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The apollo LM panel had a cross pointer that displays the horizontal velocity to the pilot. This ideally have to be made zero during landing for soft touch down by pilot

My question is, how this information is derived? Is it derived by knowledge of state vector in LM converted to local frame velocity. If that is so, then it means it is inertial propagated velocity and thus can be erroneous?

Or did apollo have three axis velocity measurements available from radar?

Edit:

After some googling I found a link LDG and Altimeter which says that landing radar had three beams to provide three axis velocity.

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  • $\begingroup$ This is a really good question. $\endgroup$ – Russell Borogove Nov 16 '19 at 17:35
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Both horizontal and lateral velocity could be displayed on the cross-pointer display.

Based on the position of the Mode Select and Rate/Err Mon switches, the velocity displayed came from one of four (for lateral velocity) or one of three (for forward velocity) sources.

  • Lateral Velocity:

    • Rate / Err Mon switch in Rndz Radar position: Rendezvous Radar Shaft Rate
    • Rate / Err Mon switch in LDG RDR / CMPTR position:

      • Mode Sel switch in LDG RADAR position: velocity derived from landing radar

      • Mode Sel switch in PGNS position: velocity computed by primary guidance system

      • Mode Sel switch in AGS position: velocity computed by abort guidance system
  • Forward Velocity:

    • Rate / Err Mon switch in Rndz Radar position: Rendezvous Radar Trunnion Rate
    • Rate / Err Mon switch in LDG RDR / CMPTR position:

      • Mode Sel switch in LDG RADAR position: velocity derived from landing radar

      • Mode Sel switch in PGNS position: velocity computed by primary guidance system

      • Mode Sel switch in AGS position: no output

Source: Apollo Operations Handbook, Lunar Module, LM10 and Subsequent

(you have to look at the 4 diagrams for the PGNS, AGS, LR, and RR separately to see where the data for the cross pointer comes from in each case) An integrated drawing would have been nice!

Rendezvous Radar

enter image description here

Landing Radar

enter image description here

PGNS

enter image description here

AGS

enter image description here

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  • $\begingroup$ Good as far as it goes, but it would be great if you could add explanation of how the landing radar and the rendezvous radar could determine the rates. $\endgroup$ – Russell Borogove Nov 16 '19 at 20:37
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    $\begingroup$ @RussellBorogove like everything electronic, radar works by magic as far as I am concerned. $\endgroup$ – Organic Marble Nov 16 '19 at 20:42
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    $\begingroup$ TIL of the existence of a NASA spaceflight topic for which @OrganicMarble isn't a walking encyclopedia $\endgroup$ – Tom Goodfellow Nov 16 '19 at 22:37

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