GPS for Dummies.pdf - Engineering Surveyor

64Part II: All About GPSUnder certain conditions — say, when weak WAAS satellite signals are beingreceived or the GPS receiver is a long way from a ground station — accuracycan actually worsen when WAAS is enabled.WAAS is only available in North America. Other governments are establishingsimilar systems that use the same format radio signals such as European Euro Geostationary Navigation Overlay Service (EGNOS) Japanese Multi-Functional Satellite Augmentation System (MSAS)Differential GPSSurveying and other work that demands a high level of precision useDifferential GPS (DGPS) to increase the position accuracy of a GPS receiver.A stationary receiver measures GPS timing errors and broadcasts correctioninformation to other GPS units that are capable of receiving the DGPS signals.Consumer GPS receivers that support DGPS require a separate beacon receiverthat connects to the GPS unit. Consumers can receive DGPS signals from freeor commercial sources.Unless you’re doing survey or other specialized work, you really don’t needDGPS capabilities. For consumer use, the increased accuracy of DGPS hasmostly been replaced with WAAS.Coast Guard DGPSDGPS signals are freely broadcast by a series of U.S. Coast Guard stations inthe United States. Whether you can receive these Coast Guard broadcastsdepends on your location.For more information on DGPS, including coverage maps, pay a visit towww.navcen.uscg.gov/dgps/coverage/default.htm.Commercial DGPSDGPS services are offered commercially for the surveying market. You canrent or purchase electronic and radio equipment for gathering precise locationinformation in a relatively small area.AntennasWell, yes, a GPS unit has to have an antenna to receive radio signals to do youany good. Several types are available, each with its advantages.