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| SA |
See Selective
Availability |
| Satellite constellation |
The arrangement in space of a set of satellites. In the case
of GPS, the fully operational constellation is composed of six
orbital planes, each containing four satellites. GLONASS has
three orbital planes containing eight satellites each. |
| Satellite Status Display |
An information screen that shows technical data about each
satellite in view. Information includes receiver channel numbers;
actual satellite I.D. numbers; status of satellite tracking
(T) or searching (S); satellite elevations and azimuths; signal
to noise ratios (SNR) (the higher the number, the better); and
dilution of precision ratings (GDOP is most important; the smaller
the number, the better potential accuracy). |
| SATNAV |
A local term referring to use of the older TRANSIT system
for satellite navigation. One major difference between TRANSIT
and GPS is that TRANSIT satellites are in a low altitude Polar
orbit with a 90 minute period. |
| Savable Plot Trails |
The capability to save your actual plot trail crated on the
plotter screen, thereby enabling the GPS user to either backtrack
the course immediately, or save and retrace the trip at a later
time. |
| S.C.I.P. |
A Site Canidate Information
Packet. These are created for use in the establishment
of new telecommunications site. |
| S-code |
The capability to save your actual plot trail crated on the
plotter screen, thereby enabling the GPS user to either backtrack
the course immediately, or save and retrace the trip at a later
time. |
| Selective Availability (S/A) |
The system used by the U.S. Department of Defense to intentionally
degrade the accuracy of satellite GPS signals being transmitted
to civilian GPS receivers. All brands of civilian GPS receivers
are equally affected by S/A. With random S/A on , the government
has guaranteed that civilian GPS accuracy levels will consistently
be 100 meters or less, 95% of the time. If S/A is turned off,
those accuracy levels will improve to 10 to 15 meters consistently. |
| Semi-major
axis |
One half of the major axis of an ellipse. |
| SEP |
See Spherical
error probable. |
| Short Static |
A computation method for deriving integer ambiguities. It
needs less data and therefore shorter station occupation times
than the classical static computation techniques. This method
differs from other rapid or fast ambiguity fixing methods because
it can use single frequency or dual frequency data. Other methods
require dual frequency data. |
| Sidereal day |
Time taken for a complete rotation of the earth. Note that
this is approximately 4 minutes less than 24 hours (a solar
day). The solar day is longer because it additionally takes
into account the movement of the earth round the sun. |
| Simultaneous measurements |
Measurements made with two or more receivers over the same
period of time, at exactly the same epochs. Alternatively, the
epochs could be so closely matched in time that any time misalignment
can be accommodated by correction terms in the observation equation. |
| Slope distance |
The three-dimensional vector distance from one station to
another. The shortest distance (a chord) between two points. |
| Slow switching channel |
A sequencing GPS receiver channel that switches too slowly
to allow the continuous recovery of the data message. |
| Solar day |
24 hours. Time between two successive upper transits of the
sun or two successive middays. |
| Space segment |
The part of the whole GPS system that is in space (i.e. the
satellites). Spread spectrum a system in which the transmitted
signal is spread over a frequency band much wider than the minimum
bandwidth needed to transmit the information being sent. This
is done by modulating with a pseudo random code, for GPS. |
| Spatial Photograph |
A three dimensional model of a physical object that has been
captured through laser scanning. |
| Speed Over
Ground (SOG) |
Digital reading that indicates your current ground speed.
(Selectable in miles per hour, knots or kilometers per hour). |
| Spherical Error Probable (SEP) |
The radius of a sphere within which there is a 50 percent
probability of locating a point or being located. SEP is the
three-dimensional analogue of CEP. |
| Spheroid |
See ellipsoid |
| Spread spectrum - GPS signal |
The received GPS signal is a wide-bandwidth, low-power signal
(-160 dBW). This property results from modulating the L-band
signal with a PRN code to spread the signal energy over a bandwidth
that is much greater than the signal information bandwidth.
This is done to provide the ability to receive all satellites
unambiguously and to provide some resistance to noise and multipath. |
| SPS |
Standard positioning service using the C/A code to provide
a minimum level of dynamic- or static-positioning capability.
The accuracy as been set as +/-100metres in horizontal. |
| Squaring-type channel |
A GPS receiver channel that multiplies the received signal
by itself to obtain a second harmonic of the carrier. It does
not contain the code modulation and is called codeless. Note
that the resultant signals are much weaker and much noisier
than either the original signal or a cross correlated signal.
They have a half cycle carrier, (12 cm) which makes rapid ambiguity
fixing much more difficult. |
| Standard deviation (sigma) |
A measure of the dispersion of random errors about the mean
value. If a large numberof measurements or observations of the
same quantity are made, the standard deviation is the square
root of the sum of the squares of deviations from the mean value
divided by the number of observations less one. |
| Standard Positioning Service (SPS) |
The normal civilian positioning accuracy obtained by using
the single frequency C/A code. Under selective availability
conditions, guaranteed to be no worse than 100 meters 95 percent
of the time (2 drms). |
| Static positioning |
Location determination when the receiver's antenna is presumed
to be stationary on the earth. This allows the use of various
averaging techniques that improve accuracy by factors of over
1000. |
| Steering Screen |
Shows a graphic "highway view" of the GPS user's
course over ground. Provides helpful instructions as to how
far off course, which direction to steer, right or left, to
make corrections, and displays related navigational data pertaining
to the waypoint. |
| Straight Line Navigation |
The standard method of navigation used by recreational GPS
products. When commanded to "navigate to a waypoint",
the unit draws a straight, dotted line from the present position
to the selected waypoint. It's the shortest, most direct route
to the destination. Caution: Straight line navigation does not
take into account any obstacles in the path; interim waypoints
may be required to navigate safely around obstacles. |
| SV |
Satellite vehicle or space vehicle. |
| Switching channel |
A receiver channel that is sequenced through a number of
satellite signals (each from a specific satellite and at a specific
frequency) at a rate that is slower than, and asynchronous with,
the message data rate. |
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