The GPC complex requires
a stable, accurate time source because its software uses Greenwich
Mean Time to schedule processing. Each GPC uses the master timing
unit to update its internal clock. The MTU provides precise frequency
outputs for various timing and synchronization purposes to the
GPC complex and many other orbiter subsystems. Its three time
accumulators provide GMT and mission elapsed time, which can be
updated by external control. The accumulator's timing is in days,
hours, minutes, seconds, and milliseconds up to one year.
The master timing unit is a stable, crystal-controlled frequency
source that uses two oscillators for redundancy. The signals from
one of the two oscillators are passed through signal shapers and
frequency drivers to the three GMT/MET accumulators.
The MTU outputs serial digital time data (GMT/MET) on demand
to the GPCs through the accumulators. The GPCs use this information
for their reference time and indirectly for time-tagging GN&C;
and systems management processing. The MTU also provides continuous
digital timing outputs to drive the four digital timers in the
crew compartment-two mission timers and two event timers. In addition,
the MTU also provides signals to the pulse code modulation master
units, payload signal processor and FM signal processor, as well
as various payloads.
The GPCs start by using MTU accumulator 1 as their time source.
Every second, each GPC checks the accumulator time against its
own internal time. If the time is within tolerance (less than
one millisecond), the GPC updates its internal clock to the time
of the accumulator, which is more accurate, and continues to use
that accumulator. However, if the time is out of tolerance, the
GPC will try the other MTU accumulators and then the lowest numbered
GPC until it finds a successful comparison.
The GPCs do not use the mission elapsed time that they receive
from the master timing unit because flag compute MET on the basis
of current GMT and lift-off time.
The master timing unit is redundantly powered by the MTU A and
MTU B circuit breakers on panel O13. The master timing unit OSC
1, auto, OSC 2 switch on panel O6 controls the MTU. When the switch
is in auto and a time signal from the MTU is out of tolerance,
the MTU automatically switches to the other oscillator. Normally,
the MTU is activated by oscillator 1 with the switch in auto .
The OSC 1 or OSC 2 position, manually selects OSC 1 or OSC 2.
The MTU is located in crew compartment middeck avionics bay 3B
and is cooled by a water coolant loop cold plate. The only hardware
displays associated with the MTU are the mission and event timers.
The mission timers are located on panels O3 and A4. They can display
either GMT or MET in response to the GMT or MET switch positions.
The forward event timer is on panel F7 and its control switches
are on panel C2. The aft event timer is on panel A4 and its control
switches are on panel A6.
The master timing unit contractor is Westinghouse Electric Corp.,
Systems Development Division, Baltimore, Md.