MOST: Architecture

December 10, 2013

The housekeeping computer, which is central to the design, is an off-the-shelf product that has been modified to meet MOST requirements. Based on a V53 processor, the computer’s crystal frequency has been increased from 9 MHz to 29 MHz to accommodate the processing demands of the mission. It interfaces with the rest of the satellite through a custom interface card that provides power, serial and digital I/O connections. The housekeeping computer’s main tasks include receiving, executing, and distributing commands and/or files uploaded from the ground, and collecting and transmitting engineering and science data to the ground.

In the figure, roughly from the V53 to the right, the satellite design is typical of AMSAT based designs. It consists of the main housekeeping computer (V53), radio transmitters and receivers including support electronics, and the power system for the satellite.

MOST_arch

MOST employs two 0.5W RF output BPSK transmitters and two 2W FM receivers. All radios operate at S-band frequencies. Sufficient downlink margin is maintained by using a 0.5 rate convolutional code, implemented on a custom board. On the uplink, FM receivers provide a simple, robust, and low-cost means to talk to the satellite. Both receivers and transmitters connect to custom telemetry and command nodes that serve as modems and telemetry collection devices. To maintain omni-directional coverage, one receiver/transmitter pair is located on either side of the satellite, connected to quadrifilar antennas. With each radio operating at its own frequency, the appropriate transmitter is selected based on which receiver is being used.

The power subsystem is based on a centralized switching, decentralized regulation topology. Power regulation occurs through switching power supplies to maximize conversion efficiency (power is very limited in a satellite of this size – 35W in fine pointing operations and only 9W in safe-hold or tumbling operations). While this poses EMC/EMI challenges for the Science DSP computer that must read its CCD Array with almost zero noise, these challenges have been met.

The power system switches are controlled via the housekeeping computer. Two levels of load shed protect the satellite from unrecoverable battery drainage, allowing contingency operations to resume in safe-hold mode. All power lines have overcurrent protection.

In terms of energy storage, a NiCd battery provides power during eclipses and supports peak power draws from equipment such as the transmitters. High-efficiency silicon solar cells on all sides of the satellite generate energy to recharge the battery and provide power for fine pointing and safe-hold operations. Peak power tracking hardware and software (run by the housekeeping computer) maximize the available power to the satellite subsystems.

To the left of the V53 computer is the equipment that makes the MOST satellite unique for a microsatellite in the scientific contribution that it can make. These are the electronics to support the telescope, and the ACS hardware and electronics. The ACS equipment consists of magnetometers, sun sensors, and a star tracker for sensing, and magnetorquers and reaction wheels for actuation. The key developments here have been the use of reaction wheels for three-axis attitude control, and the development of a star tracker that is a fundamental part of the science telescope. Combined these enable the satellite bus to maintain pointing accuracy of less than 25 arcseconds.

MOST_boardsScience and star tracker images are taken on dual 1024×1024 CCD arrays that share the focal plane of the telescope. Each CCD is connected to a pre-amplifier, and to analog and digital electronics boards. These boards are based around a Motorola 56303 DSP, and provide digital control and Analog to Digital conversion of the signals from the CCDs. The instrument computers are designed to provide nearly noiseless CCD readings while tolerating disturbances from switching power supplies.

There are four attitude control modes for the satellite:

Safe-Hold: The satellite is essentially power positive in all practical orientations. Therefore, this is an uncontrolled state in which there is no active attitude control. In this mode, the focus is nominally on commissioning or recovery operations.

Detumbling: This mode involves using the magnetometers and magnetorquers to implement B-dot control to slow the tumble rate of the satellite so that coarse pointing control can be executed. Normally this is used after kick-off from the launch vehicle.

Coarse Pointing: After the satellite is detumbled, the ACS uses sun sensors and magnetometers to determine the spacecraft attitude, while using reaction wheels to control the attitude to orient the main solar array towards the Sun and to roughly point in the direction of science interest. The magnetorquers are used to desaturate the reaction wheels.

Fine Pointing: The ACS uses the star tracker to determine spacecraft attitude to an accuracy of three arcseconds. The reaction wheels are used to control the attitude. The magnetorquers are used to desaturate the reaction wheels.

The attitude control computers (ACS nodes) are also based on the Motorola 56303 DSP. The DSP acts as the fundamental processing unit that runs the ACS software. The computers provide analog control of the magnetorquers, power and analog to digital conversion of the magnetometer and sun sensor signals, as well as RS-485 connections to the main housekeeping computer, the reaction wheels (which contain their own microcontroller), the star tracker, and the science DSP boards. Nominally, only one ACS node is operational. The second is designed as a cold spare to add redundancy where it was practical.

All computers have Error Detection and Correction (EDAC) hardware and software to correct for bit errors induced by radiation. Single event latch-ups are corrected by power cycling the affected device.

To ensure that components within the satellite operate at suitable temperatures, a combination of passive surface treatments are used including aluminum, gold, and silver teflon tapes. In the event that the satellite enters a cold state due to a disadvantageous attitude relative to the Sun, resistive heaters are used to keep the battery and trays sufficiently warm. During fine pointing operations, a passive radiator cools the telescope focal plane so as to minimize thermal noise in the CCD readout.

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