ashtech DG16 Reference Manual page 371

Taking the mean equinox as the
reference yields true or apparent
Sidereal Time. Neither Solar nor
Sidereal Time are constant, since
angular velocity varies due to
fluctuations caused by the Earth's polar
moment of inertia as exerted through
tidal deformation and other mass
transports.
Single difference
The arithmetic differencing of carrier
phases simultaneously measured by a
pair of receivers tracking the same
satellite (between receivers and
satellite), or by a single receiver
tracking two satellites (between-
satellite and receivers); the former
essentially deletes all satellite clock
errors, while the latter essentially
deletes all receiver errors.
Spherical Error Probable (SEP)
A statistical measure of precision
defined as the 50th percentile value of
the three-dimensional position error
statistics. Thus, half of the results are
within a 3D SEP value.
Spoofing
The process of replicating the GPS
code in such a way that the user
computes incorrect position solutions.
SPS
See Standard Positioning Service
Standard Positioning Service (SPS)
Uses the C/A code to provide a
minimum level of dynamic- or static-
positioning capability. The accuracy of
this service is set at a level consistent
with national security.
Static observations
A GPS survey technique requiring
roughly one hour of observation, with two
or more receivers observing
simultaneously, and results in high
accuracies and vector measurements.
Static positioning
Positioning applications in which the
positions of static or near static points are
determined.
SV
Satellite Vehicle, Satellite Visibility or
Space Vehicle.
Switching channel
A receiver channel which is sequenced
through a number of satellite signals
(each from a specific satellite and at a
specific frequency) at a rate which is
slower than, and asynchronous with, the
message data rate.
TDOP
Time Dilution of Precision. See Dilution
of Precision.
TOW
Time of week, in seconds, from midnight
Sunday GPS.
Translocation
A version of relative positioning which
makes use of a known position, such as
a USGS survey mark, to aid in the
accurate positioning of a desired point.
Here, the position of the mark,
determined using GPS, is compared with
the accepted value. The three-
dimensional differences are then used in
the calculations for the second point.
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