Page 63 - Radar Technology Encyclopedia

atmospheric emission atmospheric refraction 53

bandwidth B is equal to kT B (1 - 1/L), where k = Boltz- other storms that can create very high rates of rainfall, are
n
a n
mann’s constant, and L is the loss due to attenuation of the other examples of atmospheric irregularities that, in this case,
energy in passing (one way) through the atmosphere. Because cause severe radar attenuation and clutter at microwave fre-
of the term (1 - 1/L), this noise is greatest when the radar quencies. Strong winds and turbulence in any localized or iso-
antenna is oriented along the horizon (exposure to the maxi- lated pattern, such as that produced by the jetstream, can
mum amount of atmosphere), and least when the antenna is cause anomalies in the index of refraction that fit the category
pointed straight up (minimum amount of atmosphere). of atmospheric irregularities. (See also PROPAGATION,
Lightning strokes from storms in the atmosphere produce ATTENUATION, atmospheric turbulence.) PCH
an additional source of atmospheric RF emission, called
atmospheric loss (see ATTENUATION; LOSS, atmo-
atmospheric noise. A commonly occurring phenomenon,
spheric).
lightning radiates considerably energy at low frequencies and
over great distances. The spectrum of atmospheric noise pro- atmospheric noise (see NOISE).
duced by lightning falls off rapidly with frequency, however,
Atmospheric refraction is the term describing change in the
and is of little consequence for frequencies above about 50
direction of travel of radiation passing obliquely from one
MHz (see ATMOSPHERICS).
medium to another. Refraction is “the change in direction of
Atmospheric emissions in the form of atmospheric
propagation resulting from the spatial variation in refractive
absorption noise (thermal) and atmospheric noise (lightning)
index of the medium.” In empty space and in uniform propa-
are two of the environmental noise sources that affect a radar
gation media the ray paths are straight lines, while in most
or other RF receiver. Other sources include solar (or galactic)
media (e.g., the atmosphere) the paths deviate from straight
noise, ground noise, and man-made sources of interference.
lines due to variation in refractive index. In radar applica-
(See NOISE). PCH
tions, atmospheric refraction occurs in the troposphere and in
Ref.: Lawson (1950), pp.103–108; Skolnik (1962), pp. 368–369.
the ionosphere, chiefly as a result of variation in refractive
The exponential reference atmosphere is described by an index with altitude. In the troposphere the variation results
exponential approximation for the refractive index n of the from changing density of atmospheric gases and is essentially
troposphere as a function of altitude. For all radar frequencies independent of frequency, while in the ionosphere it is from
this can be expressed in terms of a refractivity: varying electron density and is strongly frequency-dependent
1
N(h) = n(h) - = 313exp(-0.1439h) = 313exp(-h/7) (see atmospheric refractive index). In most radar applica-
where h is the altitude above sea level, in kilometers, and the tions it is only the tropospheric effect that need be considered,
sea level value N(0) = 313. See atmospheric refraction; but when the path extends to altitudes above about 100 km it
PROPAGATION. PCH may be necessary to consider also the ionosphere, especially
Ref.: Bean, B. R., and Thayer, G. D., “On Models of the Atmospheric for radars at UHF and lower frequencies.
Refractive Index,” Proc. IRE 47, no. 5, May 1959, pp. 740–755; Blake In Fig. A93, if i is the angle of incidence (the incoming
(1980), p. 183.
wave) and r the angle of refraction (the outgoing wave), the
Atmospheric irregularities. Earth’s atmosphere affects the refraction is determined from Snell’s Law: sini = nsinr, in
transmission and reception of radar (and communications) which n is the index of refraction. Physically, n is the ratio of
signals in several ways, most importantly: attenuation of the the velocity of the disturbance in the first medium to that in
signal, bending of the radar wave from a straight path the second.
(through refraction and diffraction), and corruption of the sig-
nal with additive noise. The concept of a standard atmosphere
has been developed to describe the principal physical charac-
teristics of the atmosphere (temperature, pressure, humidity,
wind speed, etc.) as a function of altitude, for more or less
“average” conditions. Using this model, the atmospheric
effects on specific radar and communications systems are pre-
dictable.
We also know, however, that the troposphere, the
r
weather-producing part of the atmosphere, is in a state of con-
tinual change. Significant departures from the conditions i
defined for the standard atmosphere can be termed atmo-
spheric irregularities; one example of which is a temperature
Figure A93 Law of refraction.
inversion, where the earth's surface is cool compared with the
air above it. The temperature inversion may create conditions Radar waves passing through the earth's atmosphere are
suitable for the formation of a superrefracting duct, whereby bent downward by the changing refractive index of the tropo-
the energy from radars radiating into the duct at very shallow sphere and then by the ionosphere. This produces an error in
angles may be refracted along the earth’s curvature to great elevation angle measurement, the ray at the antenna having a
distances. Weather effects, such as hurricanes, tornados, and