WAAS stands for Wide Area Reference Station. There are 25 stations throughout the US that collect corrections data based on where they are located in the U.S. The further away the user is from the station, the less accurate the corrections are. The maximum recommended distance from a station is 200 miles. The closest WAAS station for RMBL is in Denver, which is straight distance of 121 miles.
After the correction data is collected, it is sent from the Wide Area Reference Stations to the Wide Area Master Station (one on each coast), where the data is packaged, analyzed, and converted to correction data. The Master Stations send the correction message to the WAAS GeoSatellites via a ground uplink station.
The WAAS GeoSatellites broadcast the WAAS-corrected signal to users. WAAS is based on line of sight, and any obstacles will interfere with the signal. Losing signal due to mountains is always a concern, but even more so with WAAS, since there are only 2 GeoSatellites (#35 East Coast and #47 West Coast) broadcasting the signal that you must pick up. In addition, the GeoSatellites broadcasting the signal are in geostationary orbit over the equator, so the further north, the more difficult it is to pick up the signal. They are also low on the horizon (#35 is low to the SE), so that they can easily be blocked by buildings, mountains, etc.
The WAAS correction information is different than RTCM (Real Time Correction Measure) corrections because WAAS examines each of the error types (position, clock, and atmosphere) individually rather than as a group. RTCM is a format for using DGPS (differential GPS) over radio to provide real-time correction of the errors. This is what you would use for high-end GPS units. WAAS is for the lower-end units. However, the same concept is used for both systems.
The FAA lists WAAS accuracy as 7 meter horizontally and vertically. However, if you are in the clear and are receiving a constant good signal from WAAS satellites, then you may improve your accuracy to 1-3 meters. Keep in mind that recreational units are tested based on 6 satellites with good geometry, even though you are only required to have 4. With only 4 satellites, you will probably not be obtaining 1-3 meter data. Also, once a WAAS signal has been lost during data collection, the GPS data collected is no longer accurate to the more precise level.
WAAS SUMMARY 1.) You may not receive the signal due to obstructions. 2.) If you do get the signal, you may lose it and if you do, the accuracy of your data is decreased. It is not always clear when you may have lost the signal. You are more likely to lose the signal in Gothic than in other places, because there are only two satellites transmitting, and the mountains will interfere 3.) The corrections are standardized and are coming from a base station 120-130 miles away. Therefore, the corrections will not be as accurate as they would be if you were close to the base station 4.) The accuracy statistics provided for personal handheld GPSs with WAAS are based on a minimum of 6 satellites. Less than 6 satellites will cause decreased accuracy. Keep in mind that WAAS was not designed for consumer GPS use, but was designed for aircraft use. WAAS is best used in open areas or in marine environments. Based on information gathered about WAAS from surveyors, GPS technicians, the web, and consultants, users should consider WAAS a tool for casual GPS users. It is neither guaranteed to work, nor to increase accuracy, and it is RMBL's recommendation that it should not be used for serious GPS work that will be used for analysis.
For visual information about WAAS, paste this address into your browser to watch ‘What is WAAS’? http://gps.faa.gov/CapHill/indexWAASpresentations.htm