MESSENGER Mission News November 13, 2003: Up or Down?

Over the past week, the MESSENGER team has run "polarity tests" on
the spacecraft’s various guidance and control systems. The tests
show whether MESSENGER’s subsystems speak the same language – for
instance, when a star tracker camera says the spacecraft is right
side up, does the gyroscope in the Inertial Measurement Unit report
the same orientation, or does it report the spacecraft as being
upside down? Do the Sun sensors say that the sunshade is facing the
Sun, or do they think the spacecraft is on its side? Missing a
direction by 90? or 180? is an easy mistake; however, the tests
make sure mistakes are found and corrected. In this week’s annotated
Webcam image a MESSENGER engineer collects data from a test pointing
of the phased array antenna. After commanding the direction and
observing the phase of the radio frequency signals in each antenna
element, engineers can easily see if the antenna beam is pointing in
the correct direction.

The integration team performs many of these basic tests on each
instrument and subsystem to quantify MESSENGER’s overall
performance. They run each test several times to fine-tune
procedures and correct problems. Once the individual tests run
smoothly, they will be combined into a comprehensive performance
test, or CPT. The CPT is very automated, but still takes about four
days to complete a checkout of the entire spacecraft.

Throughout the upcoming environmental test program, the CPT will be
repeated to verify that the spacecraft survived a given test
unscathed. The team will run a final CPT after the spacecraft is
tested at the launch site in Florida. MESSENGER will then be mated
to the Delta II rocket and placed on the launch pad for liftoff next
May.