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mikaelanderlund
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« Reply #225 on: May 11, 2018, 11:04:40 AM »

I think I have a good idea of why the need for P52 but I do not understand the need for the different options 04 06 1-4. Can anyone give a good explanation of the difference between the options and why the different options are needed as well?  Happy

/Mikael
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macieksoft
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« Reply #226 on: May 11, 2018, 01:42:34 PM »

LVLH is most used option. It orients IMU for local vertical at given time. You can use it together with ORDEAL to get idea on where the retro and pro grade is. This variant is mostly used by various manouvers like most thrusting ones and also reentry.

REFSMATT is basically to orient the thing to the actual REFSMATT, IRL there was a INU drift so after few hours platform could be off alignament. This is to "refresh" the alignament if your last was some time ago and you want to proceed with the same alignament as your previous one. So, it just reestablishes the last orientation to get rid of drift error.

Preferred... Well.. I am not sure about this one. AFAIK (i may be wrong) it orients thing to the any orientation you want (works much like GDC align, but for IMU), and it still keeps track on where the stars are, so it is better than unauthorized reorients like using CAGE switch or simply calling IMU torquing or reorientation verb/noun "manually".
« Last Edit: May 11, 2018, 01:46:33 PM by macieksoft » Logged

rcflyinghokie
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« Reply #227 on: May 11, 2018, 03:31:22 PM »

Close but not quite Tongue


LVLH is most used option. It orients IMU for local vertical at given time. You can use it together with ORDEAL to get idea on where the retro and pro grade is. This variant is mostly used by various manouvers like most thrusting ones and also reentry.

Actually this was not used very much with the predominance of uplinked REFSMMAT data.  But it's function is correct, it orients the platform to LVLH at a given time.

REFSMATT is basically to orient the thing to the actual REFSMATT, IRL there was a INU drift so after few hours platform could be off alignament. This is to "refresh" the alignament if your last was some time ago and you want to proceed with the same alignament as your previous one. So, it just reestablishes the last orientation to get rid of drift error.

This is totally correct, option 3 just realigns the platform to the current REFSMMAT to remove gyro drift error.

Preferred... Well.. I am not sure about this one. AFAIK (i may be wrong) it orients thing to the any orientation you want (works much like GDC align, but for IMU), and it still keeps track on where the stars are, so it is better than unauthorized reorients like using CAGE switch or simply calling IMU torquing or reorientation verb/noun "manually".

Preferred was used when there was a REFSMMAT stored and ready for the CMC to use, from either an uplink or from a P30-40/41.  It reorients the platform to a specified or "preferred" alignment and then gives the option to fine align it because the IMU still knows where it is in space.

Option 4 orients the platform to a lunar landing site at a given time.  This was to allow the platform to read approximately 0,0,0 RPY when the LM was landed at the prescribed landing site at the prescribed time.  The CSM uses this option so it can give the LM a docked IMU alignment prior to undocking, as well as to match alignment for when the LM lifts off from the surface (at which time, the platform is reoriented for approximately 0,0,0 in the LM when it lifts off from the surface.)

Probably a very crude explanation, but that is the basics. 
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abr35
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« Reply #228 on: May 11, 2018, 08:14:58 PM »

Is gyro drift modeled yet? Or will a proper P52 align the IMU indefinitely?
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Thymo
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« Reply #229 on: May 12, 2018, 05:16:25 AM »

It is not. Certainly would be nice to have though.
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eddievhfan1984
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« Reply #230 on: May 12, 2018, 01:50:50 PM »

Heck, that should be easy to implement, as long as we know what the gyro error distribution is.
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indy91
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« Reply #231 on: May 13, 2018, 06:09:01 AM »

It would also be fairly pointless to implement, because once our IMU can drift we would simply compensate that drift with the AGC padload, which would send torquing pulses back to the IMU to adjust the attitude every so often for the drift compensation. Same for the PIPAs. This part of the padload is of course all set to 0 right now. So it just seems like an unnecessary complication and potential source of issues. It doesn't add realism because in the end, with the compensation in the AGC, everything would be just like before.
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Thymo
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« Reply #232 on: May 17, 2018, 01:33:21 PM »

That won't be the case if the drift is random.
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eddievhfan1984
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« Reply #233 on: May 22, 2018, 07:02:59 PM »

Yeah, because P52 still has to complete the fine alignment, even with an uplinked REFSMMAT. And I don't recall V42 being done via ground uplink for any reason...
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indy91
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« Reply #234 on: May 23, 2018, 03:41:33 AM »

Yeah, because P52 still has to complete the fine alignment, even with an uplinked REFSMMAT. And I don't recall V42 being done via ground uplink for any reason...

Yeah, the IMU wouldn't be adjusted through a mission with a V42, but by updating the IMU and PIPA biases. That's the reason for the crew reporting the gyro torquing angles of their P52s to Houston. They calculated new biases from those numbers and if they were very different from what is currently in the AGC, then they would uplink updated values.
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