A Communication Ports Definition
33
carrier
A radio wave having at least one characteristic (such as frequency,
amplitude, or phase) that can be varied from a known reference
value by modulation.
carrier
frequency
The frequency of the unmodulated fundamental output of a radio
transmitter. The GPS L1 carrier frequency is 1575.42 MHz.
carrier phase
The time taken for the L1 or L2 carrier signal generated by the
satellite to reach the GPS receiver. Measuring the number of
carrier waves between the satellite and receiver is a very accurate
method of calculating the distance between them.
CMR
CMR+
Compact Measurement Record. A real-time message format
developed by Trimble for broadcasting corrections to other
Trimble mainboard receivers. CMR is a more efficient alternative
to RTCM.
DGPS
See real-time differential GPS.
differential
correction
Differential correction is the process of correcting GPS data
collected on a rover with data collected simultaneously at a base
station. Because the base station is on a known location, any
errors in data collected at the base station can be measured, and
the necessary corrections applied to the rover data.
Differential correction can be done in real-time, or after the data
has been collected by postprocessing.
Differential GPS
See real-time differential GPS.
DOP
Dilution of Precision. 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 (Position DOP) indicates the three-dimensional geometry of
the satellites. Other DOP values include HDOP (Horizontal DOP)
and VDOP (Vertical DOP), which indicate the accuracy of horizontal
measurements
(latitude
and
longitude)
and
vertical
measurements respectively. PDOP is related to HDOP and VDOP as
follows:
PDOP² = HDOP² + VDOP²
dual-frequency
GPS
A type of receiver that uses both L1 and L2 signals from GPS
satellites. A dual-frequency receiver can compute more precise
position fixes over longer distances and under more adverse
conditions because it compensates for ionospheric delays.
EGNOS
European
Geostationary
Navigation
Overlay
Service.
A
satellite-based augmentation system (SBAS) that provides a
free-to-air differential correction service for GPS.