Retrieval Model of Infrared Surface Emissity Based On
NOAA Satellite Data
Fang Yonghua and Xun Yulong
Anhui Institute of Optics & Fine Mechanics
Hefei, Anhui, 230031, China
Changchun Institute of Optics & Fine Mechanics
Changchun, Jilin, 130022, China
E-mail: yhfang@aiofm.ac.cn
Abstract
In this paper, we accomplish
research on retrieval model of surface
emissivity. We establish appropriate
calculation model and method based
NOAA satellite data to acquire actual
thermal radiation characteristic of any
area in real time.
Introduction
The infrared emissity of surface is
an important parameter that describng
the interaction of earth’s surface with
atmosphere in research on thermal
radiation characteristic of earth’s surface.
Nowadays, they’re two kinds of
elementary research approachs: the first
method is based on groundwork
observation, establishing model with
direct measure and long-term statistic.
This method is direct and reliable but
troublesome. The second method is to
adopt satellite data. Remote sensing
image data in thermal infrared band is
primary research area (such as channel 4
and channel 5 of NOAA satellite). The
application of NOAA’s image data is the
most universal on account of its high
repeat rate and simple atmosphere
correct arithmetic. We can investigate
quickly the thermal radiation
characteristic of any area in real time
according to the retrieval model in
despite of some technology difficulty.
The emissity of seawater is
approximately 1 in thermal infrared
waveband, which change hardly with the
change of wavelength, sea condition and
seawater component. Compared with
seawater, the absorption coefficient of
terrestrial surface (such as rock,
vegetation etc.) is variable along with
the surface characteristic and
wavelength. Research on infrared
waveband emissity (temperature) of land
is advanced task in infrared remote
sensing. Currently, there is no universal
and effective method. We should
establish appropriate calculation model
and method to acquire actual thermal
radiation characteristic of land.
Influence of sun reflectance in
thermal infrared waveband
Around
l=10
mm, the irradiance
of land surface from sun is about (the
atmosphere transmissivity supposed 1):
E=5×10
-1w/m
2·mm
The radiance is:
B=
rE/
p
=1.6×10
-1w/m
2·sr
·mm
Where surface reflectance
r is
supposed to 1(the strongest).
In normal temperature (300K),
thermal infrared radiant intensity of
surface on land is:
B(300K)=1.0×10
1w/m
2·sr
·mm
By all appearances, thermal
radiation occupies a leading place. Solar
reflectance can be ignored.
Emissivity e
Generally, the infrared emissivity of
most water is close to 1; the infrared
emissivity of vegetation is about 1 too.
For most rock and soil, there are some
characteristic absorption in 8-12
mm
waveband (mostly due to SO
2
oscillation). Statistically, their
reflectance spectrum look as a bell cover
(nearly Gauss distribution), which max
value is located in 9.5-10
mm, and
which peak values are not entirely
uniform. So,
e depends on surface
types, surface states and wavelength
variety.
For enough thick surface which
transimissivity is zero, we have:
r+e=1,
Therefore, we should establish
retrieval model to seek reflectance
r
or transimissivity
e.
Infrared transimissivity model of rock and soil
Our research is based on two
infrared waveband (channel 4 and 5) on
NOAA satellite. The image gray degree
D of two channels is directly related to
radiance. We can calculate radiance L
from D and instrument function of
sensor and vice versa.
L=a+k·D
Where a and k are the plus and the offset
of sensor respectively. They refer to
involved handbook.
Surface’s infrared radiance model
can be expressed as:
L'(D
4)=
e
4 B(T,
l
4)
t4
+L (1)
L'(D
5)=
e
5 B(T,
l5)
t5+L
A5 (2)
Where B is the radiance of blankbody at
the same temperature which can be
gotten from Plank formula.
l4 and
l5
are central wavelength or average
wavelength wavelength of channel 4 and
channel 5 respectively.
e4 and
e5 are
surface emissivity of two channels
respectively.
t4 and
t5 are
atmosphere transimissivity which can be
calculated from Lowtran 7 model.
Besides, L
1 is the atmosphere
thermal radiation, which don’t contain
any surface information and Should be
deducted in retrieval process.
Atmosphere influence deduction
First, near the research area, we
work over water surface or an area
having lower emissivity (approximately
black body), and calculate the
temperature T
4° and T
5°
of two channels
from Plank formula by the method of
seawater retrieval. There are some
components of atmosphere radiation in
T
4°and T
5°. Practically, there is a ratio
between two channels atmosphere
radiation. The model is such as:
Ts°=T4°+(T4°-T5°)a+b
Where a and b are content, T
s°
is the
actual temperature. We can calculate
atmospheric influence from formula (1)
and (2).
