Command and Data Handling (C&DH)
The mission of the Command and Data Handling System (C&DH) is to receive and execute commands; collect, store, and transmit house-keeping data; and support the onboard payloads. The flight computer is designed to control the satellite and manage telemetry and experiment data for a minimum of two years.
The C&DH system consists of a custom-modified MIP405-3X single board computer which included (i) 133 MHz PowerPC processor; (ii) 128 MB ECC; (iii) 4 RS-232 asynchronous serial ports; (iv) 1 Ethernet Port; (v) a PC/104 bus; (vi) a PC/104+ bus; and, (vi) a 202-D384-X Disc on Chip providing 384 MB of secondary storage. The computer board is supported by an ESCC-104 Synchronous Serial Card with 2 synchronous serial ports, and an EMM-8M-XT Serial Expansion Card with 8 RS-232/422/485 asynchronous serial ports and 8 digital I/O channels. A modified I0485 data acquisition board provides 22 analog telemetry channels and 32 digital I/O channels.
The decision to use the PowerPC based MIP405 over an x86 based board was based solely on the low power consumption of the board combined with the feature set. The choice was limited to x86, PowerPC, and ARM processor architectures because of a program decision to use the Linux operating system. The MIP405 integrates Ethernet, serial ports, and Disk-on-Chip interface on a single board while providing 128 MB of ECC memory and a powerful processor for under 2 watts. The closest x86 based system with comparable features found consumed 5 watts of power.
The M-Systems Disk-on-Chip was chosen because it was the de facto standard flash memory harddisk replacement. Flash memory was chosen over a traditional hard disk to increase reliability and reduce power. The 384 MB version was chosen to provide the storage required for the operating system and still maintain adequate margin.
The Diamond Systems Emerald-MM-8 was chosen for the asynchronous serial board based on its innate flexibility with any of the 8 ports capable of being configured as RS-232, RS-422, RS-485.
RMV's IO485 data acquisition and control board was chosen for the distributed telemetry system because of built-in support for daisy chaining and handling a large number of boards. The integrated expandability is fundamental to addressing future telemetry issues in later versions of the MidSTAR line.
The C&DH uses the Linux operating system with a 2.4 series kernel. To create an open software architecture the IP protocol stack was chosen to provide inter process, intra-satellite, and satellite-ground communications. This allowed programs created at different facilities on different hardware to be integrated with minimum difficulty.
All internal and external communications use internet protocols. TCP is used for all internal satellite communications; UDP or MDP is used on the uplink and downlink.
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