³Cat-1

On-Board Computer and Flight Software

The ³Cat-1 has a commercial single-board computer as its main On-Board Computer (OBC). With an ATM91SAM9G20 CPU (ARM9 core) running at 400 MHz with 64 MB of SDRAM, the OBC interfaces with the rest of the satellite subsystems and payloads through UART and SPI digital buses. The OBDH software architecture handles the communication protocol with each device (microcontrollers, transceivers and the like) and generates the data that they may require (e.g. telemetry packets, configuration data for payloads, etc.)

The flight software is deployed as a set of processes, libraries and drivers and is run on top of a dual-kernel Operating System: a mainline Linux kernel patched with the Xenomai real-time hypervisor to provode soft-real-time capabilities to the OS. The flight software implements most of the spacecraft functionality, including energy management, system state control, on-board task scheduling and execution of scientific experiments, the communication protocol and sensor monitoring and housekeeping activities.

Electric Power Subsystem

The EPS, designed and developed at the NanoSat Lab, is the spacecraft's main bootstrap. It autonomously deploys the antenna, and manages the satellite's energy reservoir. With a total energy storage of 1150 mAh, the Lithium-Ion batteries adaptively supply power to the rest of subsystems when the satellite is in the penumbra/umbra region of its orbit (i.e. eclipsed by the Earth). This subsystem is controlled by a centralized microcontroller, which controls localized DC/DC converters. These decentralized power converters are located in each point of load (i.e. each floor has several of them) and implement overcurrent protection to isolate subsystem faults potentially caused by SEL.

The maximum power peak of the ³Cat-1's power bus is of 17 W, whereas Point-Of-Load converters can withstand from 3.3 to 10 watts of output power. The spacecraft's solar panels have been designed with silicon cells that have been laser-cut to maximize the total surface. While silicon cells provide 18 % of efficiency, the power conversion efficiency is roughly of 87.5 %, yielding to a maximum input power of 1.2 W. When the spacecraft is not in a critical energy state, the EPS relays the power management to the OBC, interfaced through a standard serial bus (UART).  

Communication Subsystem

The communication subsystem of the ³Cat-1 comprises an amateur-band transceiver, operating at the UHF band (430-440 MHz). The transceiver is based on a COTS module from Texas Instruments, broadly used for wireless sensor networks: the CC1101. Mounted in a single board with an RF power amplifier that boosts  the signal up to 2 W, this module implements both the satellite uplink and downlink channels.

The antenna system for the UHF band is composed of a cross-dipole antenna which is deployed at the system start-up. The dipole has an omni-directional pattern, allowing the satellite to be commanded even with the attitude control disengaged, as well as circular polarization, in order to pass through the ionosphere without polarization disturbances.

Attitude Determination and Control Subsystem

Given the on-board payloads characteristics, accurate pointing is not required. Because of that, the ADCS is grounded on passive control (permanent magnets and mu-metal strips) to perform de-tumbling and attitude stabilization. Additionally, the ³Cat-1 has an active control system implemented with a single-axis magnetorquer which may be enabled when the spacecraft's attitude is stable. The attitude determination has been designed with a commercial 6-DoF IMU encompassing a triple-axis magnetometer and gyroscope.

Payloads

• Eternal self-powered beacon demonstrator: this beacon, unlike the rest of the payloads and subsystems, is totally autonomous and it is not connected to the satellite's power source or On-Computer. Thanks to the temperature gradient between the exterior side and the interior of the satellite a small Peltier cell is able to generate a small current that is acumulated ultil there is enough power to turn on a low power microcontroller and a RF stage that sends a UHF signal towards the Earth. The information contained in this signal is the satellite's identifier and temperature and can be easily received from Earth following this instructions.
• CellSat Solar Cells: one of the photovoltaic (PV) panels of the ³Cat-1 has experimental solar cells that have been developed at UPC by the Micro- and Nano-Technologies group from the Electronics Engineering Dept. The so-called "CellSat" are designed with interdigitated back contact (IBC) techniques, yielding up to 12% of energy conversion efficiency. The main purpose of this payload is to assess their degradation and performance in space. In order to do so, the satellite measures their output power and correlates that measurement with the actual irradiance and temperature of that cell group. Electrical and manufacturing details can be read in a published paper.
• MEMS-based monoatomic oxygen detector: this payload, featuring a unique MEMS devide manufatured at UPC BarcelonaTECH, is able to detect the presence of monoathomic oxygen by analysing the resonant frequency of a MEMS device which has been covered by a sensible polymer. Chemical and electrical details can be read in a published paper.
• Graphene Transistor in-space characterization: a Graphene Field Effect Transistor (GFET) is a special type of transistor where its channel is implemented with a graphene layer. These new type of devices are still in development and their performance in space conditions has yet to be characterized. This is the main goal of this payload, an experimental set-up where GFET performance and degradation is measured at several phases of the mission in order to contribute to the study of these novel devices.
• Study of plasma effects in Wireless Power Transfer links: wireless power transfers (WPT) are a key enabling technology for next generation spacecraft, allowing satellites to exchange energy and enabling innovative architectures like fractionated spacecraft or Federated Satellite Systems. Resonant Inductive Coupling (RIC) is a variant of WPT in which two magnetically coupled coils are part of a resonant circuit operating at the same frequency. The WPT experiment in the ³Cat-1 is performed with two deployable coils of a few centimeters of diameter, which exchange energy at a distance of roughly 4 cm. The receiving circuitry encompasses a power meter which will help to understand how these links are affected by plasma.
• Low-resolution CMOS camera: interfaced with the OBC through a serial bus, this payload integrates a camera capable of capturing pictures in the visible spectrum at VGA resolutions (640 x 480).
• Geiger counter: with a small neon tube of 15 mm of diameter, the ³Cat-1 is capable of counting ionized particles and perform radiation dosimetry. The radiation measured by this payload will change for each point in the orbit trajectory, allowing to have a baseline reference to correlate with the rest of the satellite's measurements. A hardware counter in the OBC's CPU is used to record the number of ionized particles detected by the device.