RazakSAT™: A small satellite platform for high resolution remote sensing Norhizam Hamzah  Ahmad Sabirin Arshad  Maszlan Ismail  Yasser Asrul Ahmad  Hafizah Nasir  Marini Abdul Hamid Astronautic Technology (M) Sdn Bhd 2, Jalan Jururancang U1/21, Hicom Glenmarie Industrial Park 40000 Shah Alam, Selangor MALAYSIA Tel: +603-55690100 Fax: +603-55690108 hizam@atsb.com.my ABSTRACT RazakSAT™ is the next generation of the small Earth Observation Satellite. It weighs less than 200kg carrying the optical sensors of the Medium-sized Aperture Camera (MAC. With an attitude accuracy of 0.21o and 3-axis stabilization using four momentum wheels, RazakSAT™ can provide an image with 10m accuracy with respect to the ground sampling distance (GCP). The RazakSAT™ is targeted to orbit the earth at 685km altitude with 9o inclination. It will serve the timely needs of Malaysian experts and users in remote sensing for various applications. INTRODUCTION The RazakSAT™ mission is to provide optimum coverage for the Malaysian region using the unique characteristic of the NEqO mission. RazakSAT™ is a small LEO satellite that carries an electro-optical payload, Medium-sized Aperture Camera (MAC). MAC is a pushbroom camera with 5 linear detectors (1 panchromatic, 4 multi-spectral). The satellite system is able to take images in near equatorial regions and shall be controlled by the Mission Control Station (MCS). Upon the request from image data users, Image Receiving and Processing Station (IRPS) shall request MCS schedule the satellite for imaging opportunity and inform IRPS the possible date, time for image download. The satellite system shall take images of desired near equatorial regions and download the image data along with attitude and ephemeris telemetry data to IRPS. RazakSAT PLATFORM RazakSAT™ is designed to be compatible with a near-equatorial orbit with altitude ranging from 600 km to 800 km. RazakSAT™ with its stowed solar panels is compatible with most of small satellite launchers as a secondary payload or as a primary payload for small satellite launch such as Falcon 1. The spacecraft bus provides a typical pointing performance of 0.2 deg. The RazakSAT™ weighs less than 200 kg including 50 kg payload mass. The average power consumption is less than 150 Watts. Figure 1 depicts RazakSAT™ in stowed and deployed configuration respectively. The origin of the satellite coordinate system is located at the geometric center of the launch adaptor bottom plane. The +Z-axis is toward the payload optical axis. +X-axis is defined along the scanning direction of MAC, which is normal to the detector lines.  Figure 1: RazakSAT™ in stowed and deployed configuration RazakSAT™ features deck-and-longeron type structure allowing easy assembly and disassembly with about 1.2 meter in diameter and 1.2 meter in height. Most of the component units are positioned on the bottom deck, while reaction wheels and gyros are positioned on the middle deck. The interface with the launch vehicle is made through an adapter (specific to each launch vehicle) bolted to the bottom of the structure. The mechanical interface with the payload Electro-Optical Subsystem (EOS) is provided through three points at the middle deck. RazakSAT™ design calls for five main subsystems which are the Electrical Power Subsystem (EPS), Command and Data Handling Subsystem (C&DH), Attitude Control Subsystem (ACS), Telecommunications Subsystem (TS), and the MAC payload.
Electrical Power Subsystem (EPS) consists of three Solar Panels, three Batteries, three Battery Charge Regulators, three Battery Discharge Regulators, two Power Conditioning Modules, and two Power Distribution Modules. Electrical power is generated by the three deployable solar panels. Triple junction GaAs solar cells are used for solar panels and each panel can generate 110 Watts at EOL. The power is regulated and distributed through electrical power bus and NiCd batteries are used to store electrical power for operation during eclipse. Command and Data Handling Subsystem (C&DH) is comprised of two On-Board Computers (OBCs), one GPS Receiver and two Telemetry and Command Modules (TCMs). C&DH performs most of its tasks through OBC. TCM performs telemetry and commanding function. GPS receiver generates a clock reference and ephemeris data for orbit determination and image processing. Attitude Control Subsystem (ACS) consists of attitude determination sensors and actuators. Attitude control software runs at OBC. RazakSAT™ is three-axis stabilized using four reaction wheels and four gyros. Two Magnetometers (MAGs), three Coarse Sun Sensors (CSSs), two Fine Sun Sensors (FSSs), two Star Sensors (STSs), and two Magnetorquers (MTs) are also be used. ACS meets the operational requirements such as cross track tilting capability for multi-path stereo imaging and frequent revisit. Telecommunications Subsystem (TS) provides communication link with the Mission Control Station (MCS) through S-band up/down links using AFSK/FSK modulation. Transmitters and receivers are fully dual redundant and share two antennas positioned at the bottom and top of the satellite. The S-band link receives commands and transmits satellite state-of-health data. X-band Image Transmission Unit (ITU) transmits high-speed image data to IRPS located in Kuala Lumpur, Malaysia. The MAC payload consists of Telescope and Focal Plane Assembly (FPA), Signal Processing Unit (SPU), Management & Memory Unit (MMU), and Thermal & Power Unit (TPU). MAC also has a direct link to ITU. MEDIUM APERTURE CAMERA SPECIFICATIONS Medium-sized Aperture Camera (MAC) system is a typical pushbroom camera produces high-resolution images in one panchromatic and four multi-spectral bands with ground sample distance of 2.5 m and 5.0 m, respectively. At the nominal altitude of 685 km, MAC has a swath width of 20 km. The MAC system specifications are summarized in Table 2 below:
RazakSATTM ORBIT AND OPERATIONS RazakSAT™ mission is designed to be inserted to Near Equatorial Low Earth Orbit or NEqO in order to acquire a more frequent imaging passes over Malaysia region other than the typical Sun high inclination low earth orbit satellites. The selected inclination which is 9° was chosen to give the optimal image coverage over the highest and lowest latitude of Malaysia. RazakSAT™ Mission Operation will be divided into several phases. The first phase is called Launch and Early Operation (LEOP) where during this period, RazakSAT™ will be launched to its intended orbit and early contact will be established. In addition, during the LEOP phase, RazakSAT™ will be going through several stages of deployment and tests; namely solar panel deployment, bus and payload tests and also in-flight calibration. LEOP is scheduled to last for about 3 to 6 months depending on the in orbit test result and calibration. RazakSAT™ will be declared as ready for its nominal operation once LEOP stage is completed. In nominal operations, RazakSAT™ will go through several operational modes, mostly depending on the schedule basis. The nominal operational RazakSAT™ modes are housekeeping mode, imaging mode and data downloading mode. During housekeeping mode, RazakSAT™ will be in idle condition, preparing itself to the next mode either imaging mode or data downloading mode by executing scenario schedule. Imaging mode is when RazakSAT™ is executing imaging scenario schedule by taking images based on the schedule file. Data downloading mode is when RazakSAT™ execute schedule for downloading image data to IRPS. If RazakSAT™ encountered some problem with its subsystem it will be automatically placed into Safe-Hold Mode. During the Safe-Hold Mode, RazakSAT™ will turn OFF some of its module to conserve its power. If the problem is severe and cannot be rectified which leads to its demise, RazakSAT™ mission will be terminated. If the error is recoverable, RazakSAT™ will exit the Safe-Hold Mode to Housekeeping Mode by MCS command. SUMMARY RazakSAT™ offers Malaysia its own platform for remote sensing capabilities. It also provides valuable lessons in developing, and calibrating the sensor. There will be many more research and applications once RazakSAT™ is launch. Engineers and scientist will benefit from RazakSAT™ opportunity to further developed satellite and remote sensing technologies. This will be the next step for a more advance future satellites and applications. REFERENCES
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