Usage-Characteristics of The Airborne Remote Sensing System
Developed at The Faculty of Transport and Traffic Engineering
Davor Franjkovic
Faculty of Transport and
Traffic Engineering,
Vukeliceva 4,
Zagreb, Croatia
davor.franjkovic@fpz.hr
Milan Bajic
Scientific Council of the
Croatian Mine Action Center,
Ul. grada Vukovara 226c,
Zagreb, Croatia
milan.bajic@zg.tel.hr
Hrvoje Gold
Faculty of Transport and
Traffic Engineering,
Vukeliceva 4,
Zagreb, Croatia
hrvoje.gold@fpz.hr
Abstract
The development of the airborne remote sensing
system at the Faculty of Transport and Traffic
Engineering has started in 1998. as support to the
postgraduate study. The system is intended for the
airborne acquisition of data of urban traffic dynamic
characteristics, for the acquisition of data of bottle-neck
phenomenon in road traffic, for determining the status of
the vegetation and the forests, and for the sensing of the
mine polluted areas. The aircraft Cessna 172R is used as
the platform for the gimbal in which more, mutually
different sensors can be placed. The characteristics of the
Sharp View Cam VL-H420S as the sensor are considered
here. The static spatial resolution of the camcorder is
determined by means of the test bars. Further, the effects
of the platform's disturbances and vibrations on the aerial
images are considered.
1. Introduction
The most important criteria of the aerial image quality
are high spatial resolution and geometric accuracy. The
spatial resolution is the function of the smallest length or
area at an original, which is separately represented at the
image, i.e. which can differentiate as discrete element of
the image - the "pixel". It is desirable that this separable
pixel be as small as possible. The geometric accuracy of
the parts of the photographed area and objects is
important for spatial relationships analysis and combining
the images from different sensors or the images taken in
the different time intervals.
The remote sensing system has its own limitations and
disadvantages. These limitations must be well known for
the quality processing of the given images. In this way,
the image quality can be calculated or foreseen in
advance. Many factors, from which some can be
completely unpredictable or random, influence on the
image quality, its spatial resolution and geometric
accuracy. The influential factors are the camera's quality,
the characteristics of the platform, the gimbals and the
navigational systems, the atmospheric conditions and the
imaging methods.
The targets, with in turns, black and white bars, each of
the different wide, are made in order to test the View
Cam's spatial resolution in the controllable laboratory
conditions. The similar target, but much bigger, is made
on the ground at airport Lučko, to test the spatial
resolution of the complete remote sensing system
including the aircraft, the gimbal and the View Cam, with
the unpredictable influence of the external factors. In
addition, the target like a circle is made to test the
influence of the disturbances and vibrations on the images
quality.
The View Cam's spatial resolution is determined after
tests conducted in the laboratory and from the airborne
tests. In addition, the image noise, because of the system's
disturbances and vibrations, is determined and discussed.
2. The View Cam's laboratory testing
The View Cam's spatial resolution is determined by
analysis of the test bar images. The test bars are in turns
black and white fields (bars) with the different, exactly
determined, width. The criterion for the spatial resolution
is the smallest width of the bars, which can be detected on
the images. It is needed to calculate the minimal width of
the test bars on the target according to the known View
Cam's parameters and the test procedure data, and then to
make the target with several narrower bars and several
wider bars. The parameters needed for test bars' sizing
are:
* the View Cam's focal length 1:
fmin = 4,5 mm (minimal "zoom")
fmax = 36 mm (maximal "zoom", enlargement 8x)
the height of the View Cam's pickup device 1:
ysen = 1/4” = 6,35 mm
* the width/height ratio is taken standard:
xsen : ysen = 4 : 3
* the width and the diagonal of the View Cam's
pickup device:
xsen = 8,47 mm
lsen = 10,58 mm
* the number of the image lines on View Cam's
pickup device is taken standard:
N = 400
* the distance from camera to the target:
H=6m
The width of the test bars pair ds which are copied on
the camera's pickup device is determined by the ratio of
the pickup device height ysen and the number of the image
lines N:
ds
y
y
(1)
 sen ;
d s  2  sen .
2
N
N
*
The term ds represents total width of one pair: one
black and one white bar. The next ratio follows from
Figure 1:
H
D H
D  ds 
;

(2)
f
ds
f
and then the needed width of the test bars pair on the
target can be obtained from (1) and (2):
D  2
ysen H

N f
(3)
The insertion of the above mentioned parameters will give
the minimal width of the test bars pair:
D  2
y sen H
6,35 6000
  2

 5,2914 mm
N f
400 36
(4)
On the basis of above result, the target for the camera's
spatial resolution determining is made. The width of 2
mm is taken for the width of the minimal test bar pair, i.e.
the first black bar is 1 mm wide. This width is enough
smaller then the calculated minimal width from equation
(4), so the target can be used for cameras with the bigger
focal length (the bigger "zoom") or for the smaller
distance from the camera to the target. The two adjacent
test bars are different for the scale factor 6 2 what is the
satisfying difference for the camera's spatial resolution
determining 5. Therefore, the width of the n-th test bar
is:
Dn  D1  6 2(n1) .
(5)
A total of the 31 black test bars are painted on the white
background. The last, 31st black bar is 32 mm wide. The
total dimensions of the target are 570,28 X 570 mm.
The angle of the camera's coverage, with the maximal
"zoom", according to the Figure 1, is equal:
  2  arcsin
y sen
6,35
 2  arcsin
 10,119 o .
2 f
2  36
(6)
The target is shot by the View Cam Sharp VL-H420S,
both placed in the equal level, at the distance of 6 m with
good lighting. The video image is transported from the
View Cam through the video recorder to the computer by
the VidCap 32 program. The digitalized images of the test
bars are saved in the JPG and TIF format (Figure 2.).
Figure 1. The geometry of the View Cam's pickup
device and target
The width of the test bars is proportional to the height
of the camera's pickup device ysen and to the distance from
the camera to the target H, and is in inverse proportion to
the number of the image lines N on the pickup device and
to the focal length f, according to the equation (3).
Therefore, the minimal width of the test bars pair is
obtained for the maximal focal length (maximal "zoom").
Figure 2. The image of the test bars shot vertically
The further processing of the images is performed by
the MicroImages TNTlite 5.6 program [6], and the
profiles and the first derivations are obtained for the
horizontally and vertically shot target. The maximums
indicate the largest intensity of the white, and the
minimums indicate the largest intensity of the black on
the profiles' diagrams (Figure 3.). On the diagrams of the
first derivations (Figure 4.), the maximums take place
where the transition from the black to the white occur,
and the minimums denote the transition from the white to
the black, looking from the left to the right.
ds  D 
f
36
 5,04 
 0,03024 mm
H
6000
(7)
The camera has the equal spatial resolution in the
horizontal and vertical direction. The width of the
narrowest separable test bar (5,04 mm) is approximately
equal to the earlier predicted width (5,3 mm). On the basis
of the equation (2) and the dimensions of the minimal
separable "pixel" on the camera's pickup device (0,03024
X 0,03024 mm), the size of the minimal separable "pixel"
on the real object or on the ground can be determined,
depending on the distance from the camera to the object
(the flight altitude for the airborne shooting) and on the
camera's focal length.
3. The spatial resolution of the View cam in
the flight
Figure 3. The profiles of the test bars
Figure 4. The first derivations of the test bars
The number of the visible (separable) test bars can be
obtained by the simple counting of the profiles'
minimums or the first derivations' minimums for both, the
vertically and horizontally shot test bars. The number of
the inseparable test bars is obtained if the number of the
separable test bars is subtracted from the total number of
test bars, i.e. 31. This is shown in the Table 1. where the
width of the minimal visible test bar is calculated
according to the equation (5).
Table 1. The analysis of the test bars'
profiles and first derivations
Test
bars...
Vertical
Horizontal
Separable
Inseparable
23
23
8
8
The width of the
narrowest visible pair
5,04 mm
5,04 mm
Therefore, the camera can separate the black/white pair
which width is 5,04 mm, for the given shooting
conditions (H = 6 m, f = 36 mm). According to equation
(2) the width of the narrowest black/white test bar on the
camera's pickup device amounts:
The calculation of the test bars on the ground for the
airborne camera testing is made for the flight altitudes of
300 and 600 meters. The test bars are painted by the
permanent white paint on the dark asphalted background.
The minimal needed width of the test bars is determined
in the same way as for the laboratory testing. Since the
minimal separable width of the test bars on the camera’s
pickup device is known now (ds = 0,03024 mm), the
minimal width of the black/white pair on the target for
above mentioned altitudes, according to equation (2),
should be:
* for H = 300 m:
H
300
D  ds 
 0,03024 
 252 mm
(8)
f
0,036
* and for H = 600 m:
H
600
D  d s   0,03024 
 504 mm
f
0,036
(9)
By analogy as for the laboratory targets, several test
bars narrower than the minimal width D = 252 mm, and
several test bars wider than the minimal width D = 504
mm are painted, so that the target can be used for the both
expected flight altitudes. If the scale factor 6 2 is again
used, it means that five test bars come between two
obtained minimal width, and the seven test bars including
these two extreme values. Beside these seven test bars,
five narrower and six wider test bars are painted.
Therefore, according to the camera's characteristics
known so far, the 13 test bars are expected to be separable
from the 300 m altitude and the seven test bars are
expected to be visible from the 600 m altitude. However,
the number of the visible test bars can be different from
these foreseen data because of the specific conditions of
airborne shooting, which can be significantly different
from laboratory conditions. The narrowest painted white
bar is 70,7 mm wide, and the last, the widest, 18 th white
bar is 504 mm wide. The total dimensions of target are
8084,2 X 8000 mm.
The camera is placed in the specially designed gimbal
with the two degrees of freedom: the rotation of 360
degrees in the horizontal plane and the rotation of 180
degrees in the vertical plane. The gimbal is hanged up on
the side of the baggage compartment of the Cessna 172R
aircraft (Figure 5.). The “zoom” and the orientation of the
camera are controllable from the aircraft’s cabin. The
preview image is also available on the special display or
on the notebook computer.
The same images are used for the analysis of the
platform's disturbances, as for the determining of the
spatial resolution from the air, but attention is now
dedicated to the target - the circle. The video of the target,
recorded in the flight at the known altitudes, is re-taped
from the Hi-8 to the VHS format. After that, the series of
the successive half-images (a total of 132 half-images) is
digitalized by the VidCap 32 program. The images are
further processed by the Adobe Photoshop 5.0 program so
that the co-ordinates of the target's center (in "pixels") are
determined. The co-ordinates of the target's center are
shown in the Figure 8.
Figure 5. The gimbal hanged up on the aircraft
Only one flight was been undertaken to determine
camera’s spatial resolution for the airborne shooting, until
now, but obtained images was of so poor quality that the
further processing of these images is impossible for this
purpose (Figure 6.).
Figure 6. The image of the target from the air
4. The determining of the disturbances from
the airborne platform
The target (a circle) for the determining of the
disturbances which the airborne platform enters in the
aerial images, must be visible from the 600 m altitude,
therefore, the target's minimal dimension must be greater
then D = 504 mm. With respect to this demand, the white
circle between the diameters of 600 and 900 mm are
painted (Figure 7, designated by arrow).
Figure 7. The target shot from the air and
dimensions needed for the calculation
Figure 8. The read co-ordinates of the target's
center and the regression polynomial (lighter)
The dimensions of the images are 767 X 575 pixels
and the co-ordinates represent the distance of the target's
center to the origin in the upper left corner of the images.
The statistical analysis is performed by the Microsoft
Excel and Analyse-It programs. The total of n = 132 data,
i.e. the 132 pairs of the x, y co-ordinates are processed.
The coefficients of the regression's polynomial (of the 6 th
order) are determined, so that the next polynomial are
obtained:
x  4,63095 10 13  y 6  1,09109 10 9  y 5 
 9,92439 10
7
 y  4,46378 10
4
4
y 
3
(10)
base for the further investigation of the platform's and
camera's dynamic characteristics.
 1,05437 10 1  y 2  1,24328 10 1  y  4,11872 10 1
The regression's polynomial is a smooth curve, and the
deviations (the rests, the differences) between the read coordinates x and the values of the polynomial represent the
shift of the target's center, which are the consequence of
the platform's disturbances and vibrations. The deviations
for every single half-image are shown in the Figure 9.
Figure 10. The distribution of the rests
On the basis of the known dimensions of the target
(8084 X 8000 mm), the measured dimensions from the
image (Figure 8, the CorelDraw program) and the known
dimensions of the image (767 X 575 pixels), the ratio of
the real length on the ground to the length of one pixel on
the image, by application of the simple ratios, is obtained:
Figure 9. The deviations for every single half-image
If the differences are classified in the intervals of the
two pixels width and after counting the number of the
rests in the every single interval, the diagram of the
distribution is obtained. The number of the rests in the
every interval is shown in the Table 2. and the diagram of
the distribution is shown in the Figure 10.
Table 2. The distribution of the rests
The range of
the deviations
The number of
the half-images
(-10,-8]
(-8,-6]
(-6,-4]
(-4,-2]
(-2,0]
(0,2]
(2,4]
(4,6]
(6,8]
(8,10]
1
3
11
19
27
40
18
12
1
0
From Figure 10. it is evident that the distribution
satisfies the form of the Gauss or normal distribution. It
means that majority of the data lies close to the
regressions polynomial, i.e. its deviations are small. The
data about deviations (Figure 9. and 10, Table 2.) are the
104 ,64 mm
8m
m

 0,408
575 pixel 3,57 mm
pixel
(11)
Knowing this ratio, all data expressed in "pixels" can
be transformed in form of the length expressed in
"meters" and further processed.
5. Conclusions
In this paper, the spatial resolution of the Sharp VLH420S View Cam is determined in the laboratory
conditions. The targets with the test bars are made, the
shooting is conducted in determined, known conditions
and the dimensions of the minimal separable "pixel" are
fixed after the images' processing. On the basis of this
fact, the dimensions of the minimal separable "pixel" on
the images, the ground or the real object can be simply
determined, if the data about the distance between the
camera and the target and applied "zoom" are known. In
the next step, the target for the determining of the
camera's spatial resolution from the air in real conditions
is made. The camera's spatial resolution from flight is not
fixed because of poor quality of the obtained aerial
images. The form and distribution of the airborne
platform's (aircraft Cessna 172R, associated gimbal,
Sharp VL-H420S View Cam and accessories)
disturbances and vibrations and their effects on the aerial
images are defined by shooting the target - a circle - from
the known altitudes. The deviations of the target's coordinates from the predicted trajectory of the aircraft
(represented by the regression's polynomial) satisfy Gauss
distribution what means that the level of the disturbances
is adequately small. Consequently, the several important
characteristics of this camera and of the airborne platform
are determined. The obtained characteristics can be
adequately used in the further exploitation of this remote
sensing system.
6. References
[1] SHARP View Cam model VL-H420S & VL-H4200S
Operation Manual, SHARP Corporation, Osaka, Japan
[2] Pilot’s Operating Handbook and FAA Approved Airplane
Flight Manual – The Cessna Aircraft Company, Model 172R,
The Cessna Aircraft Company, Wichita, Kansas, USA, 1996.
[3] M. Bajic, H. Gold, T. Fiedler: Spaceborne and airborne
remote sensing and electronic reconnaissance, Handouts,
Postgraduate Studies, Faculty of Transport and Traffic
Engineering, Zagreb, 1998. (in Croatian)
[4] M. Bajic, H. Gold, D. Franjkovic: "Development of the
Airborne Remote sensing system for the urban traffic research",
Proceedings of International Conference "Traffic in Transitional
Conditions – Intelligent Transport Systems and their Interfaces",
TTC – ITSI ’99, Dubrovnik, October 1999, Promet – Traffic –
Traffico, Supplement No. 4, Vol. 11, Portoroz, Trieste, Zagreb,
1999, pp. 137-142.
[5] R. Kuittinen, E. Ahokas, P. Järvelin: "Transportable Testbar Targets and Microdensitometer Measurements - A Method
to Control the Quality of Aerial Imagery", International
Archives of Photogrammetry and Remote Sensing, Vol. XXXI,
Part B1, Vienna, 1996.
[6] Reference Manual
www.microimages.com
for
the
TNT
products
V5.6,