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.
Last updated: November 13, 2003