GPS Accuracy and WAAS

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spectr17
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GPS Accuracy and WAAS

Post by spectr17 » Sat Jul 07, 2001 3:53 pm

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 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/i ... sld001.htm http://www.sciencenet.org.uk/slup/Cutti ... html~Jesse

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