The space shuttle crew switched the ADI frame to LVLH as soon as the roll maneuver was completed.

They waited for that time because the LVLH frame is in mathematical gimbal lock (singularity) when pitch is +- 90 deg.

But what frame was selected in the ADI before the LVLH switch?


1 Answer 1


It was initially in REF.

Source: Ascent checklist switch list, p 9-4


  • $\begingroup$ Thanks! Since REF could be changed, I wonder what that initial REF frame would have been aligned with. I imagine it would have been a convenient frame, maybe fixed to the launchpad, maybe aligned with the launch azimuth? And would it have been fixed to the launchpad (and therefore rotating with the earth) or would it have been inertial (and therefore not fixed to the launchpad except maybe at a specific time instant in the launch sequence)? $\endgroup$
    – user36480
    Jan 28, 2021 at 4:48
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    $\begingroup$ Check the SCOM but ISTR REF was an inertial frame. Can't recall what it was defined as prelaunch. SCOM might say. $\endgroup$ Jan 28, 2021 at 4:52
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    $\begingroup$ I'm pretty sure that during ascent "North" on the HSI (the lubber line) was the target insertion plane. But again, IANAP and the SCOM has detailed writeups. $\endgroup$ Jan 28, 2021 at 14:54
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    $\begingroup$ Just for ADI display. Also, I think the ADI only "came alive" 11 seconds before liftoff - often you could hear the crew say "nav init" - so the REF part only lasted a very few seconds. $\endgroup$ Jan 28, 2021 at 17:52
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    $\begingroup$ Pretty sure the real calculations ("truth data") were done in inertial. Angles for displays were derived from that by various transformations. Don't get too hung up on the ADI, it and its switches were only for crew display. $\endgroup$ Jan 28, 2021 at 18:13

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