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    Re: Sextants in space
    From: Paul Dolkas
    Date: 2018 May 18, 17:27 -0700

    It seems they are thinking ahead to using them for backup devices in the Orion spacecraft, which (like Apollo) is being designed for lunar & interplanitary flights. It's important to remember that in such missions, being able to rondezvous with another spacecraft isn't a luxury, - it is literally your only ticket home. 

    Here's the full blurb from NASA on the flight experiment:


    The human eye is a remarkable piece of optical equipment and provides many advantages over camera-based systems, including dynamic range and detail resolution. A manual sextant utilizes those advantages, providing important autonomy to the crew in the event of lost communication with Mission Control. The sextant is a mechanical instrument with an optical sight and mirrors that navigators have used for centuries in one form or another to take precise angle measurements on land, sea, and air. Astronauts can even use a sextant in space for a simple and robust navigation method that is independent from the ground. By sighting the angle between the planets and stars, the sextant can be a backup navigation source for future human spaceflight exploration missions.


    The Gemini missions in 1965-66 were the first to exercise sextant sightings from spacecraft. Following this initial technology demonstration, the Apollo designers built a sextant into their vehicle where it flew as a lost-communications navigation backup. James Lovell, on Apollo 8 in 1968, was the first person to demonstrate in space that sextant navigation could be used to navigate the Apollo spacecraft back to Earth from the Moon. Crew members conducted additional sextant experiments on Skylab in the 1970’s. The spaceflight heritage of the sextant concept, combined with its operational simplicity, made it a good candidate for further investigation into backup navigation on Orion Exploration Missions.


    Researchers previously conducted tests and analysis in 2015 at the Johnson Space Center (JSC) Electro-Optics lab to characterize the error sources of navigation-grade sextants. Reminiscent of the experiments performed in the early/mid-1960s in preparation for Apollo missions, the results of this recent study modernized and updated historical findings. In addition, the results were analyzed using Linear Covariance techniques to show feasibility in preparation for Orion missions.


    The Sextant Navigation for Exploration Missions (Sextant Navigation) research assesses the practical application for the modern spaceflight era. The sextant sighting technique is extremely flexible and is not dependent on any particular vehicle type. Onboard ISS, crew members test a hand-held sextant for emergency navigation meant for use on Orion Exploration Mission 2. The team selected the ISS Cupola module as the experimental location, as it provides a good analog for window and crew placement in future exploration vehicles such as Orion. In preparation, the crew performed training on the ground in the Systems Engineering Simulator at NASA JSC with a mockup of the Cupola and stars displayed on a projector dome. Sextant Navigation focuses on characterizing the stability and mechanics of taking accurate sextant sightings in microgravity inside a spacecraft. Researchers will use the results of the current and previous studies in high-fidelity simulations to verify that accuracy levels meet the needs for Orion navigation requirements.

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    Space Applications
    This technology is applicable as an emergency backup, or confirming navigation source, for future human spaceflight missions.

    Earth Applications
    The Earth Application for this investigation has yet to be identified.

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    Operational Requirements and Protocols
    Onboard the ISS, two crew members perform four different sighting activities to characterize the stability and accuracy of sextant measurements. The experiment takes place in the Cupola module. The technique involves measuring the angle between pairs of stars, a classic training method for celestial navigators. The key results of the experiment include photo and video of the sighting positions, the crew’s comments and evaluation, and the measurements taken using the sextant. This experiment does require that the crew manipulate the hardware to perform the task, and the crew must perform multiple skills proficiently in a limited amount of time during orbital night. The crew identifies pairs of stars using a star chart, stabilizes the sighting positions, and then acquire the pair of stars utilizing the sextant. Operations planners separate the final two activities by at least two weeks to allow the research team time to evaluate the findings and fine-tune the procedures.


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