Innovations 1983 and 1996

Nov 2005 | Comments Off on Innovations 1983 and 1996

Tidal Modeling for GPS Positioning –

In 1983, International Association of Geodesy (IAG) took a step into the future to adopt its Resolution 16:

“The International Association of Geodesy (IAG), recognizing the need for the uniform treatment of tidal corrections to various geodetic quantities such as gravity and station positions, and considering the reports of the Standard Earth Tide Committee and S.S.G. 2.55, recommends that:

• the rigid Earth model be the Cartwright – Taylor – Edden model with additional constants specifi ed by the International Centre for Earth Tides,
• the elastic Earth model be that described by Wahr using the 1066 A model Earth of Gilbert and Dziewonski,
• the indirect effect due the permanent yielding of the Earth be not removed, and
• ocean loading effects be calculated using the tidal charts and data produced by Schwiderski as working standards”.

In 1996, International Earth Rotation Service (IERS) in its Tech Notes No. 21 stated:

“To account for the effect of the permanent tide, terrestrial reference frames may be defi ned as Zero-tide Permanent or ‘zero frequency tide’ is retained. The crust corresponds to the realistic time average, which varies with the luni-solar tides. Tide-free All effects of permanent tide are removed. This is not realistic since the crust cannot be observed. Mean-tide This a “tidefree” system except the geoid is modeled with permanent tide effects.

From 2001 to 2003, I contacted Professors W. Torge, E.Groten, G. Seeber, and M. Vermeer on the IAG Resolution 16. They all agree that the present computational approach, which recommends the “Zero-tide” modeling, is still to be followed. To this view point, if we bring in the IERS defi nitions as above, it seems completely impractical and non-scientifi c to overlook IAG and IERS and follow any other practice. The most surprising aspect is that nobody knows who is that Authority in tidal modeling, who formulated the present practice of computing in the “Tidefree” non-realistic and not-observable environment.

Muneendra Kumar
Ph.D. is Chief Geodesist
(Retired), US National
Geospatial Intelligence Agency

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