Author Topic: IMU, REFSMMAT and P52  (Read 4808 times)


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« on: November 29, 2017, 06:49:53 PM »
A very brief intro of these essential concepts of the Apollo Guidance and Navigation system.
The attitude (orientation) of the Apollo spacecraft was measured by the inertial measurement unit (IMU). The IMU was a set of three nested gimbals that supported a platform at the center. The three gimbals separated the platform from the spacecraft so that the orientation of the platform remained the same while the spacecraft moved around it. The spacecraft's attitude could be calculated relative to the platform's orientation.

The IMU measurements only make sense if we know which way the platform was oriented in space. Here is where the REFSMMAT comes in. A REFSMMAT (reference to a stable member matrix) is a way to express orientationis. It is a set of numbers that describe the orientation of the IMU platform. Since the spacecraft moves substantially with respect to the Earth and Moon, an inertial reference is used. The REFSMMAT is defined with respect to the stars. 

The IMU drifted out of alignment with the REFSMMAT over time. Periodically, and prior to burns, the platform had to be realigned. This realignment is called a P52. A P52 is performed by sighting two stars with a sextant. The computer compares the star's actual position with where it thinks it should be to calculate the amount the platform has drifted.


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