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Marcel:

You are absolutely right about the EU developing EUREKA (and I can fully
understand the EU's desire to field a system under their own control).

But Gordon's "we need to keep our sextant skills up" note was addressed
to the idea of *terrorists* disabling GPS, not the US government.

As a practical matter, GPS has become deeply embedded in 21st century
life.   For at least the past decade people have regarded the ability to
accurately determine one's position as a new, universal utility.  That
was one of the reasons for disabling Selective Availability.  WAAS is a
precursor to GPS being used as a new paradigm for air traffic routing.
In the US all new cell phones are now required to give positional
information to 100 meters or less.  While there are a number of
techniques for doing this (eg, triangulation), one of the most
successful and widely adopted is embedding a GPS receiver in the cell
phone.  GPS-based mapping systems are one of the most popular options on
new cars.  I even have been told that John Deere is experimenting with
driverless, GPS-directed tractors for plowing farmer's fields!  And on
and on...

At this point, it would be difficult for even the most paranoid US
leader to turn off GPS.  As a specific example, I'll note that Selective
Availability was not re-enabled after 9/11.

Lu Abel

Marcel Tschudin wrote:
> Please correct me if I should be wrong: Since GPS is controlled by the
> US military it only needs a decision from the US administration to swich
> it off. This is, to my understanding, the reason why Europe decided to
> set in place their own system called EUREKA.
>
> Marcel
>
>
> On 4/5/06, *Lu Abel* >
> wrote:
>
>     With all respect, Gordon, it would take a lot more technology than the
>     terrorists have to turn off GPS much less to make it give incorrect
>     positions.   GPS uses satellites.  One would need satellite-killer
>     missiles to "turn off" GPS.  Only two or three countries (USA, Russia,
>     China?) have the technology (which, BTW, includes the technology and
>     infrastructure to track and identify the target before saying to the
>     missile "go get 'em").  To make GPS give the wrong position would
>     require taking over the US's GPS control centers and I'm sure the US
>     military has put a lot of thought and effort into preventing that.
>
>     There are GPS jammers available on the arms market, but they work only
>     over a small theater of operations.  Nowhere I'd be sailing, hope you
>     won't be there either.
>
>     Last but not least, terrorists are interested in, well, terror.  I
>     suspect they'd far rather kill a few thousand of their perceived enemies
>     than inconvenience them by turning off GPS....
>
>     Lu Abel
>
>     Gordon Talge wrote:
>      > Just for the heck of it, I wonder what would happen,
>      > if some terrorists managed to turn off the GPS system, or
>      > maybe even better, have it give the wrong positions.
>      >
>      > I have noticed that a lot of people seem to say they keep
>      > a sextant and tables, etc, for backup. The problem that I
>      > see with that is it takes a lot of practice to get good at using a
>      > sextant and working out sights by hand. Someone who has
>      > only tried out their sextant on a calm day or on the beach,
>      > may find it hard to get a decent shot on a pitching boat
>      > or ship, and then work out a sight where one slip of a
>      > plus or minus, or wrong column, would make it all for naught.
>      >
>      > BTW,
>      >
>      > Has anyone seen those German films of the U-Boats during World War II
>      > in the North Atlantic pitching and rolling? I start getting seasick
>      > just watching it. I saw one where a German Officer was taking a noon
>      > sight on a sub like that. They may have been the enemy, but they
>      > sure had guts. (It took guts to stand up to them too)
>      >
>      > -- Gordon
>      >
>      >
>
>

   

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