Abstract
Given that the observations from current space geodetic techniques do not carry all the necessary datum information to realize a Terrestrial Reference System (TRS), and each of the four space geodetic techniques has limits, for instance: Very Long Baseline Interferometry (VLBI) ignores the center of mass and satellite techniques lack the TRS orientation, additional constraints have to be added to the observations. This paper reviews several commonly used constraints, including inner constraints, internal constraints, kinematic constraints, and minimum constraints. Moreover, according to their observation equations and normal equations, the similarities and differences between them are summarized. Finally, we discuss in detail the influence of internal constraints on the scale of VLBI long-term solutions. The results show that there is a strong correlation between the scale parameter and the translation parameter introduced by the combination model at the Institut National de l’Information Géographique et Forestière (IGN), and internal constraints force these two groups of parameters to meet certain conditions, which will lead to the coupling of scale and translation parameters and disturbing the scale information in VLBI observations. The minimum or kinematic constraints are therefore the optimum choices for TRF.
Highlights
The International Terrestrial Reference System (ITRS) definition fulfills the following conditions [1]: 1. It is geocentric, and its origin is the center of mass for the whole Earth, including oceans and atmosphere; 2
Union of Geodesy and Geophysics (IUGG) (1991) resolutions. (The mean rate of the coordinate time TCG coincides with the mean rate of the proper time of an observer situated at the geocenter, whereas the mean rate of the terrestrial time (TT) coincides with the mean rate of the proper time of an observer situated on the geoid
Inner constraints include internal constraints and kinematic constraints, and kinematic constraints are equivalent to minimum constraints
Summary
The International Terrestrial Reference System (ITRS) definition fulfills the following conditions [1]: 1. It is geocentric, and its origin is the center of mass for the whole Earth, including oceans and atmosphere; 2. The International Terrestrial Reference System (ITRS) definition fulfills the following conditions [1]: 1. The unit of length is the meter (Le Système International d’Unités (SI)). The scale is consistent with the geocentric coordinate time (TCG) time coordinate for a geocentric local frame, in agreement with the International Astronomical Union (IAU) and the International. (The mean rate of the coordinate time TCG coincides with the mean rate of the proper time of an observer situated at the geocenter (with the Earth removed), whereas the mean rate of the terrestrial time (TT) coincides with the mean rate of the proper time of an observer situated on the geoid. Its orientation was initially given by the Bureau International de l’Heure (BIH). The time evolution of the orientation is ensured by using a no-net-rotation (NNR)
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