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Any differences found in the display of a GPS receiver must be, as Mr. Parsons 
says, due to processing variations within the software to the display.  The 
GPS signal itself is the most accurate global time source easily available 
next to the helium maser or caesium fountain standard used for 
standardisation in the bureaux of standards such as Teddington in Britian and 
Boulder Colarado USA.

I was amazed to see a number of sophisticated GPS receivers in the time 
standards department at Teddington which were used to compare the standard 
maintained there with that of the other standards facilities around the world 
- which was a serious problem until GPS allowed simple comparisons from the 
many caesium oscillators circulating the globe constantly now in the GPS 
satellites.  Each GPS has (I think) four caesium standards on board; and at 
any instant there are at least four satellites above the horizon anywhere on 
Earth, and of course quite often many more.  Making allowances for signal 
delays with the known position of the receiver, and ionospheric corrections 
from two separate frequencies allows constant comparison to a high accuracy.

I was told that the only way of doing it with sufficient accuracy pre-GPS was 
to physically take a portable caesium standard (actually not that easily 
portable and weighing quite a lot) by aircraft to the facility required.  The 
chap who's responsibility was to take it would book an extra seat on the 
aircraft next to him for the time standard in the name of Mr. C.S Clock  or 
something equally silly.
------------

A small GPS 'engine' - an integrasted chip the size of a smallish coin, easily 
available, and now at very cheap price relatively speaking, will give you a 
square wave one second output locked to GPS time within a fraction of a 
microsecond;  the best accuracy with rather more sophistication being about + 
or - 3 nanoseconds.

A friend of mine uses such a device for very sophisticated amateur radio 
transmit/receive processing methods by digital signal processing and computer 
(he produces all his own software), which enables him to use what appear to 
be unelievable nay -incredible techniques such as very narrow bandwidths in 
the order of small fractions of a Hz, enabling him to send out a 2 Watt pulse 
on his transmitter and receive the signal having travelled around the world 
not once but twice or even three times.  Also, to use exsiting transmitters 
located elsewhere to provide ionograms of the layers of the ionosphere (such 
as is done at highly costly research departments like the Appleton Laboratory 
- but he does it on his home elctronics bench at very low cost); and uses 
other transimitters as over-the-horizon radar revealing aircraft movements.

He also achieved reliable and constant accuracy of + or - approx 5 Metres 
using Loran C signal when the agreed best accuracy of that navigation aid is 
only something like + or - 100 Metres at the very best.
All relying on and due to the ability of synchronising timing to nanoseconds.  
Mind you, he is also a genius. That helps a lot.

Douglas Denny.
Chichester. England. 


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