2 Physical Principles and Technical Aspects of Remote Sensing
25
(AVHRR) of the NOAA satellites has this capability (Cracknell, 1997; Francois & Ottle, 1996).
Propagation in the Microwave Region. The microwave region offers
the advantage to observe the surface through clouds and precipitation. The
atmospheric transmissivity between 3 GHz and 300 GHz (10 em ~ ,\ ~ 1
mm) is shown in Fig. 2.9 for a clear standard atmosphere and for two different
types of clouds, based on radiative transfer calculations for a non-scattering
atmosphere. Imaging radars are operating in the spectral region below 10
GHz, which is almost unaffected by clouds. Strong precipitation may cause
disturbances for radar propagation in this region, but does not inhibit surface
imaging. The interaction of radar signals with hydrometeors is the basis for
precipitation monitoring by means of weather radar.
Microwave radiometry is applied for surface observations at frequencies
up to 40 GHz and in the atmospheric window between 80 and 100 GHz
(Ulaby et al., 1981). Above 15 GHz the atmospheric contribution becomes
increasingly important, in particular in case of clouds and rain (Liebe, 1985).
For a non-scattering atmosphere the brightness temperature TB,oo at a given
frequency measured at satellite altitude includes the following contributions
~ 1.0
.::;
'(i;
.~ 0.8
CJ)
c
co 0.6
~
0.4
0.2
0.0 3
5
10
(2.21)
,
\
30
Frequency [GHz]
Fig. 2.9. Atmospheric transmissivity in the microwave region at vertical incidence,
for clear standard atmosphere (full line), with stratus of 0.5 km thickness (broken
line), and with stratocumulus of 2 km thickness (dotted line)