User's Manual

SPSx50 Modular GPS Receiver User Guide 167
Glossary
real-time differential
GPS
Also known as real-time differential correction, DGPS.
Real-time differential GPS is the process of correcting GPS data as you collect it. This is
achieved by having corrections calculated at a base station sent to the receiver via a
radio link. As the rover receives the position it applies the corrections to give you a very
accurate position in the field.
Most real-time differential correction methods apply corrections to code phase
positions. RTK uses carrier phase measurements.
While DGPS is a generic term its common interpretation is the use of single-frequency
code phase data that is sent from a GPS base station to a rover GPS receiver and the
resultant position accuracy is sub-meter. The rover receiver can be at a long range
(greater than 100 kms) from the base station.
rover A rover is any mobile GPS receiver collecting or updating data in the field, typically at
an unknown location.
Roving mode Roving mode applies to the use of a rover receiver to collect data, stakeout, or control
earthmoving machinery in real time using RTK techniques.
RTCM (Radio Technical Commission for Maritime Services)
A commission established to define a differential data link for the real-time differential
correction of roving GPS receivers. There are three versions of RTCM correction
messages. All Trimble GPS receivers use Version 2 protocol for single-frequency DGPS
type corrections. Carrier phase corrections are available on Version 2, or the newer
Version 3 RTCM protocol, available on certain Trimble dual-frequency receivers. The
Version 3 RTCM protocol is more compact but is not as widely supported as Version 2
today.
RTK (real-time kinematic)
A real-time differential GPS method that uses carrier phase measurements for
greater accuracy.
SBAS (Satellite-Based Augmentation System)
SBAS is based on differential GPS, but applied to wide area (WAAS, EGNOS, MSAS).
Networks of reference stations are used and corrections and additional information are
broadcast via geostationary satellites.
signal-to-noise ratio
(SNR)
The signal strength of a satellite is a measure of the information content of the signal,
relative to the signal’s noise. The typical SNR of a satellite at 30° elevation is between
10.0 and 15.0 dBHz. The quality of a GPS position is degraded if the SNR of one or more
satellites in the constellation falls below 4.0.
skyplot The satellite skyplot confirms reception of a differentially corrected GPS signal and
displays the number of satellites tracked by the GPS receiver, as well as their relative
positions.
SNR See signal-to-noise ratio.
triple frequency GPS A type of receiver that uses three carrier phase measurements (L1, L2, and L5).
UTC Abbreviation for Universal Time Coordinated. A time standard based on local solar
mean time at the Greenwich meridian.