64 GPSMAP 496 Owner’s ManualAppendix > Learning about GPSCheck Beacon Wiring—the DGPS setting is enabled in theInterface sub tab on the Main Menu but no DGPS device isdetected.no Beacon Signal—DGPS receiver is attached, but nottransmitting RTCM data to GPS.tuning Beacon—the receiver is tuning manually to a DGPSfrequency.Using Differential—the unit is receiving DGPS corrections.Scanning for Beacon—the DGPS receiver is scanning for anavailable frequency.What is WAAS/EGNOS?The Wide Area Augmentation System (WAAS) is an FAA-fundedservice to improve the overall integrity of the GPS signal andincrease position accuracy for users in North America. In Europe,WAAS is referred to as EGNOS.The system is made up of satellites and approximately 25 groundreference stations positioned across the United States that monitorGPS satellite data. Two master stations, located on either coast,collect data from the reference stations and create a GPS datacorrection message.•••••According to the FAA’s Web site, testing in September 2002, WAASconfirmed an accuracy performance of 1–2 meters horizontal and2–3 meters vertical throughout the majority of the continentalUnited States and portions of Alaska.WAAS is just one service provider that adheres to the MinimumOperational Performance Standard (MOPS) for global SatelliteBased Augmentation Systems (SBAS). Eventually, there willbe several services of worldwide geostationary communicationsatellites and ground reference stations.All SBAS systems use the same receiver frequency; therefore anyoperational SBAS system should be capable of providing your GPSunit with increased accuracy at any location in the world.Currently, enabling WAAS on your Garmin GPSMAP 496 in regionsthat are not supported by ground stations may not improve accuracy,even when receiving signals from an SBAS satellite. In fact, it candegrade the accuracy to less than that provided by GPS satellitesalone. For this reason, when you enable WAAS on your GarminGPS receiver, the receiver automatically uses the method thatachieves the best accuracy. To enable WAAS, refer to page 4.For more information, go to http://gps.faa.gov/Programs/WAAS/waas.htm.
