eroyd,
you are correct, and my units are WAAS enabled, but i had never really heard the full scoop on this and was just wondering.
Slug-Gunner, thanks for the link and the info...
Here's more info on the WAAS system and improving GPS accuracy.
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. Click here and here for more WASS info.
More sophisticated and expensive GPS receivers, costing several thousand dollars or more, can provide accuracies within a centimeter by using more than one GPS frequency. However, a typical civilian GPS receiver's accuracy can be improved to fifteen feet or better (in some cases under three feet!) through a process known as Differential GPS (DGPS). DGPS employs a second receiver to compute corrections to the GPS satellite measurements.
How are these corrections provided to your GPS receiver? There are a number of free and subscription services available to provide DGPS corrections. The U.S. Coast Guard and U.S. Army Corps of Engineers (and many foreign government departments as well) transmit DGPS corrections through marine beacon stations. These beacons operate in the 283.5 - 325.0 kHz frequency range and are free of charge. Your only cost to use this service is the purchase of a DGPS Beacon Receiver. This receiver is then coupled to your GPS receiver via a three wire connection, which relays the corrections in a standard serial data format called 'RTCM SC-104.'
Subscription DGPS services are available on FM radio station frequencies or via satellite. Of course, in either case you need a separate receiver to pick up these transmissions and then send them to your GPS receiver. In some cases, the prices vary according to the level of accuracy desired.
:eek: :-)
