Subject: [IGSMAIL-1068] High-frequency polar motion variations from IGS
From: Jan Kouba, Jim Ray, Dennis McCarthy, Marshall Eubanks
Date: Mon, 25 Sep 1995 11:09:46 PDT

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IGS Electronic Mail      Mon Sep 25 11:09:46 PDT 1995      Message Number 1068
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Author: Jan Kouba, Jim Ray, Dennis McCarthy, Marshall Eubanks
Subject: High-frequency polar motion variations from IGS

It has been observed by us and others that GPS-based determinations of polar
motion differ significantly and systematically when compared with multi-
technique EOP combinations.  When compared with the IERS Bulletins A and B,
for instance, the IGS mean of 7 GPS Analysis Centers differs by up to nearly
+/- 1 milliarcsecond with clear periodicities mainly between ~7 and ~14 days.
Results from the various IGS ACs are reasonably consistent with one another.

Because Bull. A and Bull. B are formed by combining independent polar motion
determinations from SLR and VLBI, as well as GPS, the observed differences 
could be interpreted as GPS-based systematic errors.  Systematic errors
of the required magnitude would have obvious and important implications.

However, we are now convinced that the (IGS - IERS combinations) pole
differences are largely caused by smoothings applied in the process of
forming the combined polar motion time series.  Indeed, a direct comparison
of the polar motion excitations computed independently from VLBI and GPS
data shows that those two techniques agree very well since the start of
1993, at roughly the formal error level.

Even more significantly, a preliminary study by Marshall Eubanks indicates
the observed high-frequency polar motion variations are predominantly
atmospherically drive.  The "enchancement" in the retrograde component
(i.e., westward propagating motion) of polar motion, predicted by Eubanks
et al., [in _The Earth's Rotation and Reference Frames for Geodesy and 
Geophysics_, A.K. Babcock and G.A.  Wilkins (eds.), pp. 365-371, 1988],
has now been confirmed geodetically.  Results are being prepared for
publication.

[Mailed From: Jim Ray (NOAA 301-713-2850) <[email-masked]>]