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A Cubesat Communication Design For In-space Assembly

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By Author: ssla.co.uk
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Satellite architecture overview
The Data Handling Subsystem is considered as an interface between the control station and the other satellite subsystems. The Data Handling Subsystem modules include communication subsystem (antennas, modulator, demodulator) and on-board computer which controls the satellite subsystems. But how these subsystems are attached to the on-board computer, two principal architectures are distinguished.


Star Architecture: The On-Board computer (OBC) must run separate data lines to each subsystem. In this architecture, the OBC should have many peripheral interfaces equal to the number of satellite subsystems on board and perhaps there will be a problem of wiring. bus Architecture in this case, all satellite subsystems are connected to a bus which is like Local Area Network (LAN) aboard the satellite. In general, we used a Master slave protocol. The three simple Master-slave bus commonly used in the embedded system are: CAN (Controller Area Network), SPI (Serial Peripheral Interface) and I²C (Inter Integrated Circuit).

CubeSat subsystems requirements
The requirements of CubeSat-class spacecrafts are significantly different from their ancestors’ conventional satellites, and new design techniques are needed to meet these evolving requirements. The unique requirements of the Telecommunication CubeSat satellite demonstrate this evolution. The design needs to be relatively inexpensive while at the same time computationally robust. It should consume limited power and must support the space environment by reducing susceptibility to radiation and thermal effects. In a previous text, we supposed that all the satellite subsystems are intelligent. i.e.; each satellite subsystem has its own microcontroller.
CubeSat satellite platform subsystems design
Space environment
An important issue to consider, which affects all electronic devices in space environment, is radiation. It can lead to various types of problems. These problems range from operational malfunctions to physical damage of the devices. CMOS technology is preferred for space applications because of its high noise margins and low static power requirements. Scaling and integration are other advances CMOS technology has over other semiconductor technologies. On the other hand, CMOS is susceptive to two types of space radiation effects caused by electrons and protons trapped by the terrestrial magnetic field: Total Ionizing Dose (TID) and Single Event Effects (SEE). TID effects are the result of accumulated exposure to ionizing radiation. SEE are the result of a single high-energy particle that strikes the device.

Data handling subsystem design
In the satellite modular architecture (Wertz & Larson, 1999), each single subsystem has a dedicated hardware and software. The Cubesat has important constraints on cost, power and mass, and especially on size. The approach that has been taken in this research consists of the integration of the maximum subsystems within the same unit considering that single subsystems can be setup without modifying the operation of the remaining subsystems. As said before, the Data Handling Subsystem will integrate the Telecommunication Payload, the Telemetry/Telecommand functions as well as the active part of the thermal control subsystem. In this section, we will describe in detail the main considerations and solutions chosen during the design of the Data Handling Subsystem based on the fixed-point DSP processor. The design was split in two parts: The Hardware and Software Architectures.
Data handling subsystem hardware architecture
The Data Handling Subsystem hardware architecture is composed of three main parts: an on-board computer, Sensors and Control signals, and a VHF transceiver with associated antenna.

On board computer
The on-Board Computer board as illustrated is a hardware board in which is implemented the flight software. The flight software controls the whole operations of the satellite and is built around a 16 bits DSP TMS320C5416 from Texas Instrument (Texas Instrument, 2002). The hardware board includes, moreover, one analogue interface connected to the Radio Frequency module (Transceiver and antenna), two analogue to digital converters for the acquisition of the Telemetry housekeeping data, a EEPROM memory containing the flight software, a JTAG port for the flight software programming and finally the control signals, by using some DSP Input/output signals, in particular, to activate or deactivate the heater. illustrates the hardware architecture of the on-board computer. All the components on the on-board computer logic board are chosen with SMD (Surface Mounted Device) packages for space saving and high density mounting in order to minimize the weight and dimensions of the logic board.

Transceiver
A COTS amateur radio transceiver was adopted to be the main flight radio due to power, weight, and time constraints (Horan, 2002; Lu, 1996). The transceiver, integrated inside the payload, operates in amateur VHF band and consists of the “guts” of a low cost Yaesu VX1R (Hunyadi et al., 2002), arguably one of the smallest and lightest handhelds on the market. The radio is two stackable double sided PCBs measuring approximately 5 × 5 cm2. The small size of CubeSats limits the amount of energy provided by solar arrays; therefore power availability is a constraint on both the spacecraft processor and the communications systems. Power is supplied from 5 V bus to radiate only 1 Watt RF power which achieves a positive budget link (see Table 1). Current consumption for the receiver and transmitter is 150 mA and 400 mA, respectively. Only slight modifications will be required to make this component space worthy. We use only the RF parts of the VX-1R. The transceiver is interfaced with the DSP processor by means of AF (Audio Frequency) and PTT (Push To Talk) command signals. AF signals consist of transmit and receive signals of the AFSK and GMSK modems which carry data packet, whereas PTT command signal allows the DSP to choose the transmitting and receiving frequencies of the radio. The transmitting and receiving frequencies will be fixed on the 145.825 satellite frequency. The FM signal output from the transceiver is fed to an omnidirectional antenna.

Conclusion
As the satellite community transitions towards inexpensive distributed small satellites, new methodologies need to be employed to replace traditional design techniques. The ongoing research will contribute to the development of these cost saving methodologies. The goal of the integration of all the intelligences of the various satellite subsystems in only one intelligent subsystem is to minimize component expenditures while still providing the reliability necessary for mission success. Associating low cost ground terminals with a low-cost Telecommunication CubeSat-class satellite will allow universities to access space communications with a very economical system. The present work, dealing with the design of the Low-cost Telecommunication CubeSat-class spacecraft, shows hardware and software solutions adopted to cut down the system cost. The hardware utilizes commercial low-cost components and the software is optimized using assembler language. The on-Board Computer unit is small device that can be mounted on any small satellite platform to serve telecommunications applications such as mobile localization and data collection. By using a single CubeSat satellite and low-cost communications equipment, Telecommunications systems can be kept at the extreme low end of the satellite communications cost spectrum.

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