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----- 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.

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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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