Is WAAS still more accurate with Newer Chips?

[geoprincesss]

Just curious if having WAAS enabled still gives a more accurate position with the newer SIIRF 3 chips and whatever Garmin is using in the new HC series receivers. Or can the more sensitive chips provide just as accurate positions without WAAS being enabled?

[+admo1972]

That's a good question. I assume that if you have a WAAS lock with the newer chips, your position will be more accurate, but the difference will be less.

 

In MA two weekends ago, my accuracy was an average of 20 feet or so. At one cache, I got my first WAAS lock, and was pleasantly surprised to see all those sats on the sat page with the letter D in them. My accuracy was listed as 9 to 10 feet, the best I have seen yet on my Vista. So, I am guessing that with the newer chip sets, the accuracy w/o WAAS is somewhat better, and when it does get a WAAS lock it improves, but not to the same degree.

 

I'd be interested to hear some user experiences.

[+Red90]

Yes, accuracy on my 60Cx is better with a WAAS lock than without.

[+desync0]

I placed my waas enabled holux m-1000 BT receiver right on top of a cache and it worked it's way down to 1.2 meters so about 4 feet. Didn't try it with waas disabled but 4 feet is pretty dadgum good

[jonners]

Surely the Sirf III and the new 'H' series chips are just more sensitive than the older generation ones, but not necessarily any more accurate? If so, then WAAS (or EGNOS for us in Europe) will not make devices with these newer chips any more accurate than it makes devices with the older chips.

[Phoenix2001]

The signal from the satellite is a type of measuring stick. If the atmosphere slows or speeds up the radio wave from the value expected by the receiver, the measuring stick ends up being an incorrect length. WAAS helps to make the measuring stick closer to the correct length. No matter how sensitive the receiver, using "measuring sticks" that are closer to correct lengths is better.

 

The very sensitive receiver is very good at picking up signals reflected from a wall or hillside. This signal is going to be a few feet or perhaps a hundred feet long longer than it would have been if received directly. This measuring stick (being longer than it should have been) will throw the computed position off if it can't be ignored or the weighting that it is given towards the position solution reduced greatly. In other words, multipath can make the position more inaccurate in the "sensitive" receivers.

 

What I have seen with a 76Cx is it performs very well in canopy that is reducing signal strength but otherwise has good HDOP (satellite geometry). Poor satellite geometry such as satellites in a straight line plus a good hillside to reflect signals from satellites not in direct view and somewhat perpendicular to that straight line, can result in positions tens of feet from the correct location.

[+EraSeek]

Yes, WAAS still provides more accuracy.

 

What the new sirf chips do in a nut shell is grab every available signal, rejecting none. It then computes the heck out of those to figure out your position. Thus, you have your signal in tough areas though higher sensitivity, all possible input, and lots of computations.

 

This has nothing to do with what WAAS does. WAAS corrects the incoming signals for errors and delays.

 

" The biggest component of error is due to ionospheric delays followed by clock errors and ephemeris errors. In addition another significant error source was troposphere errors. To attack these error sources the WAAS system sends clock corrections, ephemeris corrections, and ionospheric corrections. It cannot compute tropospheric corrections due to the localized nature of this error but it does remove the tropospheric error component from the data it computes so that the local receiver can apply its own corrections based on an atmospheric model that is based on the current sky location of the SV...

 

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

 

The GPS uses what it is given. WAAS provides the GPS with corrections and thus a way to improve accuracy.

So no matter what kind of chip you have, the GPS sat signal still passes through the atmosphere and the ionosphere's activity will still cause delays that the chip has no clue how to fix, or even know that there are errors. The WAAS system corrects these errors and your GPS applies the corrections based on where you are.

[geoprincesss]

Wow! With so much technical understanding in this group it's a wonder there are any undiscovered caches left! Thanks to all for the responses. The short answer is YES. For the longer answers I gotta go read up on gps theory as it seems there are a few things beyond a satellite sending a signal to the gps that tells you where you are! You have peaked my interest to learn more of how it all really works!

[+GOT GPS?]

Tracklogs were more accurate with my Map60Cx than the Map60C was, and the Map60C was better than my GPS V's were, so there is an improvement with each generation of GPS.

 

I was able to retrace tracklogs on paths in a woods, with tall trees and there was good overlapping of the tracklogs, with the Map60Cx.

[John E Cache]

One thing that confuses me is if you make the radio more sensitive why doesn't the radio pick up more noise. Instead of SIRF being more sensitive, I've always thought that the SIRF chips does a better job of digital filtering out the noise as the signal approaches the noise level in cover(the signal to noise ratio gets small).