NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

2.1.5 Systems Checkout

Systems checkout includes the monitoring, testing and diagnosis of spacecraft systems to detect, and if possible to predict, failures or unsafe conditions. For most spacecraft systems, this consists of comparing current values of system parameters to their nominal ranges. When a value falls outside its acceptable range, a message or signal is sent to the crew or to the ground. However, for some systems, passive monitoring is inadequate and periodic active tests are required. An example of systems checkout is the orbital checkout mode of the Saturn 5 launch vehicle digital computer (LVDC). When the vehicle has achieved orbit, the computer checks out the propulsion system, the mid-course guidance and control system, and other related systems and sends the test results to the ground for analysis. If all tests are satisfactory, the next mission phase may be initiated from the ground (ref. 25).

The checkout of experiments is particularly important on long duration missions since some experiments may not be turned on until a substantial time has elapsed, and others may be operated throughout the mission (ref. 24). The computer requirements for checkout of experiments or other spacecraft systems depend on the number of test points, the type of testing being performed, and the frequency of the checkout. Typical requirements for a sophisticated experiment with fifty scientific data points are shown in table 2 (ref. 24). The memory requirements are dominated by the storage needed to indicate the acceptable range for each checkout point.

Checkout is not restricted to inflight needs but may be performed during certain phases prior to launch. For example, the AGC is used during prelaunch to check out the entire Apollo G&N system. Among other tasks, the AGC determines all drifts and biases in the inertial reference subsystem that can be measured in a 1-g field. In addition, the AGC is used in a simulated flight mode to check out various other vehicle and spacecraft systems. The Saturn 5 LVDC also operates in a prelaunch checkout mode to insure that all guidance system interfaces operate properly prior to flight. The checkout program includes a computer self-test, complete mission simulation, and a system test, among others (ref. 25). With the development of more flexible and powerful space- borne computers in future programs, the use of an onboard computer for prelaunch checkout of spacecraft systems is expected to grow, and thus reduce the dependence on extensive ground- based facilities. In the advanced Centaur computer, for instance, prelaunch checkout was a major design requirement (ref. 11).

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