[LINK] limits of technology in finding someone

[Paul McGowan]

Gordon wrote:

> On Sat, 9 Dec 2006 15:35, Stewart Fist wrote:
> > What do people think about the accuracy of GPS -- in the best
> > circumstances, and with the best gear.
> >
> > I remember when the USA had dithering on the system (to deliberately reduce
> > the accuracy to a few hundred metres, maybe even more).  At that time they
> > boasted that their military accuracy, with no dithering, was a matter of
> > centimetres.
> 
> My understanding is that GPS transmits on two bands, one public and one 
> encrypted. The public band also used to dithered, but isn't any more. To read 
> the encrypted military band you need special, classified, equipment.
> 
> The accuracy of both the public and military bands is significantly better 
> than just that of the public band. Although I understand it's not as good as 
> good differential GPS.
>

etc.

Something which might also be worth a mention is the difference 
between code phase GPS and carrier phase GPS.  Carrier phase is a 
heap more accurate than code phase (which is how the system was 
designed to be used) by making use of the higher frequency carrier 
waves of the GPS signal.  Carrier phase GPS is used by surveyors to 
achieve millimetre accuracy.  Note also the difference between moving 
targets and stationary ones.  Much greater accuracy can be obtained 
when the location you are trying to determine is not moving.  This, 
of course (or should that be off course?) does not apply to tourists.

There is a very good general explanation of the various techniques 
and principles on the Trimble website.  Trimble, for those who don't 
know, make GPS systems (of course).

http://www.trimble.com/gps/index.shtml

HTH,

Paul McGowan