Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Apollo spacecraft sextant
    From: Paul Hirose
    Date: 2011 Jun 28, 22:15 -0700

    NASA used sextants for star observations in both the Gemini and Apollo
    programs. During Gemini 10, Mike Collins used a handheld sextant in a
    celestial navigation experiment described in his book "Carrying the
    Fire". Results were indifferent, mainly due to the indistinct horizon
    seen from space. I'm not sure if any other Gemini missions repeated the 
    experiment. Sextants were built into the Apollo spacecraft, though.
    Recently I bought "Digital Apollo: Human and Machine in Spaceflight," by
    MIT historian David Mindell (The MIT Press, 2008). In 1961 MIT got the 
    first large contract of the Apollo program: to design the guidance and 
    navigation system. Naturally, the book has a lot to say on that subject.
    MIT's initial idea was to have a periscopic sextant covered by a door.
    But North American Aviation, the spacecraft prime contractor, didn't 
    like that. It would necessitate pressure seals around moving parts, and 
    if the optics failed to retract or the door failed to close, thermal 
    protection would be compromised during re-entry. Eventually NASA had to 
    step into the dispute and decree a fixed window, flush with the skin.
    Component location was challenging. Flexure could not be tolerated
    between the sextant and the inertial navigation platform. The two
    subsystems were mounted together in a beryllium frame. But there
    was no space for a long optical train to bring the sextant image to an
    eyepiece on the instrument panel. So the navigation station had its own
    control panel in the lower equipment bay, below and forward of the 
    couches. When standing at the nav station your body was parallel to the 
    axis of the conical spacecraft, your head toward the apex.
    The sextant line of sight could rotate in "azimuth" (called "shaft
    angle") and "elevation" (called "trunnion angle"). Trunnion angle was
    limited no more than 50°. That is, the optics viewed a conical area
    within 50° of the shaft axis (which was perpendicular to the skin).
    Often the spacecraft had to be steered to a different orientation to
    bring an object into view.
    A common task was correcting the inertial platform orientation with two 
    star observations. The computer could help. You gave it a star number (I 
    think there were 35 different stars) and it would point the sextant to 
    where the star ought to be. Assuming a fairly good platform alignment, 
    the star would be in sight. All you had to do was refine the crosshair 
    The computer was clever enough to check for blunders. Regardless of any 
    error in platform orientation, the separation angle between any two 
    stars was predictable. This value was compared to the angle calculated 
    from the astronaut's shaft and trunnion angles, and the difference 
    displayed, with an option to reject the observation.
    To measure the position of the spacecraft (vs. orientation) you shot a
    lunar distance. Or, in the early part of the mission, you shot an "earth
    distance" because a lunar is relatively insensitive to position unless
    close to the Moon.
    Of course this required viewing two bodies simultaneously. The astronaut
    had to orient the entire spacecraft to point the shaft axis at the 
    substellar point on the limb. A reticle indicated this axis. Then, by 
    rotating the shaft and adjusting the trunnion angle he superimposed the 
    star on that point and took a "mark". To accomplish this there were 
    separate joysticks at the nav station to steer the spacecraft and the 
    sextant line of sight. It helped that the computer fired thrusters to 
    hold the spacecraft rock steady in any desired orientation. Also, 
    coordinate system magic inside the computer caused both objects to 
    respond to the joysticks in an intuitive manner as seen in the eyepiece.
    Actually, there were two eyepieces and two optical systems. The
    "scanning telescope" gave no magnification but a wide field of view. The
    sextant magnified 28x.
    Apollo 8, the one that orbited the Moon on Christmas Eve 1968, gave this 
    system its most thorough checkout. Audio transcripts of the entire 
    mission are online at NASA's Apollo 8 Flight Journal site:
    Jim Lovell was in charge of navigation. His observations throughout the 
    mission involved a great deal of dialog, preserved in the transcripts. 
    The separation angle measurements utilizing the lunar or Earth limb 
    numbered more than 200. Although guidance was actually based on Mission 
    Control's navigation solutions uplinked to the spacecraft, Lovell's 
    results were excellent. Later simulations proved Apollo 8 would have 
    easily hit its reentry target with onboard observations alone. No 
    subsequent mission so thoroughly tested Apollo's capacity for self 
    contained navigation.
    I filter out messages with attachments or HTML.

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site