NSSC-1

The NASA Standard Spacecraft Computer-1 (NSSC-1) was a computer developed as a standard component for the MultiMission Modular Spacecraft at the Goddard Space Flight Center (GSFC) in 1974. The basic spacecraft was built of standardized components and modules, for cost reduction. The computer had 18 bits of core memory or plated wire memory; up to 64 k. 18 bits was chosen because it gave more accuracy (x4) for data over a 16 bit machine. Floating point was not supported.

The NSSC-1 was used on the Solar Maximum Mission, Space Telescope, and Landsat-D Missions, among others. The hardware was developed by Westinghouse and GSFC. The machine used diode-transistor logic, the lowest power parts available at the time on the preferred parts list; initially fabricated from 1700 SSI (NOR gate) packages, it was later moved to 69 MSI (medium-scale integration) chips. ^[1]

Programming and support

The NSSC-1 had an assembler/loader/simulator toolset hosted on Xerox XDS 930 (24- bit) mainframe. An associated simulator ran at 1/1000 of real time. The Xerox computer was interfaced to a breadboard OBP in a rack. (Which, of course, operated at room temperature ambient conditions). Later, the Software Development and Validation Facility (SDVF) added a flight dynamics simulator hosted on a PDP-11/70 minicomputer. ^[2]

A purpose-built NSSC-1 Flight Executive was developed and used on the Solar Maximum Mission (SMM) and subsequent flights. It time sliced tasks at 25 ms. It included a stored command processor that handled both absolute time and relative time commands. It included a status buffer that could be telemetered back to the ground. It required a lot of memory, typically more than half of that available, leaving the rest for applications and spare.^[3]

References

1. ^ Trevathan, Charles E., Taylor, Thomas D., Hartenstein, Raymond G., Merwarth, Ann C., and Stewart, William N. "Development and Application of NASA’s First Standard Spacecraft Computer," CACM V27 n9, Sept 1984, pp. 902–913
2. ^ Styles, F., Taylor, T., Tharpe, M. and Trevathan, C. “A General-Purpose On-Board Processor for Scientific Spacecraft,” NASA/GSFC, X-562-67-202, July 1967.
3. ^ Stakem, Patrick H. The History of Spacecraft Computers from the V-2 to the Space Station, 2010, PRB Publishing, ASIN B004L626U6