User's Guide

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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 CHC GPS receivers use Version
2 protocol for single-frequency DGPS type corrections.
Carrier phase corrections are available on Version 2, or
on the newer Version 3 RTCM protocol, which is
available on certain CHC dual-frequency receivers. The
Version 3 RTCM protocol is more compact but is not as
widely supported as Version 2.
RTK
Real-time kinematic. It is 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 applies to wide area (WAAS/EGNOS
and MSAS) networks of reference stations. 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 47 and 50 dBHz. The quality of a
GPS position is degraded if the SNR of one or more
satellites in the constellation falls below 39.
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
UTC
Universal Time Coordinated. A time standard based on
the local solar mean time at the Greenwich meridian.
VRS
Virtual Reference Station. A VRS system consists of
GNSS hardware, software, and communication links. It
uses data from a network of reference stations to