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"... As a specific example, I'll note that Selective Availability was
not re-enabled after 9/11. ":

  I think that it was more of a question of why bother turning S.A. back on?
WAAS exists now.
 WAAS is designed to correct for all positional inaccuracies on a
running basis, using a precision known location on earth, and deriving
the error from the GPS reported position and this known location on earth.
   The error is uploaded to the GPS constellation with serial numbers
over 100 (ie:102, 120 etc These are the DGPS sources for WAAS
receivers).  If the error was S.A. induced, I believe that the SA error
would be treated similar to the rest of the GPS inaccuracies, and the
correction applied to the DGPS signals.
  Now of course, the Pentagon could pull the plug on any satellite it
wanted to, so they still could stop transmitting.  But SA on or off
seems to be a moot point now.  Anyone know for sure about this?

  Larry DeMers
  s/v DeLaMer
  Lake Superior


Lu Abel wrote:

> 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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