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.


NASA SP-504: Space Shuttle Avionics System

Section 4  System Mechanization/Operation

Ground Checkout

The ground launch processing system controls all ground test, checkout, and prelaunch countdown operations until 30 seconds before lift-off, when control is transferred to the Orbiter avionics system. The LPS, however, makes extensive use of the features and capabilities of the onboard avionics system throughout the process. Figure 4-48 shows the major components and interfaces involved. The two MDM's, LAI and LF1, also known as command decoders, provide interfaces with the vehicle power and subsystem switches and controls necessary for remote activation and operation of the vehicle. The four SRB MDM's perform a similar function. Commands and data requests may be sent to these MDM's on one of the launch data buses by either the LPS or a GPC. Under the protocol established to avoid conflict, however, the GPC's have priority and will assume bus control when activated and loaded with the appropriate vehicle utility software. Under these conditions, the LPS acts as a bus terminal unit and issues commands and data requests through the GPC in control as part of the polling process. If no GPC is in control, the LPS may assume control of the launch data buses directly.

With the avionics systems powered up and in OPS GNC9, the ground checkout mode, the LPS exercises control through the use of test control supervisor operators. Twenty-six of these have been defined to initiate and control test operations. These can be used singly or in sequence to cause the GPC's to perform a variety of functions. In addition, they can be used to call a number of test application programs loaded in GPC memory which perform operations excessively complicated and otherwise difficult to control by way of the TCS. Examples of prestored routines are the ramp function generator, IMU calibration, the actuator positioning test, and dedicated display checkout.

figure_4_48.jpg (13522 bytes)

Figure 4-48. - Checkout configuration.


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