Subject: [IGSMAIL-5167]: ocean tide loading model
From: Jim Ray "(NGS" 301-713-2850 "x112)" <[email-masked]>
To: [email-masked], [email-masked], [email-masked], [email-masked]
Cc: [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked], [email-masked]
Date: Fri, 24 Jun 2005 08:34:00 -0400 (EDT)

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IGS Electronic Mail      24 Jun 05:51:36 PDT 2005      Message Number 5167
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Author: Jim Ray

Dear Analysis Colleagues,

According to the IERS Conventions, the instantaneous position of a
terrestrial point can be modeled apriori as a function of time, t, by

     X_m(t) = X_o + V_o*(t - t_o) + SUM_i{dX_i(t)}

where X_o and V_o are regularized coordinates and velocities (such as ITRF)
at the reference epoch, t_o, and the summation includes various predictable
motions, mostly tidal with periods near 12 and 24 hours.  The largest tidal
effect is that due to the solid Earth (body tide), with amplitudes in the
few-decimeter range.  The development of the IERS model for the solid Earth
tide is very comprehensive but easily implemented using a subroutine kindly
provided by V. Dehant and S. Mathews.

The next largest tidal displacement, up to several centimeters, is due to
ocean loading variations.  Other effects that should conventionally be
included in the summation above are the solid Earth pole tide (well modeled
in the IERS Conventions), pole tide due to the oceans (IERS model under
development), S1 and S2 atmospheric pressure loading (IERS model under
development), and tidal geocenter motion (treatment unclear in Conventions).

The IERS recommendations for ocean loading are so general and vague on
critical points that it cannot be said that a "conventional model" exists.
It is recommended that users adopt the site-specific amplitudes and phases
for the 11 largest partial tides generated by the website of M.S. Bos and
H.-G. Scherneck (www.oso.chalmers.se/~loading/), preferably using the
GOT00.2 tidal model of R. Ray.  But a specific implementation to compute
site displacements is lacking.

This is a request for coded implementations of a modern ocean loading
algorithm to be considered by the Advisory Board for IERS Conventions
Updates for possible future use in the Conventions.  The ideal routine 
should expect a station position and epoch as inputs and return the NEU
displacements.  If the Bos-Scherneck coefficients are not used, this
should be clearly explained and justified.  The code should be adequately
documented internally, preferably with supporting references and any
limitations explicitly described.  Modulation of the main tides due to
lunar nodal variation or some other interpolation method to account for
lesser tides is recommended.  On the other hand, the development should
not be so elaborate that it is unnecessarily inefficient, considering
realistic errors for the tidal amplitudes and phases.  Any additional
information you can provide to support your implementation would be
appreciated.

Please send submissions to me at your earliest convenience.  Thanks very
much for your help.

--Jim Ray (Advisory Board chair)