Is there utility in rigorous combinations of VLBI and GPS Earth orientation parameters? |
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| Authors: | Jim Ray Jan Kouba Zuheir Altamimi |
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| Institution: | (1) National Geodetic Survey& Bureau International des Poids et Mesures, 1315 East-West Hwy, Silver Spring, MD 20910-3282, USA;(2) Natural Resources Canada, 615 Booth Street, Ottawa, K1A 0E9, Canada;(3) Institut Géographique National, Paris, France |
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| Abstract: | Combinations of station coordinates and velocities from independent space-geodetic techniques have long been the standard
method to realize robust global terrestrial reference frames (TRFs). In principle, the particular strengths of one observing
method can compensate for weaknesses in others if the combination is properly constructed, suitable weights are found, and
accurate co-location ties are available. More recently, the methodology has been extended to combine time-series of results
at the normal equation level. This allows Earth orientation parameters (EOPs) to be included and aligned in a fully consistent
way with the TRF. While the utility of such multi-technique combinations is generally recognized for the reference frame,
the benefits for the EOPs are yet to be quantitatively assessed. In this contribution, which is a sequel to a recent paper
on co-location ties (Ray and Altamimi in J Geod 79(4–5): 189–195, 2005), we have studied test combinations of very long baseline
interferometry (VLBI) and Global Positioning System (GPS) time-series solutions to evaluate the effects on combined EOP measurements
compared with geophysical excitations. One expects any effect to be small, considering that GPS dominates the polar motion
estimates due to its relatively dense and uniform global network coverage, high precision, continuous daily sampling, and
homogeneity, while VLBI alone observes UT1-UTC. Presently, although clearly desirable, we see no practical method to rigorously
include the GPS estimates of length-of-day variations due to significant time-varying biases. Nevertheless, our results, which
are the first of this type, indicate that more accurate polar motion from GPS contributes to improved UT1-UTC results from
VLBI. The situation with combined polar motion is more complex. The VLBI data contribute directly only very slightly, if at
all, with an impact that is probably affected by the weakness of the current VLBI networks (small size and sparseness) and
the quality of local ties relating the VLBI and GPS frames. Instead, the VLBI polar motion information is used primarily in
rotationally aligning the VLBI and GPS frames, thereby reducing the dependence on co-location tie information. Further research
is needed to determine an optimal VLBI-GPS combination strategy that yields the highest quality EOP estimates. Improved local
ties (including internal systematic effects within the techniques) will be critically important in such an effort. |
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| Keywords: | Earth orientation Reference frames Multi-technique combinations GPS VLBI |
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