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Author
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Topic: Apollo LM altitude measurement, accuracy
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Marc05A
Member Posts: 39
From: Reims, France
Registered: May 2009
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posted 05-14-2018 08:40 AM
   
During the last phase of powered descent till landing, how was the Apollo lunar module altitude above the lunar surface measured? By the landing radar, by ground tracking or by the IMU, or was it an average of all those sources? I know that Mission Control Center was checking that on board and ground tracking data for altitude were converging during initial descent, but what about the final moments?How accurate were the altitude readings, especially during the last few minutes before the actual touchdown? And I was unable to find a picture of the ROD switch on the LM control panel. Can somebody provide a picture or a link? |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 05-14-2018 09:06 AM
The reason for the landing radar was to provide the most accurate measurement since it was measuring to the actual surface. There is no real way of figuring its actual accuracy since other methods are less accurate. IMU would be the next most accurate and ground tracking is only good for stable orbits and not during maneuvering. IMU can only provide a orbital height and hence a height above the lunar mean surface and not the actual surface. |
Marc05A
Member Posts: 39
From: Reims, France
Registered: May 2009
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posted 05-15-2018 08:17 AM
Thanks for the answer. And I was unable to locate the ROD (rate of decent) switch on any control panel diagram, or find any picture of this switch? I know it is located close to the TTCA. Can anyone provide a link or a picture?Thanks a lot! |
oly
Member Posts: 971
From: Perth, Western Australia
Registered: Apr 2015
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posted 05-15-2018 02:05 PM
Panel 5, outboard side of start push button, DES Rate. |
moonguyron
Member Posts: 191
From: Trinity, FL USA
Registered: Jan 2011
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posted 05-22-2018 02:39 PM
The landing radar essentially functioned like today's airliner's radar altimeter. For measuring absolute height above the terrain instantaneously at lower altitudes. Interestingly the LM's landing radar was manufactured by Ryan Aircraft, the same company responsible for the "Spirit of St. Louis" that made a flight you may have heard of in 1927. That same Mr. Ryan was in attendance in the VIP section for the launch of Apollo 11. My how things changed in 42 years. |
Headshot
Member Posts: 891
From: Vancouver, WA, USA
Registered: Feb 2012
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posted 06-22-2019 08:42 AM
When altitudes are given for a lunar module during their landings, are the altitudes referenced from: the tip of the contact probe to the lunar surface, the bottom of the footpad to the lunar surface, or the LM's center of mass to the lunar surface? While this is unimportant during most of the descent, it might have been critical during the last 100 feet or so. Editor's note: Threads merged. |
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