Page 15 - Radar Technology Encyclopedia

absorbing materials, screening accuracy of radar measurement, fundamental 5

Screening absorbing material is intended to attenuate unde- such coatings is usually corrugated. Ferrite radio-absorbing
sirable radiation to protect the operators of the radar station materials have wideband properties. IAM
and other groups of servicing personnel operating in the zone Ref.: Stepanov (1968), p. 59; Bhattacharyya (1991), pp. 212–220.
of the high-intensity microwave radiation. Screening radio-
ABSORPTION is “the irreversible conversion of the energy
absorbing materials are intended to absorb high-intensity of an electromagnetic wave into another form of energy as a
2
radiation (3 to 4.5 W/cm ) and to operate jointly with cooling
result of its interaction with matter.” In radar, the major inter-
systems (air and water). A multilayer structure having conical
est is in absorption along the path of propagation in the atmo-
recesses on its back surfaces is used for screening. Each layer
sphere, through precipitation, foliage, and so forth (see
consists of hollow ceramic microspheres bound by a glue-
ATTENUATION). SAL
cement. Diameters of the microspheres diminish from layer to
Ref.: IEEE (1993), p. 3; Skolnik (1990), pp.11.46–11.51; Knott (1993),
layer in the direction of the rear surface. The absorber with-
pp. 297–359; Bhattacharyya (1991), pp. 176–220.
stands a temperature of up to 1,315°C. For screening of elec-
tronic apparatus and antenna cowlings, fabric material is used accumulator (see INTEGRATOR).
with a pyramidal structure and resistive surface intended for ACCURACY is the quality of freedom from mistake or error.
2
frequencies above 2.4 GHz and having a weight of 500 g/m .
In radar applications accuracy is usually considered in regard
IAM
to the process of radar measurement and is characterized by
Ref.: Electronics, 1970, vol. 43, no. 1, p. 81; Patent CAN 3,441,933 cl. 343- measurement errors (see also ERROR). SAL
18 of 4-29-69.
The fundamental accuracy of radar measurement is that
Structural absorption material is used to achieve a combi-
corresponding to the minimum measurement error that can be
nation of physical strength and absorption. This technique is
attained for the measurements in a noise background. In other
based on replacing the original structure with composites of
words, it is the minimum error due to the fundamental limita-
absorbing material and nonmetallic structure, or combina-
tion: the presence of random noise. It cannot be reduced but
tions of filaments of wave absorbing materials and metallic or
can only be increased in a real system because of nonideal
nonmetallic structure. SAL
characteristics of radar subsystems and introduced losses. For
Ref.: Morchin (1993), p. 123.
estimation of parameters of coherent signals in a white noise
Surface-wave absorbing material is a thin layer of absorber, background, the fundamental accuracy for separate measure-
typically ferrite and synthetic rubber paint. To improve the ment of delay time (range), doppler frequency (velocity) is
performance multiple layers can be used (see Jaumann
absorber). SAL 1 1
s = -------------------------- = ----------
Ref.: Morchin (1993), p. 122. t ·· qB
00
q x t (,) ef
A tunable absorbing material is one whose absorption band
1
1
can be adjusted within certain limits. One example is a radio- s = -------------------------- = ---------
f
absorbing grid of synthetic fiber or metal wire with a diame- q ·· x f 00(, ) qt ef
ter of less than 0.1l with absorbers attached to it. Depending
on the wavelength, narrowband absorbers with the necessary where s and s are the rms errors of delay time and fre-
f
t
··
2
x
dimensions are selected, and the distance between them on quency measurement, q is the signal-to-noise ratio, (0,0) is
the grid is adjusted. the second derivative of the ambiguity function, c(t , f ) for t d
d d
These absorbers are multilayer structures consisting of = f = 0, and B and t are the effective bandwidth and dura-
d
ef
ef
reflective and absorbing layers. When such a coating is used tion of the signal. The analogous approach can be applied to
as a masking means for ground equipment, absorbers are angular coordinates measurement using the four- coordinate
combined that are effective at various wavebands from 0.1l response concept. Finally, fundamental accuracy of a basic
to 10l, where l is the longest wavelength of the radiation. radar parameter, a, can be written as:
IAM
s a = K ¤ q
Ref.: Paliy (1974), p. 197; U.S. Patent no. 3,427,619, cl. 348-18, dated 2-11- min a
69.
where for range, a = R, K = c/2B ; for doppler velocity, a =
ef
R
Wideband absorbing material is effective over a wide fre- v , K = l/2t , l = wavelength; and for either angular coordi-
v
r
ef
quency band. Depending on the application, wideband coat- nate, a = K = l/L , where L is the effective aperture
q,
q
ef
ef
ings of various types can be used. For example, for masking width for the specified angular coordinate. Hence, the radar
aerospace craft a material in the form of an elastic silicon- measurement accuracy of all radar parameters is higher when
organic foam capable of operating for a long time at high the signal-to-noise ratio increases. For a fixed signal-to-noise
temperatures (up to 260°C) in a waveband shorter than 4 cm ratio, the wider the spectrum of radar waveform, the higher
is used. For masking of stationary or slowly moving objects, the range measurement accuracy; the greater the duration of
multilayer materials may be used, made from porous rubber radar waveform, the higher the velocity measurement accu-
mixed with coal dust, or coatings of pressed grains of polysty- racy; and the wider the antenna aperture in the plane of the
rene foam surrounded by a strong coal film. The front part of