User's Manual

Glossary
166 SPSx50 Modular GPS Receiver User Guide
MSAS (MTSAT Satellite-Based Augmentation System)
A satellite-based augmentation system (SBAS) that provides a free-to-air differential
correction service for GPS. MSAS is the Japanese equivalent of WAAS, which is
available in the United States.
MTSAT Satellite-
Based Augmentation
System
See MSAS.
multipath Interference similar to ghosts on a television screen that occurs when GPS signals
arrive at an antenna having traversed different paths. The signal traversing the longer
path yields a larger pseudorange estimate and increases the error. Multiple paths can
arise from reflections off the ground or structures near the antenna.
NMEA (National Marine Electronics Association)
NMEA 0183 defines the standard for interfacing marine electronic navigational
devices. This standard defines a number of 'strings' referred to as NMEA strings that
contain navigational details such as positions. Most Trimble GPS receivers can output
positions as NMEA strings.
OmniSTAR The OmniSTAR HP/XP service allows the use of new generation dual-frequency
receivers with the OmniSTAR service. The HP/XP service does not rely on local
reference stations for its signal, but utilises a global satellite monitoring network.
Additionally, while most current dual-frequency GPS systems are accurate to within a
meter or so, OmniSTAR with XP is accurate in 3D to better than 30 cm.
PDOP (Position Dilution of Precision)
Dilution of Precision (DOP) is a measure of the quality of GPS positions, based on the
geometry of the satellites used to compute the positions. When satellites are widely
spaced relative to each other, the DOP value is lower, and position accuracy is greater.
When satellites are close together in the sky, the DOP is higher and GPS positions may
contain a greater level of error.
PDOP is a DOP value that indicates the accuracy of three-dimensional measurements.
Other DOP values include VDOP (vertical DOP) and HDOP (Horizontal Dilution of
Precision).
Using a maximum PDOP value is ideal for situations where both vertical and
horizontal precision are important.
Position Dilution of
Precision
See PDOP.
postprocessing Postprocessing is the processing of satellite data after it has been collected in order to
eliminate error. This involves using PC software to compare data from the rover to data
collected at the base station.