is referred to as residual current, and it is leak-
ing somewhere into the grounding system.
Sealed valve regulated (SVR) batteries:A
pressurized lead-acid battery that is completely
sealed, and has a pressure-controlled vent to
release excess pressure inside the battery case
(e.g., due to overcharging).
Sentence structure: In NMEA parlance,
describes the sequence and type of data infor-
mation codes that are distributed throughout
the network. (For specific sentence codes, see
the sidebar on page 151.)
Sinusoidal:Having a succession of waves or
curves, as in an AC sine wave.
Specific gravity:The ratio of the density of a
material to the density of water. In a lead-acid
battery, the specific gravity of the electrolyte is
a measure of the battery’s state of charge.
Standing wave ratio (SWR):The ratio of the
maximum RF current to the minimum RF
current on the line. It can be thought of as a
measure of an antenna system’s efficiency.208 glossary
Time domain reflectometer (TDR):A tool
used to measure time delays in signal transmis-
sion through conductors that correlate to
excessive resistance, or open or shorted circuits.
True root mean square (RMS):In an electri-
cal context, describes the algorithm a meter
must have to accurately analyze a less than per-
fect AC waveform.
Uninterruptible power supply (UPS): A
small battery that provides temporary power
to electronic equipment when the connected
power supply experiences an extreme voltage
drop or total loss.
Velocity of propagation (VOP):In an elec-
trical context, a percentage of the speed of
light that describes the speed of current
through a cable. Different conductors have
different VOP values due to their inherent
resistances to electrical current flow.