GROUND CALIBRATION OF COMPACT INFRARED CAMERA (CIRC)
FOR EARTH OBSERVATION
Ryoko Nakamuraa, Haruyoshi Katayamaa, Masataka Naitoha, Masatomo Haradaa,Eri Katoa, Koji Nakaub, and Ryota SatoaaJapan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki, 305-8505, Japanb
Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
ABSTRACTThe compact infrared camera (CIRC) is an uncooled
infrared array detector (microbolometer) with the primary
goal of detecting wildfires, which are major and chronic
disasters affecting numerous countries in the Asia-Pacificregion. Eliminating the cooling system reduces the size, cost,and electrical power of the sensor. Two CIRCs have beendeveloped, which will be launched in JFY 2013 and 2014
onboard the Advanced Land Observing Satellite-2 (ALOS-2)
and CALorimetric Electron Telescope (CALET). We havefinished the ground calibration test of the CIRC Proto Flight
Model (PFM) onboard ALOS-2 and CALET. The imaging
quality and radiometric quality have been confirmed. We
obtained the data needed for image correction andconstructed a data correction algorithm. An airborne
experiment with a ground test model was also carried out toverify the data correction algorithm.
In this paper, we provide an overview of the CIRC and theground calibration results.
Index
Terms—
microbolometer, wildfire

CIRC,

ALOS-2,CALET,

1. INTRODUCTION
Microbolometers are widely used in commercial andmilitary applications. Their advantage is that they do not
require a cooling system, such as a mechanical cooler.
Sensors without a cooling system for a detector are small insize, lightweight, and consume less power. Although thesensitivity of a microbolometer is lower than that of an
HgCdTe-based photonic infrared detectors, its advantage of
not requiring a cooling mechanism makes it suitable forsmall satellites or sensor systems whose resources arelimited.
We have developed the Compact Infrared Camera (CIRC)[1,2,3] as a technology demonstration payload for thermal
infrared imaging from space using a microbolometer. The
main mission of the CIRC is to detect wildfires, which are amajor and chronic disaster that affects many countries,

especially those in the Asia-Pacific region; therefore, theirearly detection is important. An effective means of early
detection is to increase their observation frequency. Our aim
is to realize frequent observations by loading CIRCs in as
many satellites as possible by taking advantage of there
small size, low weight, and low power consumption. Other
mission targets of the CIRC are volcanoes or heat island
phenomena in a city. The CIRC will be carried as atechnology demonstration payload of the Advanced Land
Observing Satellite-2 (ALOS-2) [4], and CALorimetricElectron Telescope (CALET) [5] , which will be attached tothe Japanese Experiment Module (JEM-EF) at theInternational Space Station (ISS).
In this paper, we present the verification results of theathermal characteristics and the calibration of the shutterless system with the CIRC Proto Flight Model (PFM). The
specifications of the CIRC is described in section 2. In
section 3, the ground calibration results of the CIRC PFM
onboard ALOS-2 and CALET is presented. The airborne
observation with the CIRC ground test model is shown insection 4. In section 5, the conclusion is provided.
2. SPECIFICATIONS OF CIRC
The baseline specifications of the CIRC are listed in Table 1.
We set the baseline specifications to meet the requirements
for wildfire detection. The detector has a large format (640 ×480 pixels) to capture a wide field of view. The spatial
resolution is an important factor for wildfire detection; it is
200 m from an altitude of 600 km (ALOS-2) and 130 m
from an altitude of 400 km (CALET). Eliminating thecooling system reduces the size (110 mm × 180 mm × 230mm) and the power consumption (<20 W) of the CIRC.
The CIRC PFM onboard ALOS-2 is shown in Figure 1. The
CIRC is based on a commercial infrared camera. We modified
the hardware design so that it can be applicable for a spaceapplication. CIRCs have key technologies, i.e., microbolometer,
athermal optics, and shutter-less system, for achieving small
size, low weight, and low power consumption.

Table 1. Specifications of CIRC
Parameter
SizeSpecifications
110 mm × 180 mm × 230 mm

Mass3 kg

Detector

Uncooled infrared detector

Wavelength

8–12 m

Number of pixels

640 × 480

Spatial resolution<200 m @600 km (ALOS-2)<130 m @400 km (CALET)

Field of view

12° × 9°Exposure

33 ms

Dynamic range

180-400 K

NEDT

0.2 K @ 300 K

FPN

0.3 K @ 300 K

Temperatureaccuracy

4K(goal : 2K @ 300 K)

blackbody images at various temperatures (–15°C to 50°C)
obtained with CIRC PFM at various environmental
temperatures (–10°C to 50°C). A flow chart of the data
correction is shown in Figure 2.
① Bad pixel correction
A bad pixel is a pixel that has a brightness lower or higher
than the surrounding pixels. We correct the brightness of abad pixel by substituting the mean brightness of the normal
surrounding pixels.
② Dummy correction
The electrical background is subtracted using dummypixels, which have no sensitivity to incident infrared rays.
③ Stray light correction
Stray light correction is performed for each pixel by using
the stray light correction coefficient.
④ Gain correction
Sensitivity correction is performed for each pixel by using
the gain coefficient.

Figure 2. Flow chart of data correction

Figure 1. CIRC PFM onboard ALOS-2
3. GROUND CALIBRATION OF CIRC PFM
ONBOARD ALOS-2 AND CALETThe experimental setup for the construction of thecalibration data table is shown in Figure 3. The CIRC wasinstalled in a vacuum chamber and enclosed a shroud to control
its ambient temperature with a heater and cooler. The
blackbody was set up in the front of the lens.

We carried out a ground calibration test of the CIRC PFM
onboard ALOS-2 and CALET. The aims of this calibration
were (i) to construct a data correction algorithm and (ii) toconfirm the imaging quality and the radiometric quality.
3.1. Constructing data correction algorithm
The stray light (i.e., the overall radiation except that fromthe target) had to be removed, because a dark image cannot
be taken as a result of the shutter-less system. We
constructed a calibration data table (stray light correction
coefficient and gain coefficient) and algorithm using

Figure.3 Experimental setup for the constructionof the calibration data table3.2. Confirming performance
We confirmed the imaging quality and radiometric quality of
the image after the data correction discussed in section 3.1.
3.2.1. imaging quality
The modulation transfer function (MTF) was measured inorder to evaluate the imaging quality of CIRC at various
operating temperatures (–10°C to 50°C). The experimental
setup for MTF measurement is shown in Figure 4. The
collimated infrared rays passed through a germanium
window on the side of the vacuum chamber. The CIRC wasable to capture images of the four-bar target. Figure 5 shows
the measurement points of the four-bar target for MTF, at thecenter, ±5° in the cross-track (CT) direction, and ±3.5° in thealong-track (AT) direction.
Figure 6. Measurement results of MTF in the CT directionof ALOS-2(top) and CALET(bottom)

Figure 4 Experimental setup of MTF measurement(0, 0)
No.1

No.2+3.5° No.3

No.4

No.5

No.6

-5°

No.7

0°

No.8+5° Cross
Track
(CT)

-3.5° No.9

Along Track(AT)(639, 479)

Figure 5. Measurement points of MTF on image
acquired by CIRC
Figure 6 and Figure 7 show the MTF results of CIRC PFM.
The MTF in the CT direction (Figure 6) are constant, whichis regardless of the environmental temperatures. In contrast,
MTF in the AT direction (Figure 7) are slightly lower in
low-temperature environments. Both ALOS-2/CIRC andCALET/CIRC show the similar trend on the environmental
temperature dependence. MTF was found to be effective atcapturing clear images across the entire range of operating
temperatures for both CIRC PFM. We confirmed theathermal optical performance of the CIRC.

Figure 7. Measurement results of MTF in the AT directionof ALOS-2(top) and CALET(bottom)
3.2.2.radiometric quality
The noise equivalent differential temperature (NEdT) andflat pattern noise (FPN) were measured, shown in Table 2.
These are the results expected from the design of the CIRC,
and satisfied its specifications.
Table 2. Results of NEdT and FPN
NEdT [K]FPN [K]
ALOS-2/CIRC
0.19
0.27
CALET/CIRC
0.18
0.21
Figure 8 shows a comparison between a raw image andcorrected image obtained by PFM for ALOS-2. Performing

data correction makes it possible to capture smooth images
without using a shutter system. The temperature accuracy isless than 2 K in a comparison of the brightness temperatureof the corrected image and the actual blackbody temperature.
This is within the specification for the temperaturecorrection accuracy. In the case of CIRC PFM for CALET,
the temperature accuracy is 0.5 ~ 2.8 K, which is also within
the specification.

Figure 9. Image captured by airborne observation:(top) raw image and (bottom) data corrected image, where
white box shows pond area
Figure 8. Blackbody image obtained by CIRC onboard
ALOS-2: (top) raw image and (bottom) data corrected
4. AIRBORNE OBSERVATION WITH THE CIRC
GROUND TEST MODEL
We carried out airborne observations using the ground test
model (GTM) of CIRC, which has optical and radiometricperformances equivalent to the corresponding PFM. The
flight altitude ranged from 300 m to 750 m. The groundsample distance (GSD) at these altitudes ranged from about
10 cm to 25 cm.
Figure 9 shows a comparison between the raw image andcorrected image captured by an airborne observation froman altitude of 750 m. We confirmed that the data werecorrected in the shutter-less system and that the actual
images could be corrected by using the ground calibrationdata.
The temperature correction accuracy was checked using theimage of the pond area shown in Figure 9 (bottom) with thewhite box. The temperature of the pond measured using awater temperature gauge was about 11°C. The corrected
data obtained by CIRC GTM revealed a temperature of
11.5°C, with the assumption that the emissivity of the water
was 0.98. The temperature correction accuracy was about
0.5°C, which is within the specification of 2 K with theenvironmental temperature of ~ 300K.

5. CONCLUSIONCIRC is an instrument equipped with an uncooled infrared
array detector, which will be launched in JFY 2013 onboardALOS-2, and in JFY 2014 onboard JEM/CALET. We havefinished the ground calibration test of the CIRC onboard
ALOS-2 and CALET, and have confirmed that its
performance is as expected and sufficient for launch. We
have also performed data correction using a shutter-lesssystem by analyzing the data obtained through airborne
observations. The temperature correction accuracy wasabout 0.5°C, which is within the specification of 2 K.
6. REFERENCES[1] H. Katayama et al., “Development of the Compact Infrared
Camera (CIRC) for Wildfire Detection,” Proc. SPIE 7458,745806-1-745806-8 (2009)[2] E. Kato et al., “Development of the Compact Infrared Camera(CIRC) for earth observation,” Proc. SPIE 8353, 8353J1[3] M. Naitoh et al., “Development of the Compact InfraredCamera (CIRC) For Earth Observation,” ICSO (2012)[4] S. Suzuki et al., “Overview of Japan’s Advanced Land
Observing Satellite-2 Mission,” Proc. SPIE 7474, 74740Q-1-10(2009).
[5] S. Torii et al., “CALET mission on ISS,” Proc. SPIE 7021,702114-1-11 (2008).