Page 62 - Radar Technology Encyclopedia

52 ATMOSPHERE atmospheric emission

Table A9
Comparison of Some Radar Astronomy Installations (from Evans and Hagfors, 1968)
Institute Location Frequency Antenna Gain Aper- Average Peak Pulse PRF T s
(MHz) diameter (dB) ture power power length (Hz) (K)
2
(ft) (m ) (kW) (kW) (m s)
Cornell Univ. Arecibo, PR 430 1,000 57.0 20,000 150 2,500 0.1– Variable 400
10
Crimean Deep Crimea, USSR 700 8 ´ 50 * 46.8 700 60 60 CW CW 100
Space Tracking
Station
California Inst. Goldstone 2,388 85 54.2 355 100 100 CW CW 30
of Tech, JPL Lake, CA
Manchester Jodrell Bank, 408 250 47.3 2,300 1.5 1.5 30 1 1,200
Univ. U.K.
Massachusetts Westford, MA 1,295 84 46.5 190 150 150 0.1–4 Variable 80
Inst. of Tech. (Millstone)
Lincoln Lab
Massachusetts Massachusetts 8,000 120 66.8 525 100 100 CW CW 100
Inst. of Tech. (Haystack)
Lincoln Lab
*This antenna consists of an array of eight 16-m parabolas fixed to a frame.

extends from the earth's surface to 600 to 1,500 km in space, ically reject weather clutter through the use of coherent wave-
but more than 75% of the atmosphere lies below about 10.7 forms and processing, which discriminate moving targets
km (35 kft) and it is in this region where most of earth's from clutter on the basis of a measurement of their doppler
weather effects occur. The atmosphere can affect radar opera- frequency. The capability of a radar to reject atmospheric
tion in several important ways: (1) by absorbing energy from clutter is defined by the radar’s clutter attenuation factor, or
the radar wave (attenuation), (2) by bending the path of the clutter improvement factor. (See CLUTTER). PCH
radar energy (refraction), (3) by contributing interfering sig-
Atmospheric ducting is a mode of anomalous propagation in
nals (clutter) due to the radar energy reflecting off rain (back-
which specific atmospheric conditions create a confined con-
scatter) and other forms of precipitation, and (4) by adding
duit, or duct, that follows the earth’s curvature. At certain
noise to the radar receiver. The magnitude of these effects is
radar frequencies, the duct acts like a waveguide, permitting
frequency (hence wavelength) dependent, and therefore the
propagation of the radar wave beyond that expected under
atmosphere is a major consideration at the design stage of a
normal atmospheric conditions and enabling the radar to
radar, in the selection of operational frequency. See PROPA-
detect targets beyond the radar horizon. Ducts occur as a con-
GATION; ATTENUATION; atmospheric refraction;
sequence of an atmospheric inversion of either temperature,
NOISE, antenna. PCH
humidity, or both, in which the gradient of the index of refrac-
Ref.: Van Nostrand (1983); Blake (1982), p. 177 tion, dn/dh, increases at a rapid rate with altitude. The large
decrease in the index of refraction with altitude causes the
Atmospheric absorption [attenuation] is the loss of radar
energy trapped within the duct formed at low altitude to prop-
energy due to absorption in the propagation medium (air,
agate along the earth’s curvature. Ducts near the ground or
clouds, precipitation, and the ionosphere). See ATTENUA-
sea surface are more common than elevated ducts, supporting
TION; LOSS, atmospheric.
only certain modes of propagation and are usually not deep in
Atmospheric backscatter from clouds and precipitation in the vertical dimension. For these reasons and others, extended
the form of rain, hail, or snow may be considered an propagation by atmospheric ducting is more likely to be expe-
unwanted source of interference, or clutter, to radars whose rienced by horizon-oriented surface radars operating at the
mission is to detect targets other than the weather itself. Rain higher microwave frequencies (UHF and above). PCH
clutter is especially important in that, on a global basis, it Ref.: IEEE (1993), p. 392; Skolnik (1980), pp. 450–456.
occurs most often, can be extended over large areas, and falls
Atmospheric emission. By virtue of its properties as a radia-
at high rates. The magnitude of the precipitation clutter signal
tion-absorbing medium, and in accordance with the law of the
as seen by a radar (i.e., its radar cross section), depends on
2
several factors, including the volume clutter reflectivity (m / conservation of energy and Boltzmann’s black-body radiation
3
m ) of the precipitation (a function of the rain rate); the theory, the atmosphere must radiate the same amount of
energy as it absorbs for the system to be in the state of ther-
dimensions of the radar resolution cell (DR ´ q´ q ); and
El
Az
a
the number of ambiguities in the radar waveform. Radars typ- mal equilibrium. If T is the ambient temperature of the atmo-
sphere, the atmospheric absorption noise power available in a