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"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."

https://www.usni.org/magazines/proceedings/2021/october/navigate-stars-beneath-waves

--
Paul Hirose
sofajpl.com

   

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