SELECTIVE AVAILABILITY (S/A)
A system whereby the accuracy of GPS is reduced. S/A is
controlled by the U.S. Department of Defense.
SEMIMINOR AXIS
The distance from the center of the earth’s ellipse to the
ellipse surface as measured along the polar axis. It is
identified symbolically with the letter B, and it varies ac-
cording to the reference datum used for position coordi-
nates.
SENSOR
A device that detects a change in a physical stimulus and
turns it into a signal that can be measured.
SET AND DRIFT
The direction and the speed of the water over ground
(current).
SIGNAL-TO-NOISE RATIO (S/N)
Quantitative relationship between the useful and non-
useful part of the received satellite signal. A high S/N
indicates a good receiving condition.
S/N
See SIGNAL-TO-NOISE RATIO
SOFTWARE
Values programmed and preloaded into memory. The
values represent a permanent set of instructions for
running the automatic functions (computations) of the
navigator.
SOG
See SPEED OVER GROUND
SOTDMA
Self Organized Time Division Multiple Access – An access
protocol, which allows autonomous operation on a data
link while automatically resolving transmission conflicts.
SPACE SEGMENT
The orbiting satellite part of the Global Positioning
System.
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