[LEAPSECS] proliferation of scales

Steve Allen sla at ucolick.org
Sat Oct 10 13:33:38 EDT 2009

While reviewing the results of CGSIC 49 I was surprised by part of
Lewandowski's summary.
He includes Recommendation CCTF 4 (2009) from the meeting last June.

That recommends the CIPM get the CGPM to adopt the ITRS, thus giving
that terrestrial reference system standing as not just scientific, but

ITRS is defined such that its units of length, time, mass are measured
in the GCRS, a relativistic reference frame co-moving with the
geocenter, but not affected by the mass or rotation of the earth.
Therefore the scale of SI metrology in ITRS is not equal to the scale
of SI metrology used by denizens on the surface of the earth. The
rotation and gravity mean that SI seconds in ITRS tick faster (they
are seconds of TCG, not TT), and SI meters in ITRS are shorter.

The various versions of ITRF through the years are the realized form
of ITRS. A look at the IERS Conventions shows that the first few
instances of ITRF were expressed in the so-called "TCG units", but
later they changed to "TT units" in acknowledgement that the real
chronometers and rulers of all the agencies contributing data were
located on the surface of the earth, not in some idealized co-moving
reference frame.

So if I take this too seriously I suppose that the CCTF just
recommended not only that GPS switch from WGS-84 to ITRF, but that
everyone change the scale of their units of measurement.

The confusion is deeper than that. The IERS Conventions document uses
the terms "TT units" and "TCG units". When I last checked the
literature I saw Sergei Klioner writing papers which challenge the
terminology of "TT units" and "TCG units" (and "TCB units", for the
solar system ephemerides). So not only is there confusion about the
scale of measurement, but also about the terminolgy which underlies
human comprehension of the distinctions being measured.

I guess we live in interesting times, and I remain unclear about
how changing UTC to omit leap seconds will reduce confusion.

Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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