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    Re: Do We Still Need to Use Sextants?
    From: Tom Sult
    Date: 2013 Apr 05, 16:41 -0500
    We could build a bunch of tall building with bright lights rotating at specific speeds so that they could be ID by the flash rate and deep voiced horns... Oh we already did that. 

    Tom Sult
    Sent from my iPhone

    On Apr 5, 2013, at 15:48, Frank Reed <FrankReed@HistoricalAtlas.com> wrote:

    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 here.


    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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