Re: GPS [MARGINAL, at best]

["P. J. Ponder" <ponder@freenet.tlh.fl.us>]

There was an article in _Scientific American_ February 1996 about GPS.  I 
couldn't find the magazine, but they had this squib on the www.sciam.com 
website:

  �The Global Positioning System�
  Thomas A. Herring
  Two dozen satellites hovering thousands of miles up can locate your 
  position on the earth's surface to within a few centimeters. 
  Originally constructed for military applications, this network of 
  space beacons today finds civilian applications--such as landing 
  airplanes in fog--that demand accuracy beyond what its designers had 
  thought would be technically possible.

According to the website, there was also a letter in the June 1996 issue 
responding to Herring�s article.  Here it is:

  MILITARY ADVANTAGE 

  I was pleased when I first saw your February article "The Global 
  Positioning System," by Thomas A. Herring. As developers and operators 
  of GPS, we in the Department of Defense and our partners in industry 
  are justifiably proud of the technology. GPS represents the best of 
  American scientific and technical ingenuity as well as being an 
  excellent example of cooperation between the military and civilian 
  sectors. But after reading the entire article, I was disappointed by 
  its unbalanced discussion of the national security aspects of GPS. 

  Yes, the Defense Department does operate GPS with unpopular security 
  features. But these features were not designed to inconvenience the 
  peaceful users of the system, as Herring implies. Rather they were 
  designed to provide U.S. and allied forces with a crucial military 
  edge. Furthermore, the Defense Department is well aware that the 
  security aspects of GPS are an additional burden for many users. And 
  while we believe such measures are still needed at this time to help 
  preserve our military advantage, we have set a goal of discontinuing 
  regular use of the feature known as Selective Availability, the 
  component that degrades GPS accuracy, within a decade. 

  Both time and resources are needed to replace the advantages Selective 
  Availability provides. In light of the revolutionary contributions of 
  GPS to both military and commercial enterprise, Herring could have 
  portrayed the technology in a more evenhanded manner. 

  PAUL G. KAMINSKI 
  Under Secretary 
  Department of Defense 

-- end of quoted material --

The article, as I recall, was about ways in which civilian users have 
found, or are finding, ways around the built-in inaccuarcy of the GPS.

I don't recall whether crypto was mentioned in the article.  My 
recollection of it was that they had been diddling with the timers or the 
clock signal or something, as opposed to encrypting anything.  But then 
again it has been a while, and my memory of it isn't too clear.

I seem to remember another crypto scheme discussed here at length about 
GPS, based on a paper by Dr. Dorothy [?] Denning, which involved having 
the intended recipient's coordinates - which were somehow involved in the 
encryption.  

The coordinates are in 3-D.  Spheres centered on three of the GPS
satellites intersect within a very small space. 
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