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Glossary
between various satellites. The C/A-
Code modulates only the L1 carrier
frequency on GPS satellites. The C/A-
Code allows a receiver to Carrier
frequency
The basic frequency of an unmodulated
radio signal. GPS satellite navigation
signals are broadcast on two L-band
frequencies, L1 and L2 is at 1575.42
Mhz, and L2 is at 1227.6 Mhz.
Carrier phase
The phase of either the L1 or L2 carrier
of a GPS signal, measured by a
receiver while locked-on to the signal
(also known as integrated Doppler).
CEP
See Circular error probable.
Channel
Refers to the hardware in a receiver
that allows the receiver to detect, lock-
on and continuously track the signal
from a single satellite. The more
receiver channels available, the greater
number of satellite signals a receiver
can simultaneously lock-on and track.
Chip
The length of time to transmit either a
zero or a one in a binary pulse code.
Chip rate
Number of chips per second (e.g., C/A
code = 1.023 MHz).
Circular Error Probable
A circle’s radius, centered at the true
antenna position, containing 50% of the
points in the horizontal scatter plot.
Clock offset
The difference in time between GPS
time and a satellite clock or a sensor
clock (less accurate).
COG
Course Over Ground
Constellation
Refers to the collection of orbiting GPS
satellites. The GPS constellation
consists of 24 satellites in 12-hour
circular orbits at an altitude of 20,200
kilometers. In the nominal constellation,
four satellites are spaced in each of six
orbital planes. The constellation was
selected to provoke a very high
probability of satellite coverage even in
the event of satellite outages.
CTD
Course To Destination
Cycle slip
A loss of count of carrier cycles as they
are being measured by a GPS receiver.
Loss of signal, ionospheric interference
and other forms of interference cause
cycle slips to occur.
DGPS
See Differential GPS
Differential GPS (DGPS)
A technique whereby data from a
receiver at a known location is used to
correct the data from a receiver at an
unknown location. Differential
corrections can be applied in real-time or
by post-processing. Since most of the
errors in GPS are common to users in a
wide area, the DGPS-corrected solution
is significantly more accurate than a
normal SPS solution.
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