Integration and Testing of

Optical Imaging System for RASAT

Ee-Eul Kim, Young-Wan Choi, Woong Choi, Hyun-Gu Kim, Sang-Jin Park, Ji-Ho Yun, Myung-Seok Kang, Seong-Keun Jeong

Satrec Initiative, eek@satreci.com

461-26 Jeonmin-Dong, Yuseong-Gu, Daejeon 305-811, Republic of Korea

Abstract - An advanced imaging system, the Optical

Imaging System (OIS) is being developed for a small Earth

observation satellite, RASAT. OIS will produce high-

resolution images in panchromatic and multi-spectral

bands and it consists of one optical and one electronics

units. Recently, we have completed the development of the

OIS engineering/qualification model (EQM). As part of

verification of EQM, functional and environmentral tests

were performed. Key system features of OIS and test result

for EQM are summarized.

I. INTRODUCTION

After the successful launch and commissioning of BILSAT

in 2003, TUBITAK-UZAY initiated the development of the

follow-on small satellite named RASAT. RASAT will be

launched on a Sun synchronous circular orbit with the nominal

altitude of 700 km. As the primary imaging payload of RASAT,

Satrec Initiative (SI) is developing an advanced high-resolution

imaging system, the Optical Imaging System (OIS).

OIS is a derivative of the IRIS system that is a high-

resolution imaging system to be flown on a Singaporean small

satellite, X-SAT [1]. OIS is a compact and rugged system ideal

for high-resolutin imaging applications on 100-kg class small

satellites. The design mission lifetime of OIS is two years.

Recently, SI has completed the development of its engineering

and qualification model (EQM) along with the structural model

(SM). SM will be used for the dynamic test of RASAT.

II. SYSTEM OVERVIEW

OIS is a pushbroom imaging system that consists of two

configurable units of the Electro-Optical Unit (EOU) and the

Processing & Control Unit (PCU). EOU consists of the

Telescope and the Focal Plane Assembly (FPA). PCU consists

of the Signal Processing Modul (SPM), the Control & Storage

Model (CSM), and the Power Supply Module (PSM).

The key system features of OIS are summarized in Table 1.

OIS will produce panchromatic images with a ground sample

distance (GSD) of 7.5 m and multi-spectral images in three

channels with a GSD of 15 m at the design orbit. The imaging

swath width is larger than 30 km at the design orbit. The

signal-to-noise ratio (SNR) will be higher than 64 for an

extended diffuse target with 0.12 reflectance and a solar zenith

angle of 45 degrees. Its peak power consumption is less than

21.0 W during imaging operation and its total mass is less than

7.0 kg.

While BILSAT produces panchromatic images of 12.6 m

GSD using a frame-type imaging system, RASAT will produce

images of higher resolution and fidelity for scientific research

and many civil / environmental applications.

TABLE I KEY FEATURES OF OIS

Parameter Feature

Imaging bands One PAN & three MS

Instantaneous field-of-view PAN : 10.7 radian

MS : 21.4 radian

Field-of-view 2.46 degrees

Aperture diameter 104 mm

Modulation transfer function

at Nyquist frequency PAN 8 %

MS 15 %

Signal-to-noise ratio 64 at specified conditions

Mass < 7.0 kg

Volume (mm) EOU : 170 170 420

PCU : 265 205 85

Power consumption Peak : 21.0 W

Standby : 13.6 W

Storage capacity 7 Gbits

Image data tx. speed 25 Mbps

The Telescope is based on the Maksutov design with all

spherical optical components made of Fused Silica. Its

entrance pupil diameter is 104 mm and the effective focal

length is about 466.7 mm. The optical design gives near

diffraction-limited performance with a modulation transfer

function (MTF) value higher than 0.3 at the panchromatic pixel

Nyquist frequency. Three baffles are used for stray-light

control.

The metering structure is made of Invar and other materials.

It will protect optical components from static and dynamic

loads during transportation and launch. It is also designed to

minimize the deformation of optical surfaces and to maintain

the optical tolerances during optical alignment / integration,

integration with the satellite bus, and operation in orbit. A cut-

away solid model of EOU is shown in Fig. 1. EOU is designed

to have the first natural frequency higher than 300 Hz.

FPA houses a linear detector array for simultaneous imaging

in one panchromatic and three multi-spectral channels. Its

proximity electronics provides clock pulses and biases for the

operation of the detector array. The pre-amplified analog video

signals are transmitted to SPM for processing.

Figure 1. Cut-away Solid Model of EOU

PSM generates all internal power from an unregulated +28 V

of the satellite bus. When +28 V power in turned on, PSM

automatically turns on CSM for communication with the

satellite bus. PSM includes solid-state power switches for other

modules and it supports CSM for the internal telemetry data

collection.

SPM generates timing pulses for the operation of FPA and

processes analog video signals from FPA. It performs gain

control for video signals, analog-to-digital conversion, and

correlated double sampling. The digitized image data are

transmitted to CSM in high-speed for storage. SPM has an

auxiliary real-time image data port for the alignment of FPA.

Within CSM, a high-performance DSP performs the overall

management of operation and the storage / transmission of

image data. The total image data storage capacity is 7 Gbits (4

for panchromatic image data). For communication with the

satellite bus, primary and redundant CAN buses are used. CSM

has four LVDS image data ports dedicated for image data

transmission for each channel and they operate at 25 MHz.

III. EQM DEVELOPMENT

A. Processing & Control Unit

PCU EQM was developed with the indentical configuration

with the flight model (FM) in order to verify its full

functionality and performance. Along with functional test,

environmetal tests performed are as follows.

- EMI/EMC test per MIL-STD-461E - Thermal cycling test - Thermal vacuum/cycling test per ECSS-E-10-03A

- Random vibration test per ECSS-E-10-03A

PCU EQM passed all functional and environmental tests

without anomaly. For these tests, a FPA test set that has the

identical design to FPA was used in order to simulate the close-

to-real test environment. Fig. 2 shows one radiated emission

profile from PCU EQM during its imaging operation over a

frequency range of 30 MHz ~ 1 GHz. The solid line is the

upper bound defined in MIL-STD-461E. Fig. 3 shows PCU

EQM being prepared for the random vibration test.

In addition, an electrical interface test between PCU EQM

and the RASAT engineering model (EM) was performed in

order to verify electrical and protocol interfaces and to

demonstrate the operation of OIS with RASAT EM.

Figure 2. Radiated Emission from PCU EQM during Imaging Operation

Figure 3. PCU EQM on Shaker for Random Vibration Test B. Electro-Optical Unit

For compact high-resolution imaging systems like OIS,

careful attention must be paid during opto-mechanical design

and analysis. The metering structure must be designed to

protect sensitive optical components from static and dynamic

loads and to meet the optical tolerance requirements for ground

and space operation. EOU EQM was developed with the

objectives to test its performance and to verify its opto-

mechanical design against dynamic loads.

During the precision alignment and assembly of EOU EQM,

a computer-aided closed-loop alignment scheme [2] was

applied together with high-precision 3-dimensional

measurement machines and specially designed alignment jigs.

FPA was alignment against the Telescope by measuring MTF

values at the pixel Nyquist frequencies. For MTF measurement,

a large collimator was used together with motorized target

slides at its focus.

Fig. 3 shows the MTF profiles for the red channel measured

at three different active pixel positions covering the near-full

Entrance

Lens

Primary

Mirror

Focal Plane

Assembly

Bus I/F Flexures

field-of-view. The MTF values are all higher than 20.0 % at the

pixel Nyquist freuqncy. MTF values measured before and after

the random vibration test showed no difference within the

measurement accuracy. Fig. 4 shows EOU EQM being

prepared for the random vibration test. The random vibration

test was performed as per ECSS-E-10-03A.

(a) Pixel Position of 123

(b) Pixel Position of 1000

(c) Pixel Position of 1924

Figure 3. MTF Profiles for Red Channel at Three Pixel Positions

Figure 4. EOU EQM on Shaker for Random Vibration Test

IV. CONCLUSION

We have developed EQM of the Optical Imaging System

and have successfully performed various functional,

performance, and environmental tests to verify its design. As

the primary imaging payload of a Turkish small satellite,

RASAT, OIS will produce high-resolution images in

panchromatic and multi-spectral channels. The image data

from RASAT will find numerous applications on remote

sensing and Earth observation.

With the successful development and verification of OIS

EQM, we have initiated the development of OIS FM. It will be

ready for the integration and test with the satellite bus by the

end of October, 2007.

REFERENCES

[1] E. Kim and et al., Development of Earth Observation Sensors for Small Satellites in Satrec Initiative, 5th IAA Symposium on Small Satellites for Earth Observation, April 2005.

[2] E. Kim and et al., Integration and Testing of a High-Resolution Camera for Small Satellites, Proc. of 2nd Int. Conf. on Recent Advances in Space Technologies, pp. 551-554, June 2005.