTrex Vista, but while I was shopping for the device noticed that no one listed signal averaging as a feature. Not that its a terribly important feature because I knew that you can always walk away from a WP and then back again to verify your results.

 

My son should be able to head out this weekend with the Vista and compasses in hand and I'm really looking forward to it.

[+Rosco Bookbinder]

When I first got my GPSr (no need to mention the brand because it doesn't matter) I had WAAS turned on. I almost swore by it. I would always get one sat and once in a great while get both. We fought and fought about if I was getting better accuracy or not. We finally tried some tests about 3-4 months ago. I HAVEN'T HAD WAAS TURNED ON SINCE!!!

 

No difference between mine or all the others we tested. It's much less a headache to have it turned off "like most people" and leave it that way.....

 

Edited for spelling>

Edited May 20, 2004 by Rosco Bookbinder

[nick_02]

on my legend when i have waas on i am 10 ft accurate but with it off it says 50 ft accurate.

does it matter if the screen says it or should i do some tests??

[+NightPilot]

What you're reading is a guess that the GPSr makes. It may or may not be accurate. The reason WAAS was implemented was to increase the vertical accuracy to allow precision instrument approaches by aircraft, which means providing a glideslope, giving vertical guidance in addition to horizontal guidance. Horizontal accuracy is already well within the necessary limits, in fact better than the current ILS (instrument landing system) equipment in use, but vertical accuracy sucks, and even with WAAS enabled, precision instrument approaches to current ILS minima aren't possible. And perhaps won't ever be. The FAA has also spent a few billion on LAAS, or Local Area Augmentation System, and it's back to the drawing boards for that, too. I use WAAS when I'm doing volunteer work for the USGS National Map Corps, getting GPS coordinates for the national map, and using an external antenna mounted on my truck. I get a good, stable WAAS signal there, but when I'm geocaching, I turn it off.

[MacBandit]

Here's a page with all you could ever want to know about GPS and how it works. I thought I knew it pretty well but I read this and now I realize that I didn't really know anything. The whole system is far more fascinating then I could have ever guessed.

 

http://www.trimble.com/gps/index.html

[+raouljan]

The idea of using a "known" to correct an "unknown" is called differential gps (or DGPS). It has been arround for a long time. There are a double handfull of DPGS provides that supply a differential signal to a FM radio type reciever that cables to your GPS to provide the differential (correction) information.

 

Here is a link to everything you would ever want to know about DGPS

 

http://www.edu-observatory.org/gps/dgps.html

[+Prime Suspect]

Thanks everyone for your help. I was pretty certain that WAAS was indeed a signal correction process, but now I know exactly what it does. I ended up ordering the eTrex Vista, but while I was shopping for the device noticed that no one listed signal averaging as a feature. Not that its a terribly important feature because I knew that you can always walk away from a WP and then back again to verify your results.

 

My son should be able to head out this weekend with the Vista and compasses in hand and I'm really looking forward to it.

Averaging was a technique used back with Selective Availability (government mandated signal "fuzzing") was active. By averaging the signal over time, one could get a better position reading. This could work because of the random nature of the errors introduced by SA.

 

SA has been turned off for years now, and there's no need for averaging. The type of errors encountered aren't of the random nature that SA had. Any benefit of averaging will only occur after many, many hours.

[MacBandit]

Thanks everyone for your help.  I was pretty certain that WAAS was indeed a signal correction process, but now I know exactly what it does.  I ended up ordering the eTrex Vista, but while I was shopping for the device noticed that no one listed signal averaging as a feature.  Not that its a terribly important feature because I knew that you can always walk away from a WP and then back again to verify your results.

 

My son should be able to head out this weekend with the Vista and compasses in hand and I'm really looking forward to it.

Averaging was a technique used back with Selective Availability (government mandated signal "fuzzing") was active. By averaging the signal over time, one could get a better position reading. This could work because of the random nature of the errors introduced by SA.

 

SA has been turned off for years now, and there's no need for averaging. The type of errors encountered aren't of the random nature that SA had. Any benefit of averaging will only occur after many, many hours.

Actually that's not true. The part of the SA being off for years. They turned it back on right after 9/11 and say they plan on leaving it that way.

[Kerry.]

Actually that's not true. The part of the SA being off for years. They turned it back on right after 9/11 and say they plan on leaving it that way.

 

Not to sure where some people dig up these myths?

 

SA was not turned back on after 9/11, SA has never been turned back on since it was discontinued in May 2000.

 

Lets get something straight, SA is not actually turned off as such, it is simply set to zero but since the use of SA was discontinued in May 2000 it has not been re-activated (moved off zero) since, technically there is simply no need to use SA again.

 

Cheers, Kerry.

[+SCOTUS]

Kerry, You're right on target. Please see the following:

http://gps.faa.gov/gpsbasics/SA-text.htm

[MacBandit]

Kerry, You're right on target. Please see the following:

http://gps.faa.gov/gpsbasics/SA-text.htm

Thanks that prompted me to do some research and you are correct they have never turned it back on.

 

I don't know where I read that is was being turned back on but I wouldn't have taken it as truth unless it had been a government statement. Maybe they were just considering turning it back on. In any case thanks for straightening me out.

[+Milbank]

I use WAAS when I'm doing volunteer work for the USGS National Map Corps, getting GPS coordinates for the national map,

 

NightPilot,

If you have time could you please email me some info on just what you do as a volunteer.

It sounds interesting.

 

Thanks,

gps@wat.midco.net

[+5nomads]

Kerry, You're right on target. Please see the following:

http://gps.faa.gov/gpsbasics/SA-text.htm

That was interesting reading but leaves me with a question about what makes the technological difference b/w civilian and military units that makes SA no longer necessary.

 

As the article states, "Now, GPS is accurate within 40 feet, or much better. Military GPS is even more precise and has a margin of error of only a few centimeters."

 

Thanks,

[+WeatherMaker]

The GPS system (of which WAAS is not a part, since it's run by the FAA) actually broadcasts on 2 frequencies. One, the "civilian," has always had the same accuracy rating. It had been selectively made worse by the Defense Department through the use of SA (Selective Availability). As described above, that technique hasn't been used since May of 2000.

 

The military has always used BOTH frequencies with their units. The data stream in the second frequency is what allows their units to be much more precise. Civilian units don't receive this frequency. Ironically, from what I've read in other posts here & elsewhere, many soldiers prefer to use civilian GPS units instead of their issued ones because they are so much more user friendly & smaller, and they don't necessarily need the extra accuracy.

 

There haven't been any technological advances making SA no longer necessary. Rather, it was a combination of economics, politics and a reality check. Clinton shut it off just prior to a US/European summit on the future of GPS, GLONASS and Galileo. Anyway, as you might imagine many nations aren't too fond of building systems dependent upon the good will of the US defense establishment and want to pursue their own systems. It was felt that in a effort to maintain American dominance, or at least a seat at the table, turning off SA would be a good political move. Economically, navigation has benefited enormously from SA being shut off, and this affects everyone. The reality check was partially that even within the US government, other agencies were actively working to get around the SA errors for the benefit of the civilian population (e.g. FAA's WAAS system being planned, as well as USCG's DGPS network). There are other security issues as well, but that's it in a nutshell.

[+EraSeek]

Correct, except for "There haven't been any technological advances making SA no longer necessary".

 

They have the ability to jam or use SA in local areas, rather than system wide now. Think battle fields, high security areas. Thus SA is no longer necessary except in a large scale nuke attack.

[+NightPilot]

Why would SA be necessary in a large-scale nuclear attack? Close counts in horseshoes, hand grenades, and nukes. A few meters one way or the other with a 100 megaton device makes no difference.

[+EraSeek]

Why would SA be necessary in a large-scale nuclear attack? Close counts in horseshoes, hand grenades, and nukes. A few meters one way or the other with a 100 megaton device makes no difference.

Well, that was the whole purpose of SA. Missle silos could withstand all but a direct hit. SA would throw off that direct hit. You and me....toast!

[+EraSeek]

Here is a track with SA on:

 

 

And with SA off:

 

Edited September 1, 2004 by EraSeek

[+EraSeek]

The White House May1, 2000:

"The decision to discontinue SA is coupled with our continuing efforts to upgrade the military utility of our systems that use GPS, and is supported by threat assessments which conclude that setting SA to zero at this time would have minimal impact on national security. Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened. This regional approach to denying navigation services is consistent with the 1996 plan to discontinue the degradation of civil and commercial GPS service globally through the SA technique. "

[+NightPilot]

And now with the massive use of GPS by civil aviation, the military has little if any chance of being able to turn SA back on. Almost every instrument approach in the country has a GPS overlay at least, and many airports are served only by GPS approaches. The nondirectional radio beacon network is not being maintained, and there are probably more NDBs out of service than in.

[+EraSeek]

Yes. They have stated they never will turn it back on (ever!), other than the possible use of regional SA under threat.