User's Manual

Using GPS
32 GeoExplorer 2005 Series Getting Started Guide
Differential GPS
Use differential GPS to correct errors in your collected data.
Differential GPS (DGPS) requires two or more receivers. One receiver,
called the reference station, is located at a known point to determine
the GPS measurement errors and compute corrections to these
errors. An unlimited number of mobile GPS receivers, called rovers,
collect GPS data at unknown locations within the vicinity of the
reference station. Errors common at both the reference and rover
receivers are corrected with DGPS either in real time or during
postprocessing.
Real-time DGPS
In real-time DGPS, the reference station calculates and broadcasts
the error for each satellite as each measurement is received, enabling
you to immediately see differentially corrected data on the map
displayed in the data collection software.
Real-time DGPS sources include external beacon and radio sources,
as well as Satellite Based Augmentation Systems (SBAS) such as
WAAS in the US and EGNOS in Europe, and Virtual Reference Station
(VRS
™
) systems. SBAS and VRS systems use multiple reference
stations in a network to calculate the DGPS corrections which are
then delivered to the user from a Geostationary satellite (SBAS) or
from a radio or cell phone (VRS).
Factors that affect real-time DGPS accuracy include how often the
corrections are updated, and whether the coordinate system used by
the correction source matches the coordinate system used by the
GPS receiver.
Postprocessed DGPS
In postprocessed DGPS, the collected GPS data is transferred to an
office computer, and measurements from the reference station are
downloaded. Postprocessing software such as the GPS Pathfinder
Office software or the Trimble GPS Analyst extension for ESRI ArcGIS
software is used to differentially correct the collected data.