JWG1.1: Tie vectors and local ties to support integration of techniques

Terms of Reference

Tie vectors are crucial elements of the combination of space geodetic solutions. Their accuracy straightforwardly refers to their capability of expressing the relative position of the co-located space geodetic instruments to conventional reference points. They enter the combination of space geodetic solutions as additional observations and can be eventually regarded as the fifth technique involved in the combination. Their role is not strictly limited to the provision of the necessary information for the combination since accurate tie vectors can also highlight the presence of technique- and/or site-specific biases. In particular, tie vectors can support the investigation of technique-dependent systematic errors and the ability of each technique to connect electronic and conventional reference point together.

The whole process related to the planning of on-field operations and the surveying carried out with either terrestrial or GPS methods (i.e. the local tie procedure) needs to be constantly revised and when necessary improved. The definition of new local tie approaches remains central to the activity promoted by the working group. As a group of experts, the working group aims at promoting discussion and at serving as forum for the evaluation of existing as well as new procedures. Equally important, the working group aims at improving the existing analysis strategies applied to the local tie observations and to the data post-processing. The ultimate scope is to achieve the utmost level of accuracy of the tie vector. The consistency between tie vectors and space geodetic solutions is recognized as a crucial aspect on which the improvement of the International Terrestrial Reference Frame depends.

An important by-product of the combination of space geodetic solutions is represented by the post-fit residuals. They are empirically determined during the combination process and express the discrepancies between space geodetic solutions and tie vectors. Particularly, the values of the post-fit residuals of the combination can serve as preliminary indicators of the quality of the space geodetic solution and/or tie information at one specific site. As such, they can highlight the necessity to investigate further the co-location and the techniques involved in the tie. This specific use of post-fit residuals is extremely interesting and promising although their handling requires much attention. Many factors may combine to the final post-fit residual value. The accuracy of the tie vectors has to be certain to effectively use the co-location post-fit residuals for inferring about site dependent or technique dependent systematic errors. This clearly highlights the necessity to tempt a revision of the quality of the existing tie vectors and possibly verify their formal accuracy not only empirically, on the base of the post-fit residuals themselves, but on the base of other factors such as their age and their length.

The working group should serve as reference for the know-how and the good practice on co-location issues for the whole geodetic community. It should be ready to support new survey activities, assisting and advising remotely, and it should promote re-surveying if necessary. The link with the relevant branches of GGOS has to be ensured during the terms of the working group activity. Finally, the link between the working group and the single IAG technique services is considered essential and a permanent flow of relevant information must be constantly pursued.

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