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Navigation Stars used in the AOT

Commentary Copyright © 2011 by Eric M. Jones.
All rights reserved.
Last revised 21 January 2012.

Apollo Stars

Each of the LM crews had onboard a mission-specific G & N (Guidance and Navigation) Dictionary that included a two-part star chart and a separate list of 41 objects for use when establishing establishing the LM orientation on the lunar surface. The LM Guidance Computer (LGC) held celestial coordinates for 37 stars plus the Earth, Sun, Moon, and, as number 00 in the star list, another suitably located object*. The astronauts used the Alignment Optical Telescope (AOT) to establish the location of a selected object relative to the structure of the LM. Sightings on two objects then allowed the AGC to determine the LM's orientation on the surface. Alternatively, a sighting on one appropriate object plus a determination of the direction of the lunar gravity vector made by integrating output from accelerometers in the Inertial Measurement Unit (IMU) allowed the LGC to determine the LM's orientation on the surface. * Apollo 12 LM Lunar Surface Checklist page Sur-6 contains a handwritten change indicating they were going to use Pollux rather than Procyon for their second star.  Data to be loaded in Noun 88 for Pollux are also handwritten.  There is also a note "Pollux 200" which indicates that Pollux was expected to be in detent 2 and that it would be designated Star 00.



How Navi, Regor, and Dnoces Got Their Names

As detailed in the postlanding chapter of the Apollo 15 Lunar Surface Journal, three of the stars on the Apollo list that had relatively unfamiliar names were renamed in a prank by Gus Grisson to honor his Apollo 1 crew. The new names were simply parts of an Apollo 1 astronaut's name spelled backwards: Gamma Cassiopeia became Navi for Virgil 'Ivan' Grissom; Suhail (Gamma Velorum) became Regor for 'Roger' Chaffee; and Talitha (Iota Ursa Majoris) became Dnoces for Ed White II ('Second').

Stellar Alignments made on the Lunar Surface

The AOT gave the astronauts a choice of six fields-of-view, each 60 degrees wide and with the centers separated by 60 degrees. The centers were also 45 degrees down from the plus-X (vertical) axis. The AOT could not be moved continuously but only from one fixed position (called a detent) to another. The six detents are shown in the following figure.  Note that, at stellar elevations of around 45 degrees, there is a fair bit of overlap between neighboring fields-of view.


Locations of the six AOT fields-of-view
Position to the six fields-of-view, relative to the spacecraft axes.
Each field-of-view is conical with a width of 60 degrees.

Selected Uses of the AOT


In the transcripts and checklists, the detents are often referred to by number, from 1 to 6.  Computation of the lunar azimuth and elevation of the star used in a particular instance can be used together with the known LM yaw with respect to down-Sun to calculate location in the Y-Z plane relative to plus-Z.  That, in turns lets us identify the corresponding detent and match the numerical designation with the positional (front, right front, etc.)  The result is that left-front detent - usually referred to as 'left' in the transcripts - is detent No. 1; 'front' = 2; and so on, clockwise around to 'left-rear' = 6.  These identifications have been added to the diagram above.  Note that the Sun-facing, rear detent (No. 5) was not used, except as a stowage position.

Mission
Reported LM Orientation
GET
UTC
Star
Lunar El/Az
Detent
Pitch
Roll
Yaw
LM plus-Z Azmth
hh:mm
dd/mm/yy

El.
Azmth
Angle from plus-Z
Planned
Used
Apollo 11 4 0 -13 257 103:22:54
22:14
20/07/69
Rigel
43
133
236
6
6








Capella
43
56
159
4









Navi
42
2
105

4






122:17:10
15:49
21/07/69
Capella
51
50
153

4














Apollo 12
-3
-1.4
10
280
110:56:00
07:18
19/11/69
Sirius
35
225
-55 (305)
1
1





111:06:44
07:29
19/11/69
Pollux
50
287
7
2
2














Apollo 16
2.5
0
0
270
104:59:14
02:53
21/04/72
Altair
52
339
69
3
3


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