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    Re: Role of CN at sea
    From: Glendon
    Date: 2004 Oct 14, 11:21 +1000

    ----- Original Message -----
    From: "Dan Allen" 
    > Dan Allen replies:
    
    > A friend of mine sailing from San Francisco to Hawaii and although he knew
    > CN, he instead took 4 separate GPS receivers as his means of redundancy.
    > But what if the same kind of electronics failure that hit my GPS40 years
    ago
    > hit one or more of the GPS satellites themselves?  Does anyone know about
    > actual mortality rates of the US GPS satellite system?
    >
    > Dan Allen
    > N  39d 59' 50"
    > W 111d 45' 27"
    
    And Jared wrote in response:
    
    Dan-
     I don't know the mortality of the GPS birds, offhand I hadn't heard of ANY
    failures in them. And, there are spare birds kept in orbit so that even if
    one or two failed "now" they could be replaced as soon as the spares shifted
    orbit, no need for launching anything.
     It is, after all, a hardened military system designed for combat
    performance--not a consumer device allowed to casually fail.
    
    ----------------------------------------------------------------------------
    ----------
    Some comments from me:
    
    There are lots of sites documenting the failures of satellites, their being
    taken out of and put back into service, and the effects on GPS precision. It
    is difficult for the layman (including myself) to sort through the mass of
    information. One of the more readable sites covering performance over the
    last 5 years is:
    http://www.schriever.af.mil/GpsSupportCenter/index.htm
    look up advisories on that site, and also the notes on interpretation of the
    data.
    
    Another site covering an earlier period is:
    http://tycho.usno.navy.mil/gps_datafiles.html
    but not so easy to make sense of. There is, however, a lot of very useful
    GPS related information on that site.
    
    Basically, satellites go in and out of service all the time....either as
    part of routine management, or due to failures. Sometimes the effect can be
    temporary degradation of GPS precision over particular areas...see the PDOP
    charts on the first site I mentioned. As I understand it, there have been
    very, very few complete outages of the system. I am only aware of one....due
    to a combination of circumstances there was a degradation of precision over
    central USA in April 2000....culminating in the GPS System becoming unusable
    over parts of Oklahoma,Kansas and Nebraska for a period of 18 minutes.
    
    I also understand the US military may jam the signal for civilian use, over
    very small areas (radius 10-40 miles) from time to time.
    
    The system is very reliable. It has to be, so much of current civilian life
    is riding on it. It is now accepted in Government circles around the world
    that provision of the GPS System has become provision of the fifth utility
    to the public, after gas, water, electricity and telephone. A major reason
    for the European Commision proceeding with their duplicate Galileo System is
    recognition of the importance of of the GPS system, and thus the need for
    redundancy in the system.
    
    Galileo will become increasingly available from late 2007, with full service
    in 2008. Then, with two systems in space, and, say, three GPS sets on board,
    the yachtsman will have satisfactory redundancy in his/her navigation tools.
    Surely enough to satisfy all but the most recalcitrant of Luddites!
    
    Finally, I recount a conversation at dinner with a retired merchant marine
    officer some 7 or 8 years ago. He tapped his nose conspiratorialy and
    announced "they turn it off from time to time, you know, just so we don't
    forget how to navigate!". Absolute codswallop! The system has never been
    turned off. Just another myth.
    
    Hmm, probably best back to CN after that rant!
    
    Lee Martin
    
    
    

       
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