Page 66 - Radar Technology Encyclopedia

56 attenuation be clear air attenuation in ground penetration

tion coefficient k for the average sea-level atmosphere as a attenuation by fog (see attenuation by clouds).
a
function of frequency. DKB
Attenuation by foliage is a severe problem in radar systems
Ref.: Barton (1988), p. 279.
that must observe targets through even a thin line of trees.
Attenuation by clouds is a function of frequency and cloud Attenuation coefficients are measured in dB/m, rather than
3
density, as measured by condensed water density in g/m , or dB/km. Typical values for different frequencies are shown in
approximately by the visibility in meters. Figure A97 shows Table A10 and Fig. A98.
the two-way attenuation coefficient for different cloud or fog
Table A10
conditions. DKB
Attenuation in Foliage.
Ref.: Barton (1988), p. 285.
Frequency Two-way k a
(MHz) (dB/m) Reference
82 0.05 Jakes
210 0.08 Jakes

9,400 2.2 Currie and Brown
35,000 3.5 Currie and Brown

95,000 4.5 Currie and Brown

Figure A97 Attenuation coefficient of clouds and fog (from
Barton, 1988, Fig. 6.1.6, p. 285).

Attenuation by chaff is generally negligible. A chaff reflec-
tivity h implies that this fraction of power entering a 1m
v
cube will be scattered by the chaff, with 1 - h transmitted
v
through the cube. Thus, in chaff with the relatively high den- Figure A98 Measured and calculated attenuation coefficient
- 6
sity h = 10 , the fraction scattered in passage through 1 km for trees vs. frequency (from Currie, 1992, Fig. 2.19, p. 81).
v
will be 0.001, leaving 0.999 transmission, giving an effective
attenuation coefficient k = 0.004 dB/km for the transmitted The calculated values represented by continuous curves
a
wave. Even if the chaff were to absorb, rather than scatter, the in Fig. A98 represent the results when the dielectric constant
incident wave, the same attenuation coefficient would apply. of the leaves is matched to fresh or salt water, or to an inter-
Only in the immediate vicinity of chaff-dispensing apparatus mediate mineral water model. At frequencies above UHF, the
does the attenuation become significant. DKB attenuation is such that a typical treeline may be regarded as
Ref.: Barton (1988), p. 285. at impenetrable obstacle, beyond which the field strength may
be calculated by assuming knife-edge diffraction. DKB
clutter attenuation (see MOVING TARGET INDICA-
Ref.: Currie (1987), pp. 170–174; Jakes (1974), pp. 107–110; Currie (1992),
TION). pp. 77–82
Clutter attenuation refers to the rejection of clutter in an attenuation by gases (see attenuation by clear air).
MTI or doppler processor. The normalized clutter attenuation
Attenuation in ground penetration refers to the ability of
is defined as the ratio of clutter-to-noise ratio at the processor
the transmitted signals to penetrate through the surface and
(
input to that at the output: CA º C/N) /(C/N) . (See MOV-
i
o
ING TARGET INDICATION; RADAR, doppler.) DKB into the depths of the ground or other medium, such penetra-
tion being quite limited. Penetration can be characterized by
Ref.: Barton (1988), p. 244.
the penetration depth for a given attenuation and by the atten-
The attenuation coefficient is the attenuation per unit dis- uation coefficient, which is the attenuation per unit depth (see
tance along the path in a given medium, usually expressed in Table A11). An estimate of the penetrating capability is
dB/km. It is expressed either as a one-way or two-way value, needed when the radar targets are located beneath an attenuat-
the latter applicable to the radar case where the wave ing medium. IAM
traverses the path in both directions. Ref.: Mel’nik (1980), p. 71