Geoscience Reference
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appropriate choice of the detector material, the desired spectral sensitivity of the
instrument can be achieved. Indium antimonid, for instance, is suited to the detec-
tion of near infrared radiation, while cadmium tellurid, on the other hand, is sensitive
to longer infrared radiation (Strangeways
2003
).
Radiation intensity is a function of the temperature of the emitting object
as described by Planck's law. On the low frequency side (
h
ν/
(
kT
)
>>
1) the
Rayleigh-Jean approximation is valid, where
2
c
2
,
E(
ν
)
=
2
kT
ν
/
(3.21)
10
−
23
JK
−
1
is the Boltzmann constant,
T
is
absolute temperature, and
c
is the speed of light. The so-called brightness tempera-
ture,
T
β
can be obtained from (3.21) by an inversion. For a black body (emissivity
=
here
,
ν
is frequency,
k
=
1.381
×
1), brightness temperature equals the true temperature of the object. For lower
emissivities, the brightness temperature is lower than the true temperature, where
T
b
=
ε
T
.
(3.22)
Radiometers sited at the ground detect a brightness temperature depending on
the zenith angle,
according to
T
b
(
)
=
T
bg
exp(
−
i
(
h
1
,
))
+
sec
α
(
h
)
T
(
h
)exp(
−
τ
(
h
,
)) d
h
,
(3.23)
where the integral is taken over the whole depth of the atmosphere. Here,
T
bg
is the
background temperature (the radiation temperature of space) and
α
(
h
) the absorp-
tion coefficient at a height,
h
. The optical depth,
, of a layer up to the height,
h
,is
determined from the integral over the absorption coefficient from the surface to this
height as given by
τ
(
h
)d
h
.
τ
=
α
(
h
,
)
sec
(3.24)
A particular radiometer that scans the whole hemisphere above it, and whose data
processing is based on (3.24), is described in detail in Martin et al. (
2006
).
Radiometers operating in the microwave spectral range between 5 and 80 GHz
(microwave scanning radiometers, MWSR) are important for the detection of ver-
tical profiles of temperature, humidity, and water droplets (Dabberdt et al.
2004
),
because they can be used day and night with profiling interrupted only by rain.
Validations and enhancements are still ongoing (Rouffieux et al.
2006
). In the
microwave spectral range, the emissivity
of the surface varies strongly with sur-
face properties so that microwave radiometers on satellites detect mainly emissivity
differences. This is in striking contrast to the infrared spectral range where the
emissivity is nearly constant and the received signal is primarily dominated by
temperature differences of the surface.
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