User's Manual
Page
56
X360 GIS Reference
Receiver
User
Guide
D.
Glossary
GLOSSARY
base station
Also called station. A base station in construction, is a Receiver placed at a
known point on a jobsite that tracks the same satellites as an RTK rover, and
provides a real-time differential correction message stream through radio to
the
rover, to obtain centimeter level positions on a continuous real-time basis.
A base
station can also be a part of a virtual station network, or a
location at which GPS
observations are collected over a period of time, for
subsequent postprocessing to
obtain the most accurate position for the
location.
carrier
A radio wave having at least one characteristic (such as frequency, amplitude,
or
phase) that can be varied from a known 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.
C
M
R
C
M
R+
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.
differen
tial
correcti
on
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.
elevation mask
The angle below which the Receiver will not track satellites. Normally set to
10
degrees to avoid interference problems caused by buildings and trees,
and
multipath errors.