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

[MexicoDoug 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