The gov't has eliminated selective availability as of May 1, 2000.   (http://www.ngs.noaa.gov/FGCS/info/sans_SA/docs/statement.html)    But what I am talking about is the mapping software that is installed on your computer.  After marking a point (for instance my house) and then downloading it to the software on my computer, my house could be nearly 500 foot from the software road.  That is a software error, and not a GPS error.

Eric Aldhizer
  ----- Original Message ----- 
  From: Paul Moring 
  To: [log in to unmask] 
  Sent: Monday, October 06, 2003 9:14 AM
  Subject: A Note about GPS accuracy...


  The following statement was in an earlier post:
  "I have found that the software can be off as little as 10 feet, and as =
  much as 1,000 feet."

  The network of nearly 30 satellites for global positioning coordinates was
  developed, launched and is controlled by the Defense Dept for military use.

  The "error" factor in GPS accuracy is controlled by the US Defense
  Department. It's not within a particular units software.

  It is called "selective availability", which, in a nutshell means: when the
  D.O.D. has the error factor "turned on", your readings, depending on which
  satellites you're locked on, will give you a position OFF by several meters
  to close to a kilometer. This is to make "enemies" equipment less accurate
  for their targeting purposes. "Accurate" postions of one meter
  are "available selectively" via the D.O.D.'s big hand on the switch.

  Of course, if a nuke hits 1000 meters away from the desired target....does
  it really matter? There's the rub and the controversy.

  The D.O.D.'s turning off and on the "SA" over the years has been a point of
  contention for GPS manufacturers and industries that use GPS heavily....ie:
  shipping and maritime companies, as well as a robust pleasure boating
  industry. Those users should realize the system was NOT developed for their
  use. It was developed for accurate targeting and guidance purposes
  on "smart" bombs.
  The D.O.D. has said that eventually the SA will be eliminated.

  To overcome the error in maritime navigation via GPS, units with "D-GPS"
  capability have been popular for about 10 years. The "D-GPS" units
  (Differential GPS) use LORAN land based transmitters to fall back on when
  SA is causing error. The LORAN system uses triangulation between
  transmitter signals and calculates signal delay between the signals at the
  LORAN receiver to determine postion. It is VERY accurate, but will be
  phased out completely soon.
  LORAN is controlled by the Coast Guard.

  Now that y'all are in a coma from my boring discertation, I'll be quiet now.

  Regards,
  Paul W. Moring
  Va to SC to IN

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