Computers in Spaceflight: The NASA Experience

- Chapter Three -
- The Skylab Computer System -
The Reactivation Mission
[81] The Skylab Reactivation Mission represents one of the most interesting examples of the autonomy and reliability of manned spacecraft computers. The original Skylab mission lasted 272 days with long unmanned periods. The reactivation mission, flown entirely under computer control, lasted 393 days. Therefore, the bulk of the activated life of the space laboratory fully depended on the ATMDCs.
When it was obvious that the Workshop was going to fall to the earth long before a rescue mission could be launched, NASA began studying methods of prolonging the orbital life of the spacecraft. Even though the atmosphere is very thin at the altitude Skylab was flying, the drag produced on the spacecraft was highly related to its attitude with respect to its direction of flight (velocity vector). During most of the manned mission periods Skylab flew in solar inertial (SI) mode, in which the lab was kept perpendicular to the sun to provide maximum exposure for the solar collectors. Momentum desaturation maneuvers were done on the dark side of the earth to compensate for bias momentum buildup resulting from noncyclic torques acting on the spacecraft. The SI mode was high drag, so engineers devised two new modes, end-on-velocity-vector (EOVV) and torque equilibrium attitude (TEA). EOVV pointed the narrow end of the lab in the direction of flight, minimizing the aerodynamic drag on the vehicle. TEA could control the re-entry, using the gravity gradient and gyroscopic torques to counterbalance the aerodynamic torque. Only in this way could the Workshop be controlled below 140 nautical miles altitude66.
Use of the new modes required that they be coded and transmitted to the computers in orbit. First it was necessary to discover whether or not the computers still functioned. Since the ATMDC used destructive readout core memories, there was some concern that the software might have been destroyed during restart tests if the refreshment hardware had failed. On March 6, 1978, NASA engineers at the Bermuda tracking station ordered portions of Skylab to activate. On March 11, the ATMDC powered up for 5 minutes to obtain telemetry confirmation that it was still functioning. The software resumed the program cycle where it had left off 4 years and 30 days earlier. As far as the computer was concerned, it had suffered a temporary power transient67!
When IBM began to make preparations to modify the software, it discovered that there was almost nothing with which to work. The [82] carefully constructed tools used in the original software effort were dispersed beyond recall, and, worse yet, the last of the source code for the flight programs had been deleted just weeks beforehand. This meant that changes to the software would have to be hand coded in hexadecimal, as the assembler could not be used-a risky venture in terms of ensuring accuracy. Eventually it became necessary to repunch the 2,516 cards of a listing of the most recent flight program, and IBM hired a subcontractor for the purpose68.
Engineers could not test this software with the same high fidelity as during the original development. They abandoned plans for real time simulations because they could not find enough parts of any of the original simulators. Interpretive simulation could be performed because the tapes for that form of testing had been saved. However, the interpretive simulator ran 20 times slower than real time, so less testing was possible69.
IBM approached the modification using the same principles as in the original production. The baseline software for the reactivation was Flight Program 80, including change request 3091, which was already in the second computer. Software changes for reactivation were simply handled as routine change requests. They placed the EOVV software in memory previously occupied by experiment calibration and other functions useless in the new mission. TEA replaced the command and display software70.
When the software was ready for flight, NASA uplinked it to a reserve area of memory and then downlinked and manually verified it. If it passed the verification, engineers gave a command to activate it. The reprogramming was generally successful. The four people assigned to the software revision maintained IBM's record of quality throughout the reactivation mission71.

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