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    Re: GPS as a time authority
    From: Douglas Denny
    Date: 2009 Sep 15, 10:26 -0700

    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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