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Bill-
 I think part of your confusion is because the GPS equipment has changed
over the years and some things are (and were) done invisibly in the system.
 "This offset is also given in the navigation (NAV) message" IIRC the "Nav"
message is given once every 14 minutes or so, it contains information that
the receivers need for full correction. Without the information in that
message, the GPS receivers are "almost right" but not as good as they can
be. I think the oldest receivers did not always store and process that
information, but relied on getting it live during operation, versus the new
receivers that have more onboard memory, backup power, and more advanced
calculations.
 As a user, you never see this information (or the 14-minute "cycle") and
you never are aware of it. In order to have faith in a GPS position when you
don't know whether your GPS has this data current, you can just make sure it
has been on and working for longer than 15 minutes. A zero-warmup would
be just as good--if you knew you GPS had recent information in it. Which, I
think, all the newer ones do. Perhaps someone else can be more definite
about that.


There were also some errors in the old/original units because of the way
they calculated cumulative time on the system, they counted up to 1024 weeks
and then rolled over to start at 0 weeks again? And some failed to restart
properly. Again, the rollover problem was one of inadequate programing that
new units should not suffer from. (Ha.)

"Ignore the chip behind the curtain." 

   

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