[LEAPSECS] Leap smear

[Steve Allen]

On 2011 Sep 29, at 15:25, Paul J. Ste. Marie wrote:

> Both GPS and VLBI require knowledge of UTC and UT1 for implementation.

> How does the GPS pick up the UT1 corrections?

> VLBI, at least when I was involved with it many years ago, depends on the IERS circular.


UT1 is required as part of the orbit model for GPS satellites.
UT1 is required as part of the correlation for VLBI in order to
ensure that you get fringes.

That said, GPS is used to determine short period variations in UT1
because the orbit model is reasonably stable over a short period and
GPS measurements are cheap.  For the longer periods, and for the
accuracy, VLBI is the means of determining UT1.  The VLBI sessions
are done at longer intervals, but they serve as absolute and precise
reference for the GPS orbits and the earth's crust in order to remove
systematic drifts in those models.  Add in LLR to tie those reference
frames together with the moon.  At longer intervals still, the
same DSN antennae which do VLBI also track spacecraft in orbit around
the other planets, and that ties everything in the solar system
with respect to the quasars that define the universe of ICRF.
                                                                             
--
Steve Allen               <sla at ucolick.org>              WGS-84 (GPS)
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