I.

INTRODUCTION

The Geostationary Ocean Color Imager II (GOCI-II) is the next generation of GOCI, which is one of the main payloads of the Korean COMS satellite. GOCI was the first ocean color sensor in the world operating on the geostationary orbit. Since COMS launch in June 2010, GOCI is monitoring ocean color around the Korean Peninsula in order to detect, monitor, quantify, and predict short term changes of coastal ocean environment for marine science research and application purpose.

GOCI-II has improved functional, radiometric and geometric performances in comparison with GOCI. It acquires Earth images in 12 visible and near-infrared narrow spectral bands between 380 nm and 865 nm with a spatial resolution of about 300 m over the Korean sea (250 m at Nadir). It provides a full Earth Disk coverage as well as Moon and star imaging capabilities.

As GOCI instrument and COMS satellite, GOCI-II is developed by AIRBUS Defence & Space in close collaboration with the Korean Aerospace Research Institute (KARI) and Korea Institute of Ocean Science and Technology (KIOST). It will be delivered in 2017 and launched in 2019 on the Geostationary Earth Orbit Korea Multi-Purpose Satellite: GEO-KOMPSAT-2B simply called GK2B. It will be operated on a geostationary orbit located at 128.2°E.

The paper presents the main GOCI-II design features, with a specific focus on the new developments dedicated to GOCI-II. This concerns in particular the 7.3 Mega-pixels CMOS Image Sensor, specifically designed, developed and qualified for GOCI-II.

II.

GOCI-II OVERVIEW.

E.

Main challenges of GOCI-II development

Although the overall instrument architecture and operation principles are inherited from the first GOCI, the specified performance enhancements required major new developments of key components:

• • New dedicated CMOS Image Sensor to fit the smaller sampling distance with increased number of pixels while keeping high level radiometric and MTF performances. This detector was developed in partnership between AIRBUS D&S and ISAE, as presented in details in chapter III.

• • New 2-axes Pointing Mechanism (POM - fig 7 below) to manage the enlarged instrument Field of Regard and the increased pointing mirror size and mass, while keeping a high stability, an accurate restitution of the pointing direction and the capability of being launched without Launch Locking Device. The POM was designed and developed by AIRBUS D&S in Toulouse. A fully representative qualification model has been build and the qualification tests will be completed before the end of the summer 2016.

• • The electronic unit (IEU) that has to manage more detection chains and has to provide higher level of autonomy and flexibility for the user, while keeping low size and mass (7.5 kg). The IEU was developed by AIRBUS D&S electronics division. The full functional and performance validation was reached in 2015 on an EM model and the Flight Model has been delivered mid-2016.

• • The dedicated TMA telescope with enlarged pupil and improved straylight performances (Field stop implementation) while keeping compactness and diffraction limited performances. The SiC telescope structure and mirrors are designed by AIRBUS D&S and manufactured by MERSEN BOOSTEC. Mirror polishing is done by AMOS (Belgium) and coating by SCHOTT (Switzerland).

• • The optical filters were newly designed by OBJ in Germany with barrels made by SODERN. After full qualification on dedicated models, the Flight Models have been delivered mid-2016.

• • Two Sun diffusers made of HOD material are also implemented to enhance radiometric calibration

• • In addition, several key elements (harness, structures, EGSE) have been subcontracted to the Korean industry

Fig. 1:

Example of GOCI images for monitoring of turbidity.

Fig. 2:

Comparison of GOCI and GOCI-II requirements

Fig. 3:

Functional architecture of the GOCI-II instrument

Fig. 4:

GOCI-II instrument overview

Fig. 5:

Two imaging modes are available: Local area acquisition and Full Disk acquisition

Fig. 6:

Typical example of GOCI-II imaging operation over 24 hours

Fig. 7:

CAD model of GOCI-II 2-axes Pointing Mechanism

III.

THE GOCI-II CMOS IMAGE SENSOR

In order to meet the demanding radiometric performance requirements, a 7.3MPixels CMOS image sensor has been specifically designed for GOCI-II mission. Die design and architecture have been carried out in partnership with the “Institut Supérieur de l’Aéronautique et de l’Espace” (ISAE). Following design phase, the silicon wafers (Fig-10) have been manufactured and processed in an Asian semiconductor foundry.

Fig. 8:

Telescope during alignment

Fig. 9:

Picture of GOCI-II filter wheel

Fig. 10:

Wafer

The 7.3MPixels CMOS image sensor features:

Fig. 11:

Die architecture

Fig-12 illustrates one of the first GOCI-II detector mock-ups. The manufacturing and test flows follow the ECSS-9020 standard. A dedicated ceramic package has been designed (Fig-13). Wafer sawing, die gluing and wire-bonding (Fig-14) are performed in ISO5 environment. Two dedicated test benches have been designed by Airbus D&S one to test the electrical behavior of the detectors during the manufacturing (Fig-15) and the other to operate detectors during the burn-in phase (Fig-16). A key performance linked to manufacturing is die flatness: die gluing optimization process leads to very low flatness (<5μm) fully in line with GOCI-II instruments requirements (Fig 17).

Fig. 12:

Detector mock-up (temporary window)

Fig. 13:

Package drawing

Fig. 14:

Wire bonding

Fig. 15:

Test bench for functional test

Fig. 16:

Set-up for burn-in

Fig. 17:

Die flatness lower than 5μm peak-to-valley

Once manufactured, the detectors have been tested at Airbus D&S Toulouse facilities using a high performance electro-optical test bench in-house designed and developed (Fig-18). Tests are performed in ISO5 environment The test bench is fully automated and allows the determination of noise, CVF, dark current, linearity, sensitivity and its non-uniformity for the 12 GOCI-II spectral bands (Fig-19). Test sequence duration is about 1 hour per detector including automated post-processing sequence for electro-optical parameters extraction.

Fig. 18:

Detector mounting onto proximity board

Fig. 19:

12 GOCI-II spectral bands characteristics

Temperature is controlled at 20°C thanks to Thermal Electric Cooler equipment.

For selection of detector flight model, screening on several tens of detectors has been carried out: this includes a specific data analysis taking into account the GOCI-II operating mode. Main selection criteria are SNR, cosmetic defects and visual inspections (Fig-20 and Fig-21).

Fig. 20:

Cumulative histograms for data analysis

Fig. 21:

Cosmetic defects determination for FM selection

A qualification program has been conducted on several models of the 7.3MPixels image sensors following space standards. It includes:

Fig. 22:

Detectors during mechanical tests

Fig. 23:

Detectors during TID irradiation

Fig. 24:

Detectors during latch-up test (heavy ions

Electro-optical tests as well as visual inspections are performed prior and after each qualification test. The qualification also includes a Destructive Physical Analysis.

All qualification models successfully passed the qualification tests.

Before final integration onto GOCI-II instrument, the flight detector model has to be assembled onto focal plane. The assembly activities consist of:

Fig. 25:

Detector reported onto PCB with flexible links

Fig. 26:

Detector positioning during gluing onto SiC piece

Fig. 27:

GOCI-II detector assembly

IV.

GOCI-II DEVELOPMENT SCHEDULE.

The GOCI-II development started in July 2013 and the instrument Flight Model delivery is scheduled mid-2017. The design, procurement and integration phase take place in AIRBUS Defence & Space in Toulouse (France) with the help of a detached Joint Development Team from KARI and KIOST.

A GOCI-II EM instrument made of EM electronics, including fully representative detector and mechanisms was built for early verification of the detection chains and full instrument functional validation. The EM instrument was delivered to KARI beginning of 2016.

The completion of the GOCI-II Flight Model integration is scheduled by the end of 2016. The first set of performance acceptance tests will be done in AIRBUS D&S premises in ambient conditions. Then the instrument will be shipped to KOREA beginning of 2017 and all the environmental tests (mechanical, thermal vacuum, EMC) will be performed with the KARI facilities in close collaboration between the French and Korean teams.

After the completion of the acceptance tests, the instrument will be formally delivered to KARI. It will then be mounted on GK2B satellite and launched aboard ARIANE V in 2019.