Re: GPS and security

[tcmay@netcom.com (Timothy C. May)]

kootz@well.sf.ca.us (Brenden Kootsey) wrote:
> 
> Like Mike, I don't claim to know much about the GPS, and I didn't see the 60
> Minutes report, but it was my understanding that at least a portion of the
> signal is encrypted. The signal available to us mortal (non-U.S. military)
> types has a built-in inaccuracy; while it is still accurate enough for some
> general use, it is not nearly as accurate as the signal available to the
> military. Anyone have more of the technical details, especially regarding
> the accuracy?

This is come up a couple of times, and I think it was Phil Karn who
summarized the situation well.

Basically, there have been two modes used simulaneously, a precision
mode for U.S. military use (cruise missiles, for example) and a
general mode for everyone else (boaters, campers, Iraqis, etc.).

During the Gulf War, I have heard, the encryption was turned off, or
whatever it took to make the general channel as precise as the
military channel. (This is not as dumb as it sounds, as it let the
military use lower-cost Trimble Navigation-type units in large
quantities, among other things, and certainly Saddam Hussein was in no
position to take advantage of this sudden situation in any significant
way).

I also understand that the general channel can give near-military
results by straightforward crunching of some of the general channel
results (to null out positional errors, the "dither" introduced to
degrade the positional accuracy).

--Tim May


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