INDEX satellite system
The INDEX satellite adopts the three-axis stabilization system, which is suitable for taking images, because the science mission requires fine-structure images of the aurora. Its enveloping area is 60 x 60 x 70cm and its weight is 60kg. Within its limited space, a variety of instruments are installed: sensors such as Star Tracker (STT), Spin/Non-Spin Sun Aspect Sensor (SSAS/NSAS), Geomagnetic Aspect Sensor (GAS) and three-axis Fiber Optic Gyro (FOG); actuators of Reaction Wheel (RW) and Magnetic Torquer (MTQ). Considering the mission requirements and available onboard space, only one axis of the RW is installed: three-axis attitude control will be performed via bias momentum. The target of attitude-control accuracy is 0.5deg. and attitude-determination accuracy is 0.05deg.
In the initial attitude-acquisition mode just after release from the launcher, spin- angular momentum must be given to the satellite within 10 hours from release. The solar array paddle must also be pointed toward the Sun. The result of initial attitude-acquisition consisted of liberation-damping control, to constrain separation disturbance; spin-up control, to obtain spin-angular momentum; and sun-acquisition control, to capture the direction of the sun, will be confirmed by first visible operation about half a day later. This is the first highlight in INDEX satellite operation just after launch.
In the INDEX bus system, the new integrated control system uses a single computer to execute almost all the tasks relating to satellite control, including computation for attitude control. In an ordinary scientific satellite, computers are used exclusively, for example, for guidance control and data handling. INDEX’s Integrated Controller Unit (ICU) executes all computations, including attitude control, command handling, telemetry generation, acquisition of house-keeping data, power control, and observation-data compression. A commercial-type RISC processor (SH-3), which is widely used in game machines, is used for the CPU, and its radiation hardness is enhanced by majority logic of three redundancies.
S-band is used for both up- and down-link communication, adopting no-coherent communication system with no-ranging function. To determine the INDEX orbit, we will use “Two Line Element” data, which are published on the web of the North American Aerospace Defense Command (NORAD), and data of the onboard commercial car-application GPS. With the recent advances in hand-held communication equipment, digital signal-processing technologies and new electronic components are put to practical use. In order to incorporate such new technologies into satellite onboard communications, we developed the S-band Receiver (SRX) and S-band Transmitter (STX). For the ground station, the wind radar-driving system and 3m-parabola antenna’s primary mirror were moved from the Uchinoura Space Center (USC) and installed on the roof of the Research Building at Sagamihara Campus (Fig. 1). Using this antenna, we can carry out the usual satellite operations (by PC base!) in the INDEX operation room being built at Sagamihara Campus.
The satellite has still other unique systems. The power system, for example, features thin-film reflection panel concentrating sunlight to the solar paddle (generating about 1.2 times more electric power) and manganic-acid lithium ion batteries. The thermal control system uses variable emittance device (thermal louver without movable part) made of materials whose thermal emittance becomes smaller as the temperature lowers. Its performance will be evaluated in orbit.
In the design of a highly functional small satellite, the first hurdle is how to stow the required instruments in a limited space. In the INDEX program, by using professional-use 3D-CAD for instrument allocation, the efficient design was conducted including instrument interference checks, mass-property adjustment, and field-of-view securing for optical sensors. In a small satellite, the weight of mounted cables also has a large effect on mass-property. Taking this into account, we adjusted mass property.