The Command and Control System

The command and control system of the ballon borne SBI is directly derived form the system used for FGE. Actually, several components (like the MAX1, MaX3, and microcontrollers, and the GPS) are the same used for FGE.

There are two main computers on-board: the Command and Control Computer (CCC) and the Digital Acquisition Computer (DAC). Both computers use a commercial ATX mother board with a 1GHz Pentium III.

The CCC runs two separate processes: the Autonomous Control Executive (ACE) process and the Instrument Control (IC) process. The ACE is responsible for properly scheduling the operations performed by the gondola and to carry on the observational program. It can either operate totally autonomously or execute commands received directly from a ground control station via UHF radio link. The IC's main task is to provide a uniform interface for the ACE to a series of instrument subsystems. It also handles all the communications: it collects and transmits the housekeeping data and the I/O with all the instrument controllers.

The DAC controls the SBI detector, and is responsible for handling the stream of images coming from the frame grabber. It can transfer the image data to one of the two 80 GB hard drives (of high shock type), and it can perform simple data manipulations if needed: like averaging, subtracting, multiplying, or dividing frames. It communicates directly to the CCC via an ethernet link. It can handle commands arriving directly from the ACE process and can deliver images to the ACE for example to perform tasks such as autofocus, pointing calibration or for downlinking images to the ground.

The IC process int the CCC interfaces with two instrument controllers, Max1 and Max3, and the UHF radios, via RS232. Max1 is a Motorola 68HC11E2 microprocessor which handles the secondary focus actuator and the offset pointing motors. Max3 is a special-purpose board designed around a Dallas 87C520 (an upgrade to the 87C51 microprocessor). It collects a large fraction of the housekeeping data for the gondola, including temperatures, pressures, currents, and voltages. It also supplies the control voltages for the servo amplifiers that drive the three torque motors (elevation, reaction wheel, and momentum dump), and the discretes that switch such items as the stow latch. The most critical function of Max3 is the pointing and control system. In this, Max3 combines input data to determine an "error," and from it and the current state of the payload, produces an output for either the elevation or reaction wheel drives to compensate.

The CCC, the DAC, the hard drives and other microprocessors are all commercial electronic products, thus not specifically designed to operate in a vacuum environment. They are all housed inside three pressurized vessels that maintain a stabilized pressure of 1 atm.