[meteorite-list] Can you give me a second? [2] (GPS, a kiss & time)

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Tue Dec 27 15:10:54 2005
Message-ID: <2c3.10ee72d.30e2f9c4_at_aol.com>

Hello, this will only take a second...

I received some very kind feedback for the post on the "leap second" from a
few nice list members, thanks!

First, let me provide the link showing the memo from the Men-In-Black - the
ones that decided this past summer that the world need to wait a second:
ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat
Just love that memo, directed "To the Authorities Responsible")

One list member who chooses to remain anonymous, pointed out that GPS clocks
are automatically updated for the "leap second", so there should never be a
problem. I don't get Sky & Tel. anymore, and he volunteered a link for a
recent article on this very same subject. Wish I had seen it before. The
article was great though I thought, a little misleading on this point:
http://reference.aol.com/space/skyandtelescope?id=20051221113209990001

There was another article on the S&T web site, but, this was even more
misleading when it said:
"If everything goes right, personal computers, GPS receivers, and
radiowave-updated clocks around the world will automatically adjust themselves. But
there is some question about this, because the last time a leap second was added
was on December 31, 1998, before most of today's computers were
manufactured."
see: http://skyandtelescope.com/aboutsky/pressreleases/article_1645_1.asp

My original statement stands (though not as generally as I originally
thought):
"You're GPS unit might not show you the proper time you think it does. This
is because GPS has not put any of the leap seconds into its epoch (time
reference point), which was in January of 1980. Internally, GPS units are not
using Universal Time, they are 13 seconds fast due to the sum of the "leap
seconds" since 1980. The GPS time is the first time it acquires. Not always a
good idea to set your atomic clock by this because:

When and why would your GPS not show the right time? Well, that happens
whenever you loose your GPS almanac:

Not used for a long while.
Not stored with batteries that keep the memory.
Moved more than ~250 kilometers.

If your almanac is not in there, it takes at least 12.5 minutes to download
this signal from the satellites. Meanwhile, you get GPS time as a default,
which is 13 seconds faster - until your GPS is orientated:). It could be
worse if you happen to lock on to an unhealthy satellite, too. The Men-In-Black
disable the time correction information if they are making an orbital
correction and they call the satellites unhealthy. Sick satellites are more
frequent than you think. So your GPS has to find a new satellite to get this info
from.

Once the almanac is downloaded, a Universal Time correction for the "leap
seconds" is also added to the calculation for the time your GPS displays. I
just tried my GPS and compared it to the US official clock at www.time.gov
My GPS is two seconds slower. That would indicate it has its almanac
loaded. Well, there is a fraction of a second uncertainty on the web, but why is
it slower by even 2 sec? Ans: The software that displays the time is not that
fast. It's a Trimble unit I've had since 1995, so I am not complaining. It
now has a mental 2-second software correction understood.

Finally, and to better understand the uncertainties in dealing with an
atomic clock, why not check out Doug's Atomic Clock. It's not me, but I sure
don't mind being identified with my namesake. Here is my tocayo's web page where
he shows off the personal Cesium Atomic clocks HE BUILT, and evaluates the
uncertainty that was associated with the SA (selective availability), and
other things, mucking up accuracy of our units from the collaboration of the
Men-In-Black with the Pentagon:
http://www.niceties.com/atomic.html

So the bottom line, is to bottom up for the extra second like Ben would -
drinking French Champagne (sparkling apple cider for those against drinking) in
Paris, London or New York. And it is also not to set your second hand on
your watch when you first see the time on your GPS unit when its been a while or
you travel away on meteorite hunting expedition. If you think one second is
no big deal, just think,, for every nano-second of imprecision, your GPS
unit loses a meter!! (Not a METEOR, thank goodness, that's just a METER), who
needs GPS anyway;-)

Saludos, Doug
Received on Tue 27 Dec 2005 03:10:44 PM PST