R
RADAR SIGNATURE—Visible patterns of radar
return echoes that are commonly associated with
certain phenomena, usually correlated with severe
weather events.
RADIAL VELOCITY—The component of velocity
toward or away from a radar that is parallel to the
radar beam.
RADIOMETER—The scanning sensor aboard
satellites sensitive to only a small range of
electromagnetic wavelengths; that is, water vapor,
infrared, visible light, and microwave.
REFLECTIVITY (Z)—A measure of the efficiency
of a radar target in intercepting and returning radar
energy.
REFRACTION—Changes in the direction of energy
propagation (due to changes in speed) as a result of
density changes within the propagating medium.
REFRACTIVE INDEX—A measure of the amount
of refraction. Numerically equal to the ratio of
wave velocity in a vacuum to wave velocity in the
medium (the atmosphere).
RESOLUTION—In radar, the minimum separation
in angle or in range between two targets that the
equipment is capable of distinguishing. With
satellite imagery, resolution is a measure of the
smallest detail visible in the display. It is usually
measured by the number of pixels that can be fit
horizontally and vertically on the display.
RH—Usual abbreviation for relative humidity.
S
SAR—Acronym for Search and Rescue.
SCAN—One complete rotation of a radar antenna at a
single elevation angle.
SCATTERER—Any object capable of reflecting the
radar signal.
SECOND-TRIP ECHO—A radar echo received
from a target beyond the normal maximum range
of detection. Also called range folding.
SHADOW ZONE—A zone in which the intensity of
electromagnetic energy is negligible due to
diffraction or refraction.
SHEAR—Speed or directional variation in a wind
field.
SHF—Super-high frequency. (3 GHz to 30 GHz).
SIDE LOBES—Concentrated elements of focused
power outside the main radar beam caused mainly
by diffraction near the antenna. Ground clutter is
one common result from side lobes.
SIGNAL-TO-NOISE RATIO—A ratio of the
intensity of the minimum signal capable of being
detected to the amount of interference generated
by the radar.
SIPRNET—Secure Internet Protocol Routing
Network.
SKY WAVE—That portion of a transmitted
electromagnetic wave that travels upward and
outward and is not in contact with the ground. Part
of a sky wave is refracted by the ionosphere and
returns to earth.
SPACE WAVE—At frequencies of greater than 30
MHz, the ionosphere has no effect on
electromagnetic waves, thus allowing the waves to
travel in straight lines eventually reaching space.
The space wave is limited to line-of-sight
transmission.
SQUALL LINE—Any line or narrow band of active
thunderstorms.
SUBPOINT— The point on the earth’s surface
immediately below a satellite at a specific point in
time. Also, nadir.
SUN SYNCHRONOUS—An orbit in which the
satellite passes overhead at essentially the same
solar time throughout the year.
SUPERCELL—A large, long-lived thunderstorm
cell consisting of one steady updraft/downdraft
system that is capable of producing the most
severe weather (tornadoes, high winds, and large
hail).
SYNCHRONIZATION— The process of matching
the line scanning rate in the display equipment
with that of the image transmitting system.
SYNOPTIC—In general, pertaining to or affording
an overall view. In meteorology, this term has
become specialized in referring to the use of
meteorological data obtained simultaneously over
a wide area for presenting a comprehensive picture
of the state of the atmosphere.
T
TAC-4—Tactical advanced computer, version four.
AI-5
