collapse

Author Topic: IMU, REFSMMAT and P52  (Read 6038 times)

ApolloGirl

  • Full Member
  • ***
  • Posts: 118
    • View Profile
IMU, REFSMMAT and P52
« 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.

 

Recent Posts / Topics

looking for Chinese translators 中文翻译 by ApolloGirl
[July 17, 2018, 07:04:30 AM]


Question for what something is by Kmassie92
[May 30, 2018, 01:32:36 PM]


Maneuvering the spaceship by Vinny002
[May 26, 2018, 06:45:08 AM]


ISS Docking by David
[May 19, 2018, 12:06:53 PM]


Space Shuttle dock with ISS by ApolloGirl
[May 16, 2018, 03:56:42 PM]


from saturn to the moons by astro_tm
[May 11, 2018, 12:44:17 PM]


Update for android? by TragicMike
[April 21, 2018, 06:59:58 AM]


spacecraft staying in orbit by ApolloGirl
[April 10, 2018, 11:12:17 AM]


cockpit lights demo by jdietrichson
[April 01, 2018, 03:39:06 PM]


Just an idea by JessieR
[March 25, 2018, 02:33:09 AM]


menu text size by ApolloGirl
[March 09, 2018, 11:33:17 PM]


Steam Autopilots by jcarrion
[March 09, 2018, 07:47:32 AM]