DUTHSat

Components & Subsytems

Owing to its multi-national nature, a key aspect of the CubeSat initiative is that all critical components of the CubeSat are available as Commercial-off-the-shelf (COTS) components without export restrictions, contrary to most aerospacequalified components. This significantly reduces costs, while the standardization of all subsystems has lead to greatly minimized integration efforts. However some COTS components that are commonly used by CubeSats in space have limited capabilities, such as low bit-resolution, high power consumption, and also are not radiation hardened: Space systems operate in conditions that involve plasmas and high-energy electrons, protons and heavier ions that are hazardous to the electronics of common technological systems, which are vulnerable to Single-Event Upsets (SEU), Single- Event Latchups (SEL) and Total Ionizing Doze (TID).
The Space Research Laboratory of the Democritus University of Thrace (DUTH/SRL), with a longterm experience in the development of space ASICs components that are miniaturized, ultra-low-power, high-resolution and also radiation hardened, is demonstrating through DUTHSat solutions that can be used in any future CubeSatbased missions in more harsh environments than QB50, enhancing their capabilities and reliability, and also expanding the region in space where they can safely operate.

  • On-board Computer (OBC)
    The On Board Computer (OBC), the “brains” of DUTHSat, is responsible for all functions of the spacecraft, including deploying the antennas and mNLP booms, spacecraft telemetry data collection, attitude determination and control execution, constructing or deconstructing a file in order to upload or download it to the Ground Station, controlling the power in every subsystem, receiving and executing commands from the Ground Station, automated failure recovery, high-level system and payload control, etc.
    The OBC also monitors spacecraft temperature and housekeeping parameters, and plays a supervisor role for the power subsystem by interfacing with battery monitors and recording voltage and current levels of the batteries, automatically switching off non-critical subsystems in case of low levels of power. The OBC is interfaced with the Communications subsystem through which it receives ground station commands and transmits data and satellite status information.
    DUTHSat uses a CubeComputer OBC designed by Stellenbosch University, which is based on a high performance, low power 32-bit ARM Cortex- M3 based processor. Other features of the OBC include: PC104 bus connector, Flash Memory Data Storage, MicroSD card support, Power monitor/power-on reset, CAN bus interface, UART interface, SPI interface, GPIO pins, ADC interface and I2C interface. Single Event Upset protection is implemented by means of an FPGA based flow-through EDAC, and Single Event Latchup protection is implemented by detecting and isolating latchup currents.
    For robustness, no operating system is used in the DUTHSat design. This decision creates several difficulties in the design process, as all low level drivers (e.g. CAN,UART,SPI, GPIO, ADC,I2C) are created from scratch. Also, if an operating system is not used, techniques such as threading and schedulers that are used by default in every operating system cannot be used in the satellite. The advantages are that the code can be much more time and energy efficient, and that system engineering has much larger flexibility in the design of satellite operations.
  • Electrical Power Subsystem (EPS)
    The Electric Power Subsystem (EPS) includes high-efficiency solar panels placed at the sides of the CubeSat, a Power Distribution Module and Battery. Power is expected to be 4.6W at ambient temperature with a supply voltage of 3V3 and 5V. A Li-Ion battery of 2600mAh has been selected after extensive orbital and subsystem simulations.
    The power is supplied to all subsystems through the NanoPower P31U power supply designed by GOMSpace. The power capabilities of this power supply are for missions with power demands of up to 30W. The power coming from the solar panels and/or from the battery is used to feed the output power buses of 3.3V@5A and 5V@4A. Each of these buses has three individual output switches with over-current shutdown and latch-up protection. Finally, a heater is automatically switched on to protect the batteries from very low temperature and increase battery life.
  • Communications Subsystems
    The Space Segment of the Communications Subsystem is formed from the antennas and the TRXVU VHF/UHF Transceiver, which enables the system to have full duplex capabilities with telemetry, telecommand and beacon capabilities on a single board. The peak power consumption of the transceiver is < 1.7W while it only uses < 0.2W on receive only mode with an average transmit power of 22dBm. The transmitter frequency is controlled from the installed crystal and ranges between 400-450 MHz while the receiving frequency ranges between 130-160 MHz. DUTHSat’s communication frequencies are 436.420 for downlink and 145.810 for uplink. The modulation scheme for downlink is RRC-BPSK (Root-Raised Cosine Binary Phase-Shift Keying).
    For uplink it will use the AFSK scheme on FM with 1200 b/s bit rate. The supported data rates from TRXVU are 1200 to 9600 bits per second with the protocol AX.25 for the communication channel. The antennas have a crossed UHF/VHF dipole configuration and deploy from inside an enclosure through a command from the OBC that is issued upon ejection of the satellite from its P-POD.
  • Attitude Determination and Control Subsystem (ADCS) and GPS
    An Attitude Determination and Control Subsystem is used by DUTHSat, in order to achieve the following: a) Alignment of the long axis of the satellite with the velocity vector so that the experiment package is pointed in the direction of motion (velocity-vector stabilization). b) Control of the attitude so that there is less than 5o between the long axis of the satellite and the velocity vector, down to 250 km altitude (velocity- vector pointing accuracy). c) Measurement of the satellite velocity-vector attitude to within ±1o at the time of receiving data from the payload (velocityvector attitude knowledge). d) De-tumbling and stabilization during commissioning phase within 2 days. e) Recovery from tip-off rates of up to 100ο/sec (tipoff rate recovery).
    An ADCS that is tailored to the above needs has been developed at the University of Surrey together with Stellenbosch University. This ADCS is modular and in its full configuration it can achieve 3-axis stabilized attitude control, accurate position, velocity & time from a GPS, < 1° roll, pitch, yaw stability. The sensors that are used in the various modes of operation of the ADCS include a Y-axis aligned rate sensor, Magnetometer, Coarse Sun Sensors, Sun Sensor, Nadir Sensor.
    This configuration has relatively low power consumption (2W for 3-axis mode), compact size (0.4U), and low cost. DUTHSat will also feature a GPS receiver for timing and position determination purposes. The Novatel OEM615 GPS receiver module with special Space Firmware is used. The GPS receiver will also interface to the ADCS to assist the attitude determination process as well as in obtaining a timing stamp for the CubeSat.
  • Ground Station at DUTH
    The Ground Station of DUTHSat is located at the laboratories of the Department of Electrical and Computer Engineering of the Democritus University of Thrace, at address: Vasilisis Sofias 1, 67100, Xanthi, at a Latitude of 41° 08’32.81” and a Longitude of 24° 53’24.83”. See figures at the bottom of the page.
  • Satellite bus, Structure and Thermal Subsystem
    For the QB50 project a 2U CubeSat structure is baselined, which has an aluminum chassis with an outside envelope of 100 × 100 × 227.0 mm. The QB50 Science Unit will be accommodated at one end of the CubeSat structure, in the spacecraft ram velocity direction. The thermal control subsystem is particularly important in the QB50 mission: During the gradual re-entry large temperatures will develop, and eventually the CubeSat will ablate due to friction with the upper atmosphere.
    The operational range of the primary Science Unit is –20o to +40o, and, in order to extend measurements at as low altitudes as possible within the largely unknown lower layers of the Thermosphere, thermally conducting plates will be used in the design to serve as a passive heat dissipation and thermal control system: this will be done by attaching struts, plates and heat conducting wires, which in turn will be attached to a heat dissipation plate, at the lower end of the CubeSat (anti-ram direction).
  • Primary Payload: multi-Needle Langmuir Probe, Thermistors, Magnetometer
    The main payload of DUTHSat is a multi-needle Langmuir probe system. Langmuir probes have been widely used to determine plasma electron density and temperature in space. The Langmuir probe works by placing an exposed conductor in a plasma, biasing it relative to a reference potential and measuring the collected current.
    A swept bias probe sweeps the bias voltage from a negative to a positive value, and the collected probe characteristic makes it possible to determine electron density ne, electron temperature Te and the spacecraft potential. In addition, Thermistors will be monitoring the Temperature at various locations on the spacecraft and a magnetometer will provide magnetic field measurements along the orbit.
  • Secondary Payload: DUTH ASICs Board and ULPDAQ chip
    In addition to the mNLP Science Unit, DUTHSat will house an Ultra-Low Power Data Acquisition Unit, ULP-DAU, which is designed as a prototype smartsensor data acquisition system-on-achip for use particularly in micro/nano/ pico-satellite missions, but with features that will also be attractive to larger missions.
    This prototype will be flown in the DUTH/SRL developed nano-satellites of the CubeSat standard so as to demonstrate that the boards used currently by CubeSats can be replaced with ASICs,drastically reducing the size, weight and power requirements of the CubeSat avionics and I/O units. Thus, this mission also provides an opportunity to flight-test ASICs microchips developed at DUTH, which would accelerate their acceptance as standard and flight-proof components for other larger space missions; it is expected that the DUTH ASICs microchips will prove to be an invaluable asset in micro/nano/pico-satellite missions, where space, power and reliability are critical.