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Space shuttle training manuals say the GNC system obtains state information from the IMU platform at a low rate (0.25-0.5Hz) and that state propagation algorithms estimate the vehicle's state vector at a higher rate (12.5-25 Hz).

At the beginning of each cycle, when new state information from the IMU is available, that information replaces the estimates from the state propagation algorithm, which starts over from the updated IMU data.

If I got this right, I wonder: What state information does the GNC system obtain from the IMU? Is it just the velocity increment provided by the integrating accelerometers? Does it include position itself?

If IMU position is obtained by numerical integration of velocity (itself the integration of acceleration over a fixed time interval), then, well... isn't that just what the state propagation algorithm does?

So it seems the IMU would have nothing new to offer in terms of position (nothing that the GNC couldn't calculate), and that the updated state info would include only the velocity increments plus attitude info from the gimbals.

Is this right?

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Q:

What state information does the GNC system obtain from the IMU?

A:

The IMUs provide inertial attitude and velocity data to the GNC software functions.

Shuttle Crew Operations Manual p 2.13-6

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  • $\begingroup$ Thanks @OrganicMarble. So it seems the position estimate from the state propagation algorithm is not corrected at the start of each cycle---only velocity and attitude are. Right? This would mean that position integration errors accumulate faster than velocity integration errors (the first remains uncorrected for, the second is corrected for each cycle). How often is position data corrected and where does the corrected data come from (GPS? Radar? Star alignment?) Is position at all corrected before orbital insertion? Thanks! $\endgroup$
    – user36480
    Feb 6, 2021 at 19:36
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    $\begingroup$ @uhoh space.stackexchange.com/questions/49977/… $\endgroup$
    – Organic Marble
    Feb 7, 2021 at 0:52

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