Page 67 - Radar Technology Encyclopedia

attenuation in ground penetration attenuation by snow 57

Table A11 Attenuation for laser radar is sensitive to the wavelength
and atmospheric conditions. Atmospheric windows exist in
Penetration Capability of Radar in Various Media.
the bands 8–14 m m, 3–5 m m, 0.7–2.5 m m, and in the visible
Wave- Attenuation Penetration spectrum. Within these windows, strong absorption regions,
Medium length coefficient, depth for 20 dB due to water vapor and CO , are present near 1.4, 1.9, 2.7,
2
(cm) (dB/m) attenuation, (m) and 4.3 m m. The attenuation coefficients for other atmo-
spheric components are shown as a function of wavelength in
Snow 10 0.3 67
Fig. A100.
100 0.036 520
Frozen soil 300 4.2 4.5
Dry soil 500 0.8 25
Dry sandy soil 3 300 0.07
with 3% 60 3 6.7
moisture
Dry clay soil 3 300 0.07
60 14 1.4
Attenuation by hail is critically dependent on the size distri-
bution and surface condition of the hailstones. Presence of a
water film greatly increases the attenuation. Values calculated
for typical hail with diameter of 2.9 cm are shown in
Table A12. DKB
Ref.: Sauvageot (1992), pp. 109–110.
Table A12
Attenuation in Hail

Frequency Two-way k (dB/km)
a
(GHz) Dry Wet
Figure A100 Approximate variation of attenuation coefficients
9.4 3.3 7.6 with laser wavelength, at sea level for various atmospheric
conditions, excluding H O and CO components (from Jela-
2
2
5.5 0.67 5.2 lian, 1992, Fig. 2.3, p. 65).
Attenuation in fog, rain, and snow has little dependence
3.0 0.07 2.4
on wavelength but is dependent on water droplet density or
attenuation by ice (see attenuation by hail). precipitation rate, as shown in Fig. A101. DKB
Ref.: Jelalian, 1992, pp. 72–74.
Attenuation by the ionosphere is negligible above the UHF
band, and small even at VHF. Figure A99 shows the total one- Attenuation by particulates, smoke, and aerosol has been
way attenuation through the ionosphere, as a function of fre- calculated under various assumptions, the results generally
quency, for paths at 0 and 90° elevation. DKB indicating that there will be little effect on radars even in the
millimeter-wave bands. DKB
Ref.: Berkowitz (1965), p. 376.
Ref.: Currie (1992), pp. 82–90.
1000
attenuation by precipitation (see attenuation by hail,
snow, or rain).
Frequency (MHz) 500 E = 0, daytime Attenuation by rain is a severe problem for radars in the
upper microwave and millimeter-wave frequencies. Figure
a
o A102 shows the two-way attenuation coefficient k as a func-
200 E = 90 , daytime tion of frequency for different rainfall rates. DKB
Ref.: Barton (1988), p. 283.
E = 0, nighttime
Attenuation by snow is much less than that for rain with the
100
0.01 0.02 0.05 0.1 0.2 0.5 1.0 same water content. Figure A103 shows the one-way attenua-
Attenuation (dB)
tion coefficient as a function of precipitation rate (in mm/h of
Figure A99 Ionospheric attenuation as a function of frequency, water) for dry snow. DKB
one-way transmission path (after Berkowitz, 1965, Fig. 1.32, Ref.: Sauvageot (1992), p. 110; Blake (1982), p. 214.
p. 376).
attenuation by trees (see attenuation by foliage).