About the AACS

The AACS uses a single Applied Technologies Advanced Computer (ATAC) which is a 16 bit computer made up from four 4-bit bit-slice computers. The computers memory consists of 2 kbytes of ROM and 64 kbytes of RAM which is divided into two 32 kbytes halves which contain identical code and either of which can be mapped into either the low 32 kbyte or high 32 kbyte portion of the ATAC's 64 kbyte address space. The portion mapped into the lower 32 kbyte address space is considered "on-line" and contains the code which is actually performing the AACS operations. The portion which is mapped into the high 32 kbyte address space is considered "off-line" and is not executing any code in the current design (but that will be changing as is discussed later).

The AACS was programmed in a combination of assembly language (a custom assembly language for this computer that was modified especially for Galileo) or HALS, the same language used to program the U.S. Space Shuttle. HALS has strong matrix manipulation capability which was desired for the kinds of computation that must be done to support attitude and articulation control of a dual spinning spacecraft such as Galileo. The HALS language was adapted for the ATAC computer by a company in the U.S. called Intermetrics.

The real time operating system for the AACS was custom designed here at the Jet Propulsion Laboratory. It is called the Galileo Real-time AaCs Operating System (GRACOS). GRACOS is an interrupt driven system that schedules AACS activities around subdivisions or multiples of a Galileo system-wide 66 & 2/3 millisecond time unit known as a Real-Time Interrupt (RTI) which is broadcast by the CDS to all subsystems on the spacecraft to coordinate activities. Various things that the AACS needs to do are scheduled at frequencies of: 2 times/RTI, Once/RTI, or Once/10-RTIs. A few events are also asynchronous and for which special ATAC interrupts are provided to let the AACS know when those event have occurred.

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