Leica Geosystems GPS Basics - Manual (Page 50)

50
GPS Basics -1.0.0en Glossary
Cutoff angle
The minimum elevation angle below
which no more GPS satellites are
tracked by the sensor.
Cycle slip
A discontinuity of an integer number of
cycles in the measured carrier beat
phase resulting from a temporary loss of
lock of a GPS satellite signal.
Data message
A message included in the GPS signal
that reports the satellite’s location, clock
corrections, and health. Included is
rough information on the status of other
satellites in the constellation.
DGPS
Differential GPS. The term commonly
used for a GPS system that utilizes
differential code corrections to achieve
an enhanced positioning accuracy of
around 0.5 - 5m.
Deflection of the vertical
The angle between the normal to the
ellipsoid and the vertical (true plumb
line). It is usually resolved into a compo-
nent in the meridian and a component
perpendicular to the meridian.
Delay lock
The technique whereby the received
code (generated by the satellite clock) is
compared with the internal code (gener-
ated by the receiver clock) and the latter
shifted in time until the two codes match.
Differenced measurements
GPS measurements can be differenced
across receivers, across satellites and
across time. Although many combina-
tions are possible, the present conven-
tion for GPS phase measurement
differencing is to perform the differences
in the above order: first across receiv-
ers, second across satellites and third
across time.
A single difference measurement
(across receivers) is the instantaneous
difference in phase of a received signal,
measured by two receivers simultane-
ously observing one satellite.
A double difference measurement
(across receivers and satellites) is
obtained by differencing the single
difference for one satellite with respect to
the corresponding single difference for a
chosen reference satellite.
A triple difference measurement (across
receivers, satellites and time) is the
difference between a double difference
at one epoch of time and the same
double difference at another epoch of
time.
Differential positioning
Determination of relative coordinates
between two or more receivers which
are simultaneously tracking the same
GPS signals.