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    Submarine celestial through periscope
    From: Paul Hirose
    Date: 2023 Mar 1, 10:37 -0800

    "Navigate by the Stars—From Beneath the Waves" by Lieutenant Matthew G.
    Homeier, USN, October 2021, US Naval Institute Proceedings
    "Submarines typically use inertial navigation systems, such as ring
    laser gyroscopes (RLGs), in conjunction with inputs from an electronic
    speed log, fathometer generated seafloor topography, and periodic GPS
    position updates when the boat comes to periscope depth to accurately
    determine the ship’s position...
    "But the military operates in a dynamic geopolitical environment, and
    the possibility that an adversary could render GPS inaccurate or
    inoperative is something submarine crews can prepare for... Even though
    there are two independent RLG units for redundancy, underway failure of
    one would place the boat in a precarious position. And a shipboard
    casualty could potentially take out both—a fire, for example, could
    claim both units.
    "The Navy requires that submarines carry marine sextants. My submarine,
    the USS Key West (SSN-722), had three, but only two crew members knew
    how to operate them—the assistant navigator, an electronics technician
    navigation (ETV) senior chief petty officer with nearly 20 years in the
    Navy, and myself, a submarine warfare officer who had celestial
    navigation training at the U.S. Merchant Marine Academy and on
    commercial cargo vessels.
    "Submarine ETVs no longer receive marine sextant operation, theory, and
    training because the Navy has deemed the skill all but obsolete thanks
    to GPS and inertial navigation. The only celestial navigation training
    they receive is how to obtain azimuths of the sun from a bearing on the
    periscope and how to input that bearing, current position, and time into
    the System to Estimate Latitude and Longitude Astronomically (STELLA)
    computer program designed by the U.S. Naval Observatory. In the event
    STELLA is rendered inoperable (unlikely, because the program is
    installed on numerous laptops and a copy of the original CD is
    maintained by the ETVs), paper copies of the Nautical Almanac are also
    available. The Sight Reduction tables, which were not available in paper
    copy on board my submarine, consist of tabulated calculations that, if
    necessary, could be replicated by use of the original formulas in the
    paper copy of the American Practical Navigator (also known as
    “Bowditch,” after its original editor, Nathaniel Bowditch) that is
    maintained on board.
    "But celestial navigation using a sextant is impractical and inefficient
    for a submarine, which would have to surface from her haven beneath the
    waves and send personnel into the sail. Those sailors would have to
    carry a sextant up the ladder carefully to avoid damaging the finely
    tuned instrument. Then, the observer would have to take the sights
    quickly, before rapidly descending with the fragile sextant so the
    submarine could submerge expeditiously. This might have made sense in
    World War II, when submarines spent most of their time surfaced, but
    today, while it is possible, it is impractical and could expose the
    submarine to unnecessary risk."
    [In the classic WW2 novel "Run Silent, Run Deep" by Edward L. Beach (a
    sub skipper during the war), the protagonist describes the experience of
    surfacing for first time off the Japanese coast. Coming up early will
    give the executive officer (who was also the navigator) a better horizon
    for his star shots, he explains, but it's more dangerous. You have to
    compromise. By the way, if you have seen the movie with Clark Gable and
    Burt Lancaster, only the names are the same. The plot is totally
    different from the book!]
    "On the Key West, I hypothesized that the optical periscope’s internal
    camera that was tied into the BYG-1 Fire Control System could provide a
    practical and efficient tool to perform celestial navigation. The
    periscope can be trained on any bearing and to a high elevation. I also
    hypothesized that star fixes would be the best option, because the
    periscope’s optics can turn on a telemeter to assist the operator with
    zooming in and marking precise bearings and elevations of celestial
    bodies. When the periscope operator presses the “MARK” button on the
    right-side handle, the fire-control system captures a camera image of
    what the periscope was looking at. Unfortunately, the camera cannot
    capture the dim light of stars, but this empty image contains excellent
    metadata that provides ship’s course, speed, depth, position data,
    bearing, and elevation that the periscope was looking at when the image
    was taken... Since the periscope does not need to have a reference
    horizon, this enables the navigator to obtain a celestial fix overnight,
    with the cover of darkness improving stealth and safety for the submarine."
    "Several officers made observations and attempted to get celestial fixes
    with accuracies within 10 nautical miles of GPS position. Of the 14 who
    attended a one-hour wardroom officer training session that covered very
    simplified celestial navigation theory, 3 achieved first-time fixes
    within 10 nautical miles. Three others executed celestial observations
    routinely and achieved respective average accuracies about half again as
    good. Across the entire wardroom’s 30 total fixes, the average accuracy
    was well inside 10 nautical miles."
    Paul Hirose

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