GPS Accuracy and WAAS

spectr17

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How accurate is GPS, really? A typical civilian GPS receiver provides 10 to 60 feet accuracy with selective availability off and 60 to 225 feet accuracy with selective availability on. The accuracy of your GPS unit also depends on the number of satellites available, and the geometry of those satellites. I'm getting about 10 to 15 feet average position error on my Garmin III+.

WASS. There is a new upgrade to the GPS system we have used in the past called WAAS (Wide Area Augmentation System). The Garmin eTrex Legend, GPSmap 76 and Vista are currently the only civilian GPS models that are capable of using WAAS. The only drawback to using WAAS right now is that your geographic location and the terrain you're in will affect how well it works. From a review by Joe Mehaffey and Jack Yeazel, "There are only 2 WAAS satellites as of July 2001, one of these is low on the SE horizon from the Eastern USA and the other low on the SW horizon in the western USA. If you cannot see at least ONE of these WAAS satellites, WAAS will not operate."


WHAT IS WAAS? The basic GPS service fails to meet the accuracy (the difference between the measured position at any given time to the actual or true position), availability (the ability of a system to be used for navigation whenever it is needed by the users, and its ability to provide that service throughout an air flight operation), and integrity (the ability of a system to provide timely warnings to users or to shut itself down when it should not be used for navigation) requirements critical to safety of flight. In order to meet these requirements the FAA is developing the Wide Area Augmentation System or WAAS. WAAS is a safety-critical navigation system that will provide a quality of positioning information never before available to the aviation community. It is what the name implies, a geographically expansive augmentation to the basic GPS service. The WAAS improves the accuracy, integrity, and availability of the basic GPS signals. This system will allow GPS to be used as a primary means of navigation for enroute travel and non-precision approaches in the U.S., as well as for Category I approaches to selected airports throughout the nation. The wide area of coverage for this system includes the entire United States and some outlying areas such as Canada and Mexico.

The WAAS is based on a network of approximately 35 ground reference stations that covers a very large service area. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist. The WAAS will improve basic GPS accuracy to approximately 7 meters vertically and horizontally.

Do-it-Yourself Accuracy Test from Sam Wormley

Measure the accuracy of your GPS receiver by following these four steps. A set of measurements is worth a thousand expert opinions! Forget about EPEs! Trust your own plot.

1. Find any convenient unobstructed place.
2. Record the UTM coordinates for that place. Don't throw away any data points!
3. Make a graphic plot of Eastings and Northings (pencil and paper works really well for this).
4. Repeat steps 2 and 3 (at different time of day and night) for at least a week.

WAAS info links,

http://joe.mehaffey.com/waaslaas.htm

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

http://www.avweb.com/articles/satnav.html

http://waas.stanford.edu/~wwu/rfuller/iongps98/sld001.htm

http://www.sciencenet.org.uk/slup/CuttingE...navigation.html
 

Tinhorn

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Since this report was written, Garmin has went great guns on including WASS in their GPS units.  They have a bunch listed as WASS capable on their site:

http://www.garmin.com/aboutGPS/waas.html

But Magellan has not been so busy, they only had one model listed (or actually 3 types with this one model number)  Map 330, Map 330M, and Map 330X

Since I don't care about Map capabilities (or rather I don't want to pay for it), I am considering the Etrex Venture at about $170

I'm thinking it may be too early to select one tho, I think a bunch will appear with the WASS thing, so won't be in too much of a hurry, but I am excited about 9ft accuracy (if it's true)

Tinhorn
 

spectr17

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Thanks for the link Tinhorn. I think if I was shopping for a GPS I would get a WAAS capable GPS. It may not work to good now but next year it should be better. Then again, I'm pretty happy with my 15 feet average position error I get right now. Now I just went and confused myself again.
 

Richard Webb

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According to my "real life" tests at a survey marker, most of the time GPS is 30 feet to far south, and the east/west error isn't enough to worry about. When I say "most of the time", I mean you can count on it. Because, once in a while the GPS position will creep up towards the marker, but it soon fades back to its 30 feet too far south position.  What WAAS "real life" correction is to be expected in these new units?
 

Tinhorn

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Richard

What kind of accuracy did you getting for elevation in your test?

Just curious.....

Jesse's post above "The WAAS will improve basic GPS accuracy to approximately 7 meters vertically and horizontally."

I think I read somewhere to expect 9 Meter accuracy, Horiz and "Vert"  ( I said 9 feet in the above post-oops!)

Tinhorn

(Edited by Tinhorn at 1:34 am on July 9, 2001)
 

Tinhorn

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I was just flipping thru the Cabela Fall Catalog I just got and they have <NO> WASS compatable GPS's

Bet they will be "Dumping" Non-WASS in-stock units soon.  Some units were already marked down in this catalog...

Just a guess

Tinhorn
 

Richard Webb

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Tinhorn - I didn't keep altitude records during my GPS test, because after working a life time for the airlines I knew better.  One of the reasons I quit posting on the GPS News Group is I noticed that some math experts there drift back & forth between airline math where safety prohibits statics, to civilian math where 90% of the time you can count on something being correct. For all practical purposes, my conclusion is that the laws of statics are on our side, but I wouldn't believe the altitude reading on any GPS Receiver.
 

Richard Webb

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The mailman just dropped off my copy of Western Outdoor News, and West Marine has featured the Garmin WAAS GPS Receivers. So, I guess they are in. The GPS 76 is $219.99, and the GPS 182 Cartographic is $699.99.  Has anyone checked out the accuracy of these units? Is it worth it to upgrade?

By the way, Martin Strelneck has a good article about Eastern Sierra bear encounters. Sounds like a good place to plant a cam.
 

Tinhorn

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this says accuracy to "3 Meters"  which is about 9 feet ? ? ?



from Garmin's Site:

You've heard the term WAAS, seen it on packaging and ads for GARMIN products, maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.
 

Topclamdigger

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I just started using the Garmin GPS 76 with the WAAS technology. The accuracy was impressive . The unit has other features worth the upgrade. Get close to the waypoint the zooom down to 5 meters to close on the point. Easy to follow heading to waypoint: I was testing after sunset. even buttons are lighted. I could get to within 3-4 meters of  Nav points. You can find your treestand with this unit. Put a few Brighteyes in the tree- no problem, find it in the dark. Try one, to see how easy land navigation is using this unit.  
 

Topclamdigger

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I used the Garnin GPS 76 all day today.  I'm a Captain working for Towboat US. Went 50 miles offshore today to tow a sportfisherman home. Had my handheld GPS 76 operating beside a NorthStar GPS  chart/plotter. A good comparison- I'm amazed at the capability and accuracy of that GPS 76. I'd take that unit on an ocean trip anytime. Even as I returned to the inlet, the navigation aids were displayed perfectly on  the Garmin. Navigation was simple and "right- on". I'd say this unit is a good one. Check it out.  
 

Tinhorn

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Topclamdigger

That's good to hear Topclamdigger, I was hoping some one would make a report on the WASS system from personal experiences.

Nice to know there are "wreckers" on the sea, to tow broken down rigs.   How long does it take to go 50 miles on the ocean?  Did the guy have coordinates for his location?  was wondering how easy it was to find him........

as you can tell, we don't have much big water here in Missouri and I'm completly ignorant of the ocean

Tinhorn
 

spectr17

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Thanks for the report TCD,

I've been wanting to get my grimey hands on one of these WAAS receivers. Now ya got me sitting here looking at my old decrepit Garmn III+. Hmmmm.

I bought my last 2 GPS at TVNav, good prices and service. http://www.tvnav.com/navhome.html The GSP 76 was in stock.

Tinhorn, this ocean out here, just think of it as a big pond, a VERY big pond. Getting lost in the fog kind of puckers you up when you realize you may be heading to the south Pacific. I love those Vessel Assist guys, sure beats swimming for it.
 

Topclamdigger

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Tinhorn    If radio comunications are good, the vessel that needs assistance can give an accurate postition( lon/lat) its easy to find them. Otherwise Radar can help in combination with a Radio Direction Finder(RDF) which will determine a bearing to the disabled vessel. The towboat is an ex Coast Guard 41' with twin Detroit 8V92 diesels that cruises at 14 knotts(14 nautical miles per hour). So the ETA at 50nm is about 3.5 hrs. Towing speed is about 6 Knots. This trip took a total of about 11 hrs. There is some time that the boat is not underway .
 


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