User's Manual Part 2

5 Using the GPS receiver
74 GeoExplorer 2008 Series User Guide
Differential GPS explained
Use differential GPS to correct errors in your collected data. Differential GPS (DGPS)
requires one or more additional receivers, called base stations or reference stations,
which are located at known points. Data collected at the base stations is used to
determine 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 base station. Errors common at both the base
station and the rover receiver are corrected with DGPS either in real time or during
postprocessing.
Real-time DGPS
In real-time DGPS, the base station calculates and broadcasts the error for each
satellite as each measurement is received, enabling you to apply corrections while in
the field and collect accurate GPS data. DGPS corrections are available from a variety
of public and commercial sources. They can be generated and broadcast in real-time
by privately or self-owned GPS base stations, or by a wide range of government
agencies.
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 VRS networks. SBAS and VRS networks use multiple base stations to calculate the
DGPS corrections that are then delivered to the user from a Geostationary satellite
(SBAS) or from a radio or cellular phone (VRS networks).
Factors that affect real-time DGPS accuracy include how often the corrections are
updated, how far you are from the base station, 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 base station are downloaded. You can postprocess GPS
data collected with Trimble GPS field software using:
• the GPS Pathfinder Office software version 4.10 (with the latest updates) or
later.
• the Trimble GPS Analyst extension for ESRI ArcGIS software version 2.10 (with
the latest updates) or later.
Typically, postprocessed DGPS uses only one base station. However, when
differentially correcting data collected with GeoXH receivers using H-Star technology,
you can select multiple base stations to correct the file against for improved accuracy.
Factors that affect the accuracy of postprocessed DGPS include the type of receiver
and antenna used at the base station, the distance between the base station and the
location where the rover data was collected, the accuracy of the base station position,
and the logging interval at the base station.