User's Manual Part 2

5 Using the GPS receiver
66 GeoExplorer 2008 Series User Guide
Ensuring the accuracy of your GPS data
GeoExplorer 2008 series handhelds are high performance GPS receivers that calculate
very accurate GPS positions.
To correct errors in the collected data and to further improve the accuracy of the GPS
positions, use differential GPS. 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. These corrections are
then applied to data collected in the field either in real time or during postprocessing.
For more information, see Differential GPS explained, page 74.
The three different models in the GeoExplorer 2008 series offer three levels of DGPS
accuracy:
• The GeoXM handheld provides reliable accuracy to within 1 to 3 meters with
real-time or postprocessed differential correction. To achieve this level of
accuracy, the DGPS infrastructure needs to provide single-frequency (L1) GPS
corrections from a base station within 500 km.
• The GeoXT handheld uses EVEREST multipath rejection technology to provide
submeter accuracy with differential correction. To achieve this level of accuracy,
the DGPS infrastructure needs to provide single-frequency (L1) GPS corrections
from a base station within 200 km. The GeoXT handheld can achieve better
postprocessed DGPS accuracy by using longer occupations, a dual-frequency
(L1/L2) GPS base station within 80 km, and carrier postprocessing techniques.
• The GeoXH handheld uses EVEREST multipath rejection technology as well as
H-Star technology to provide decimeter (10 cm) to subfoot (30 cm) accuracy
with real-time or postprocessed differential correction.
To achieve decimeter accuracy in real time, the DGPS infrastructure needs to
provide dual-frequency (L1/L2) GPS corrections, either from a VRS
™
network
or from a base station within 30 km. When using a single base station within 30
to 80 km, typical real-time accuracy is subfoot or better. When postprocessing,
decimeter accuracy is achieved when using a single base station within 30 km,
and subfoot accuracy is achieved when using a single base station within 30 to
80 km (or three base stations at a distance of up to 200 km).
When using an external Zephyr antenna during data collection, postprocessed
accuracy improves to 20 cm when using a single base station within 30 to 80 km
(or three base stations at a distance of up to 200 km).
Where the DGPS infrastructure does not meet subfoot accuracy requirements
( for example, the base station provides L1 corrections only, or is at a distance
greater than recommended), the GeoXH handheld typically achieves submeter
accuracy.
Note – To achieve decimeter or subfoot accuracy with a GeoXH handheld, data must be
collected using Trimble field software (TerraSync software, the GPScorrect extension for
ESRI ArcPad software, or a custom application developed using the GPS Pathfinder Tools
Software Development Kit).