Airborne remote sensing validation for the separation of the polarized effects
between objects and atmosphere based on atmosphere neutral point method
YANG Shang-qiang1,2, GUAN gui-xia1, ZHAO Hai-meng2,YANG Bin2, ZHANG
Wen-kai1,2, WU Tai-xia3, YAN Lei2,
1. College of Information Engineering, Capital Normal University, Beijing 100048; 2.
Beijing Key Lab of Spatial Information Integration and 3S Applications, Peking
University, Beijing 100871; 3. Institute of Remote Sensing and Digital Earth, Chinese
Academy Of Sciences, Beijing 100101
Abstract Based on the object’s polarization effects, polarization remote sensing is a
newly emerging method in the field of remote sensing. Both objects and atmosphere
have polarization effects, however, the atmosphere’s polarization effects are much
stronger than that of objects’. Consequently, atmosphere polarization effects will
interfere or even cover objects’ when observing with sensors. How to maximally
eliminate the polarized effects generated by the atmosphere is a crucial problem in
polarization remote sensing. Atmospheric neutral point is an area where the degree of
atmosphere polarization is next to zero; therefore, if sensors are set up in this area,
atmosphere polarization would be greatly eliminated, which is the main content of
separating the effects between objects and atmosphere by atmospheric neutral point
method. In this paper, after processing and analyzing the experimental data got from
the first polarization remote sensing flight experiment with atmosphere neutral point,
the degree of polarization images captured in neutral and non-neutral point area were
obtained, and it can be seen that the main value of polarized degree of images got in
neutral point area was obviously smaller than that in non-neutral point area. The
results showed that the theory mentioned above was logical and practical. An
innovation in our study is that the requirements needed in polarization remote sensing
flight with neutral point were clarified. In the meantime, a qualitative conclusion was
drawn that observing with longer wavelength is more applicable to polarization
1
Foundation: National Natural Science Foundation of China ( 61174220) (41101328)and Project of Beijing Municipal
Education Commission(KM201210028002)
Author: YANG Shang-qiang, male, Master candidate, Engaged in polarization remote sensing and atmospheric neutral point.
Telephone: 18810442532
E-mail: tinginsky@126.com
Corresponding author: YAN Lei, Professor, Peking University. Telephone: 13910821927
E-mail: lyan@pku.edu.cn
remote sensing.
Key Words
polarization remote sensing; atmospheric neutral point; separation the
polarized effects between objects and atmosphere
primary scattering will generate positive
Introduction
As one of the major characteristics
of Electromagnetic Wave [1], polarization
effects can occur when natural light
reflects from objects
[2,3,4]
, and thus
polarization remote sensing provides
new potential information with this kind
of feature. This made polarization
remote
sensing
emerging
and
become
a
newly
highly
concerned
observation method in recent years,
which is applied to practice. However,
the atmosphere also has polarization
effects much stronger than the object’s
polarization[5];
therefore,
atmosphere
polarization effects will interfere or
cover objects’ when observed with
sensors. Then how to eliminate the
influence of atmosphere polarization on
objects polarization is one of the crucial
problems
in
polarization
remote
sensing[6].
When the sun light crosses through
the atmosphere, it is scattered by
atmospheric particles, and then becomes
polarized light. Generally speaking,
polarization
scattering
value,
will
polarization
and
multiple
generate
negative
value;
while
in
some
particular area, which is the so-called
atmospheric
neutral
point[7],
the
polarization value may turn to zero
when
the
positive
and
negative
polarization counteracted with each
other. Consequently, if sensors are set up
in
the
area
to
observe
objects,
atmosphere polarization effects will be
eliminated, and more information of
objects will be obtained, which is the
main content of separating the effects
between objects and atmosphere based
on atmospheric neutral point theory
[8]
.
In our study, the polarization remote
sensing flight experiment firstly reported
in
China
was
carried
out
and
requirements needed in polarization
remote sensing flight with neutral point
were clarified. A series of images both in
neutral and non-neutral point area were
obtained by making comparisons. The
data showed that polarization degree of
images obtained in neutral point area is
smaller than that got in non-neutral point
area,
proving
the
rationality
and
Fig. 1.1 Positions of three neutral points
Optical parameters of clear sky can
practicability of the theory.
be characterized by Rayleigh scattering.
1 Atmospheric neutral point and
However, the actual sky polarization is
very different from Rayleigh model,
separation
effects
of
between
the
polarized
objects
and
Arago, a French astronomer, firstly
discovered the phenomenon of sky
polarization in 1809. Soon afterwards he
found an area called Arago Neutral Point,
in which the polarization was close to
zero. Thirty years later, Babinet, a
French meteorologist, discovered the
second neutral point called Babinet
Neutral Point. The third neutral point
was found years later by Brewster, a
named
Brewster
Neutral Point[9,10]. The positions of the
three neutral points are shown in Fig 1.1.
The height of three neutral points will
simultaneously change according to the
position of the sun.
anisotropy,
aerosol
distribution and ground reflection. One
1.1 Atmospheric neutral point
physicist,
multiple scattering of aerosol particles,
molecular
atmosphere
Holland
called polarization defect caused by
obvious character of the defect is the
atmospheric neutral point [11].
Stokes vectors can be used to
describe light intensity and polarization.
Stokes
vectors
consist
of
four
components, I, Q, U and V. I component
indicates the total intensity of the light,
Q component indicates the horizontal
and vertical linear polarization, U
component indicates the ±45°linear
polarization, and V component indicates
the laevorotatory and dextrorotatory
circular
polarization[12].
Being
very
small, V component is commonly
ignored, so we can get all Stokes vectors
just by solving three equations.
When the light crosses through an
ideal polarizer, its Miller matrix is:
 1

1 cos 2
Mp  
2  sin 2

 0
cos 2
cos 2 2
cos 2 sin 2
sin 2
cos 2 sin 2
sin 2 2
0
0
0
0 
0

0
(1.1)

is
the angle
between the
of multiple scattering, the neutral point
polarizer and reference direction.
Stokes components of scattered
is a region in which the polarization is
close to zero, rather than a separate point;
light S out are:
Sout
two meanings: first, due to the presence
Second, the neutral point in the sky is a
 I '
I 
Q '
Q 
    Mp  
U '
U 
 
 
V ' 
V 
(1.2)
circular cone(Fig. 1.2) in which the
polarization is always zero at any
height[8,13], rather than a fixed area.
The intensity of scattered light is:
1
I out  I '  ( I  cos 2  Q  sin 2  U ) (1.3)
2
Since V component is very small,
we just calculate the DOLP (Degree Of
Linear Polarization) of the object. If
 is set as 0°, 60° and 120°, respectively,
three
equations(1.4)
Afterwards,
observed
are
Stokes
points
obtained.
components
are
obtained
of
and
DOLP(1.5) can be calculated by using
2

 I  3 ( I 0  I 60  I120 )

2

Q  (2 I 0  I 60  I120 )
3

2

( I 60  I120 )
 U
3

1.2
the
polarized effects between objects and
atmosphere is: the observation sensors
are put in the neutral point areas, then
greatly
eliminated.
Finally,
the
information of objects polarization is
(1.4)
acquired as much as possible
2 Airborne authentication
2.1 Experimental instruments and
Q2  U 2
I
Separation
The method used to separate the
the atmosphere polarization effects are
these Stokes components.
DOLP 
Fig. 1.2 Neutral Point Area
(1.5)
flight requirements
polarized
Some
requirements
and
experimental instruments must be met in
effects
between
objects
and
atmosphere
The atmospheric neutral point has
remote sensing observation experiments,
described as follows.
2.1.1 Experimental instruments
The experimental instrument is
independently contrived by Beijing Key
Lab of Spatial Information Integration
and 3S Applications, Peking University.
It consists of three D200 digital cameras,
Fig. 2.2 Three digital cameras
one engineering control machine and
one liquid-crystal display (LCD) (Fig.
2.1). The three digital cameras are fixed
on a mechanical bracket which can
rotate at specific angles (Fig. 2.2). In the
front
of
each
digital
camera
a
customized polarizer is fixed. The
angles between the light-transmissive
axis and reference direction are 0°, 60°
and 120° (Fig. 2.3), respectively. The
cameras
are
controlled
by
the
engineering control machine in order to
shoot at the same time, after which
images
are
transmitted
into
the
engineering control machine and can be
viewed through the LCD.
Fig. 2.3 Angles of three polarizer
2.1.2 Neutral point selection
There are three common neutral
points in the sky: Arago Neutral Point,
Brewster Neutral Point and Babinet
Neutral Point. As the neutral point is
distinctly affected by external conditions
such as the sun’s location, atmospheric
conditions,
surface
albedo
and
observation band, it is thereby necessary
to select a suitable neutral point. All
things considered, Babinet Neutral Point
is chosen as the observation neutral
point, which is always within the range
of 20 ° in the vicinity of the sun, and
coincides with the sun at noon. The
relation between Babinet Neutral Point
elevation angle and solar elevation angle
Fig. 2.1 Experimental instrument
is shown in formula 2.1. Solar elevation
angle
always
nevertheless
it
changes
can
be
in
a
day,
calculated.
Formula 2.2 illustrates how to calculate
elevation angle is complementary with
solar elevation angle. In the formula, h
camera deflection angle.
indicates solar elevation

angle,
represents latitude and longitude, and
solar declination angle
checked
out
by
the

can be
Astronomical
Almanac.
Fig. 2.5 Camera deflection angle
2.1.5 Route spacing
 y  0.9 x  18(0  x  30)

 y  0.75 x  22.5(30  x  90)
Images
(2.1)
where x indicates solar elevation angle, y
indicates neutral point elevation angle.
sinh  sin  sin   cos  cos  (2.2)
in
both
neutral
and
non-neutral point area are needed to
make a comparison. Supposing that we
are observing in the neutral point area
when the airplane flies from south to
2.1.3 Flight direction
As shown in Fig. 2.4, flight
direction should be perpendicular to the
line between the sun and object, and the
north is set as zero while defining solar
north, images shot in non-neutral point
area can be obtained when it returns. In
order to observe the same objects, a
distance limited by flight height H and
camera deflection angle  should be
azimuth.
made between the round-trip routes (Fig.
3.6).
Rout e spaci ng = 2 t an  H
(2.3)
Fig. 2.4 Flight direction
2.1.4 Camera deflection angle
Fig. 2.6 Route spacing
In order to make the observation
area within the neutral point region, the
cameras should deflect a certain angle.
Fig.
2.5
demonstrates
the
camera
deflection angle. The neutral point
Neutral
point
elevation
angle
always changes along with the solar
elevation angle, while the solar elevation
angle changes over time. Flight direction
and
camera
deflection
angle
are
(Table 1), from which it can be seen that
determined by both solar and neutral
neutral point coincides with the sun at
point elevation angle. All these factors
noon; therefore, this period of time
considered, a parameter lookup table
should be avoided.
was drawn for convenient research
Table 1 Parameter lookup table
Neutral
Solar
Solar
Solar
Time
zenith
Camera
point
elevation
azimuth
angle
Flight
Observing
Camera
Rout
direction
angle
direction
spacing(H=3km)
8.54
West
2219.76
8.19
West
1917.44
8
West
1623.70
8.04
West
1337.04
8.49
West
1056.07
9.76
West
780.33
13.16
West
509.37
25.36
West
249.11
23.42
West
113.90
-19.05
East
334.14
-11.61
East
599.67
-9.2
East
872.36
-8.27
East
1149.44
-7.99
East
1432.17
deflection
elevation
angle
angle
angle
10:30
27.07
81.46
62.93
69.6975
20.3025
10:45
23.63
81.81
66.37
72.2775
17.7225
11:00
20.19
82
69.81
74.8575
15.1425
11:15
16.75
81.96
73.25
77.4375
12.5625
11:30
13.31
81.51
76.69
80.0175
9.9825
11:45
9.88
80.24
80.12
82.59
7.41
12:00
6.47
76.84
83.53
85.1475
4.8525
12:15
3.17
64.64
86.83
87.6225
2.3775
12:30
1.45
336.58
88.55
88.9125
1.0875
12:45
4.25
289.05
85.75
86.8125
3.1875
13:00
7.61
281.61
82.39
84.2925
5.7075
13:15
11.03
279.2
78.97
81.7275
8.2725
13:30
14.46
278.27
75.54
79.155
10.845
13:45
17.9
277.99
72.1
76.575
13.425
north by
west
north by
west
north by
west
north by
west
north by
west
north by
west
north by
west
north by
west
north by
west
north by
east
north by
east
north by
east
north by
east
north by
east
14:00
21.34
278.04
68.66
73.995
16.005
14:15
24.78
278.29
65.22
71.415
18.585
14:30
28.22
278.68
61.78
68.835
21.165
14:45
31.65
279.15
58.35
66.2625
23.7375
15:00
35.08
279.70
54.92
63.69
26.31
north by
east
north by
east
north by
east
north by
east
north by
east
-8.04
East
1721.04
-8.29
East
2017.47
-8.68
East
2323.03
-9.15
East
2638.50
-9.7
East
2966.68
2.2 Data analysis
The experiment, which was carried
3.5km (Fig. 2.6), which was observing
out in Zhuhai(113.4°E，22.1°N) on
in non-neutral point area. The remaining
June 18, 2012 when the sky was clear
was free flight time.
with few clouds, was the first polarized
remote
sensing
experiment
with
Images captured in neutral point
area are shown in Fig. 2.7, and Fig. 2.8
atmospheric neutral point in China. The
shows
the
images
flight height was 3km, the speed
non-neutral point area. There are some
250km/h, the shooting interval 6s, and
differences between images captured by
the time 16:30pm~17:30pm. Camera
three cameras, because the cameras’
deflection angle was 40.18°, facing the
location fields of view are different, and
east. The airplane flew from south to
it is impossible to avoid image distortion.
north first, and the angle between flight
Therefore, it is necessary to register the
direction and the north was 14.7 ° ,
images first. Using the software ENVI,
which was observing in neutral point
the registration images captured both in
area. After 15 minutes’ flight, the
neutral (Fig. 2.9) and non-neutral point
airplanes returned with a distance of
area (Fig. 2.10) were obtained.
Fig. 2.7 Images captured in neutral point area
captured
in
Fig. 2.8 Images captured in non-neutral point area
Fig. 2.9 Registration images captured in neutral point area
Fig. 2.10 Registration images captured in non-neutral point area
Fig. 2.11 and Fig. 2.12 show
non-neutral point area with a percent of
polarization degree of images got in
5.17%, which proved that atmospheric
neutral point area and non-neutral point
neutral point can be used to eliminate
area respectively, along with its
atmosphere polarization effects in
histogram. By comparison, it can be
polarized remote sensing in order to get
seen that most values of polarization
more information of objects.
degree are smaller than 0.4 in neutral
Fig. 2.13 shows R-band, G-band
point area, while most are bigger than
and B-band polarization degree of
0.5 in non-neutral point area. Besides,
images got in neutral point area. It can
the object details in neutral point area
be observed that the polarization degree
are much more than those in non-neutral
of R-band and G-band is smaller than
point area. Based on the statistics, the
that of B-band, and more details are
value of polarization degree is smaller
obtained in the first two bands. For
than 0.4 in neutral point area and the
example, the building in upper left is
percent is 84.45%, whereas the value of
missing in B-band polarization degree
polarization degree is smaller than 0.4 in
image, and there is more information in
R-band than G-band image. Fig. 2.14
acquired while being observed with
shows R-band, G-band and B-band
longer band. What’s more, longer
polarization degree of images got in
wavelength band is conducive to
non-neutral point area. The same
eliminating atmosphere polarization
conclusion as neutral point area
effects. Further research on the
mentioned above can be drawn.
relationship between observation band
Therefore, it can be concluded that
and the amount of image information is
qualitatively more information can be
needed in the future.
Fig. 2.11 Polarization degree of images got in neutral point area (left) and its histogram (right)
Fig. 2.12 Polarization degree of images got in non-neutral point area (left) and its histogram
(right)
Fig. 2.13 R-band (left), G-band (middle) and B-band (right) polarization degree of images got in
neutral point area
Fig. 2.14 R-band (left), G-band (middle) and B-band (right) polarization degree of images got in
non-neutral point area
Although the height and brightness
get images of the very same objects in
of the sun always changed during the
the round-trip flight. However, as the
flight, the flight time was so short that
effect of eliminating atmosphere
the influence on the results can be
polarization effects with neutral point
ignored. We can only get images of
was very obvious, the experiment was
objects nearby, for it was too difficult to
reliable.
3 Conclusions
Based on the analysis above, the
sensing.
information of objects in neutral point
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