Abstract
Montana State University's Space Science and Engineering Laboratory (SSEL) under support from the Montana NASA Space Grant Consortium is engaged in an earth orbiting satellite student project that will carry a reproduction, using current-day technology, of the scientific payload flown on Explorer-1 in 1958 into a 650 km sunsynchronous polar orbit. On-board operations will be commanded by a Motorola MC68HC812A4 (HC12) microcontroller, chosen for its ease of use, processing power, and intrinsic features. Accompanying this will be an Integrated Device Technology CMOS Supersync First-in, First-Out (FIFO) IDT72291 150 Kbyte RAM chip, used for storing scientific data and system telemetry before downlink to ground station. The RAM was selected for the simplicity of the FIFO data flow. The HC12 is responsible for controlling antenna deployment, communication handling, and other system parameters. Using in-house Assembly code, the system has been designed to run an abbreviated main loop, using hardware and software interrupts to call subroutines, thereby allowing the mission critical tasks to be on the main loop for nearly constant monitoring. System interrupts allow the HC12 to seamlessly collect payload data, attitude data, battery conditions (voltage, current, charge state, and temperature), bus voltage, bus current, processor temperature, and stability of the payload high voltage power supply. This interrupt method allows new payload routines to be added with little code modification, maximizing satellite modularity for future experiments aboard Montana State University Cubesats. The design has the advantage of utilizing a powerful microcontroller therein eliminating the need for many of the external components needed by other systems (analog-to-digital converters, serial communications interfaces, slave microcontrollers, etc.), while remaining simple to program and implement. This paper describes MEROPE's computer subsystems in detail.
Abstract
Montana State University's Space Science and Engineering Laboratory (SSEL) under support from the Montana NASA Space Grant Consortium is engaged in an earth orbiting satellite student project that will carry a reproduction, using current-day technology, of the scientific payload flown on Explorer-1 in 1958 into a 650 km sunsynchronous polar orbit. The off-the-shelf emphasis of the MEROPE component selections has required the power system to adapt to widely different electrical needs across subsystems. The size limitations of the Cubesat-class specifications confine body-mounted solar arrays to approximately 64 cm^2 per side, restricting overall power production and necessitating the use of an extremely efficient power bus. MEROPE will employ dual-junction GaAs solar cells (19% efficiency) to produce the 5W necessary for satellite operation and battery maintenance. Included in the system will be two Li-ion battery cells chosen for their high energy density, rapid charge characteristics, low mass, and lack of memory effects. The power system is responsible for providing a highly regulated 5 V bus to the microcontroller subsystem and a 6 V bus to the communication subsystem. In addition, the Geiger tube scientific payload aboard MEROPE requires a stable 500 V high voltage power supply (HVPS) to operate the experiment. This will be accomplished using a prototype HVPS requiring +/- 5 V busses. This paper describes the challenges and solutions involved with powering a successful scientific mission within the boundaries of the CubeSat-class design specifications.
Abstract
The MEROPE communications subsystem consists almost entirely of commercial off the shelf components. Simple and robust, it is centered around a Paccomm Picopacket terminal node controller (TNC) operating continuously in "transparent" mode, whereby all serial data from the processor are immediately packetized and transmitted through a Yaesu VX-1R dual-band radio. The entire subsystem weighs less than 140 grams and occupies a total volume (including antennas and interconnects) of 180cm3--less than 1/5 of the total spacecraft weight and volume budgets with a hardware cost of less than $400. MEROPE communications uses the AX.25 packet radio protocol at 1200baud. Uplink is at a frequency of 145.835 MHz with 20 kHz of available bandwidth. Downlink is at 437.445 MHz with a 30 kHz bandwidth. Communications flow is controlled by the Motorola HC12 flight processor, which is linked through a 9600 baud RS232 serial connection to the TNC. The entire communications link (ground-MEROPE-ground) is seamless, initialized by a single encrypted uplink command. Upon contact with MEROPE, the ground station instructs the processor to dump the contents of its memory into the TNC, which packetizes the binary data and keys the transmitter. The TNC consists of a single shielded printed circuit board (PCB) measuring 8.45 cm long by 6.17 cm wide, weighing 57 grams. It is powered at 7-14Vdc and draws between 50mA and 70mA during continuous operation. The transceiver consists of the "guts" of a Yaesu VX-1R, arguably one of the smallest and lightest handhelds on the market. The radio is one double-sided PCB measuring 8.32cm long and 4.20cm wide, weighing only 47 grams. The shielded RF module stands 1.20cm high and occupies one half of the PCB. Power is supplied from a 5V bus for 1W of RF output. Current consumption for the receiver and transmitter is 150mA and 400mA, respectively. Transmission duration for the expected 100 Kbytes/pass of telemetry and payload data at 1200 baud is about 11 minutes, comparable to the above-the-horizon window. The MEROPE antenna is a center-fed dipole tuned to the 2m uplink, which is nearly harmonic with the 70cm downlink. The antenna consists of two 48cm-long nickel-titanium (?Nitinol?) tape measure to avoid binding to the attitude control magnets. Each element is uncoiled on orbit from opposing sides of the spacecraft. The choice of a single dipole is fourfold: (1) it allows for enhanced directional gain over an omni antenna since a dipole does not require a ground plane and is not shadowed by the spacecraft; (2) the use of one antenna for both up-/downlink frequencies avoids the addition of duplexer hardware; (3) it is relatively simple to tune to both uplink and downlink frequencies; (4) it reduces the number of potential points of failure, and will radiate if only one element deploys. The MEROPE project ground station will consist of a COTS amateur base station radio and antenna assembly.