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    Re: Do We Still Need to Use Sextants?
    From: David Cortes
    Date: 2013 Apr 5, 19:26 -0400

    Did I get you right, that in location with various structures that would
    allow  the drawing of lines of perspective, as an artist would draw them,
    the "true horizon" for purposes of using a sextant can be derived that
    simply?  Would the ghost of George Huxtable (God bless his soul) say
    anything contrary?
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
    Sent: Friday, April 05, 2013 4:49 PM
    To: dcortes@rwlw.com
    Subject: [NavList] Re: Do We Still Need to Use Sextants?
    I am replying under this thread title since I think it's closer to the
    topic. In another thread, Yves Robin-Jouan wrote:
    "About GPS-GLONASS jamming (addressed by FER), and CelNav as a fold-back, I
    would say something about a French project where I am involved. Both SatNav
    and Celnav should be integrated into a common computer, connected to a Sat
    receiver and to an optronic turret. Same algorithm (Method of Coplanar
    Vertices) is applied to any celestial object. And landmarks optronic
    measures can be added to the process when coastal navigation is concerned.
    It is anticipated as running efficiently as an hybrid navigation system."
    This sounds similar to the automated "sextant" that Sems Aktug alluded to
    (and other similar ideas that have been discussed over the years in NavList
    messages). May I ask, regarding the "optronic turret" which you mention, how
    does it get its vertical? Do you do this with accelerometers and band
    filtering? Another option which I have considered over the years is the
    possibility of using near-horizon refraction to determine the vertical. The
    constellations are significantly distorted when they are near the horizon.
    Three cameras pointed low would look for the pattern of altitude variation
    that comes from refraction near the horizon and then a camera pointed high
    (they don't need to be separate cameras, but I'm imagining something built
    off-the-shelf) would always be able to locate the true zenith among the
    stars that it images. The rest is easy.
    A system like this could handle certain kinds of problems with GNSS jamming,
    but  celestial is always at the mercy of the weather. It seems to me that
    the biggest risk of GNSS jamming is criminal intent. If a criminal
    organization wants to jam satellite signals, perhaps to hold a port hostage,
    or maybe so they can drive a tanker full of crude oil onto a reef off a
    valuable tourist destination if they're not "paid a billion dollars by
    midnight", then they could just wait for a cloudy day --not much of an
    impediment! The celestial backup is then useless, of course, no matter now
    matter how sophisticated. If that's the risk, then a celestial backup system
    would only work during a brief window of years when it's not widely known
    that it exists.
    Does anybody know if "dual band" GNSS jammers already exist? Can nefarious
    no-goodniks already take out GPS and GLONASS at the same time? I assume the
    technology is so similar that this is the case. But again, I'm just guessing
    PS: Or we could place the sensors above the weather! We position very
    high-altitude drones with GPS and celestial backup every hundred miles or so
    along major coasts. They report their positions with higher power signals
    that are difficult to jam. Then ships below could triangulate off that.
    PPS: Wait... I have another idea! We position old barges off major coasts.
    Each will fire a "star shell" once every hour with different colors for the
    explosions at different locations. Observers below will time the interval
    between the visible flash and the arrival of the sound from the explosion
    thus independently determining their positions. It's brilliant! Now give me
    back the Lucasian Chair.
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