I was on a crew recently using a Thales GPS handheld unit (Thales Navigation, Inc. now part of the "Magellen" brand named after the circimnavigator of the earth who perished April 27, 1521, a day that occurred during the fieldwork using it at Quantico, VA for a proposed tree harvest to aid "Marine One" and other air traffic's radar signatures) which can also be attached to an antenna worn on a belt that receives "Differential GPS" NMEA "beacon" signals. These "beacon" signals will be expanded in the coming years and are more accurate than LORAN C in cloudy weather and storms mostly as aids to navigation. The small antenna can either have a cable or is WiFi with the handheld computer which was running "MobileMapper Office" software which held color ESRI "shape files" for transect area layouts used by one of the crew members, familiar with forest survey in the Pacific Northwest. WAAS used in aircraft usually requires line of sight I thought I read.
Unfortunately the listed "experimental" beacon at Alexandria, Virginia was not on line, which would have helped (the unit can find them automatically) and the Annapolis, MD beacon was used a little further away (with the city of Washington, D.C. in between). The trees there are so large and tall I imagine a few months later would have provided even more interference. Ironically the: "...centralized Command and Control unit is USCG Navigation Center, based in Alexandria, VA. The USCG has carried over its NDGPS duties after the transition from the Department of Transportation to the Department of Homeland Security. There are 82 currently broadcasting NDGPS sites in the US network, with plans for up to 128 total sites to be online within the next 15 years." (Wikipedia "Differential GPS"). It was an interesting unit my first encounter with one.
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