Geophysical applications of very-long-baseline interferometry

Abstract

Very-long-baseline interferometry (VLBI) is a novel observing technique for measuring the relative positons of widely separated points on the surface of the Earth with centimeter-level accuracy. Such accuracy is two or three orders of magnitude better than was available with classical techniques only a few decades ago. This enormous improvement in accuracy has opened up for study a broad new spectrum of geophysical phenomena. The new measurements allow direct observation of the tectonic motions and deformations of the Earth's crustal plates, observations of unprecedented detail of the variations in the rotation of the Earth, and direct measurement of the elastic deformations of the Earth in response to tidal forces. These new measurements have placed significant constraints on models of the interior structure of the Earth; for example, measurements of the variations in the Earth's nutation have been shown to be particulary sensitive to the shape of the core-mantle boundary. The VLBI measurements, coupled with other space-based geodetic observing techniques such as the Global Positioning System, allow construction of a global reference frame accurate at the centimeter level. Such a frame will be essential to studying longterm global changes, especially those changes related to sea-level variations as recorded by tide gauge measurements.

VLBI: A fascinating technique for geodesy and astrometry

“…The effect of large planets such as Jupiter and Saturn is also appreciable. At an elongation angle of 2.5 • to the Sun, which was the minimal angle of VLBI observations until 2002, the differential deflection reaches 150 milliarcseconds (Robertson, 1991) causing a significant effect on the observed group delays. With respect to the noise floor of the source coordinates of about 40 as for the ICRF2 (Fey et al, 2009), analysis of source observations in the vicinity of the Sun allows determining the post-Newtonian parameter ('light deflection parameter'), which characterizes the space curvature due to gravity (see, e.g., Heinkelmann and Schuh, 2010).…”

“…The idea that underlies geodetic VLBI observations is conceptually quite simple: the fundamental observable is the difference in arrival times (time delay) of a signal from an extragalactic radio source received at two (or more) radio observatories (Robertson, 1991). This observable only depends on fundamental physics as it is derived from a realization of the atomic second and a clock synchronization convention.…”

“…The state-of-theart of the VLBI techniques at the end of the last century along with its historical development is described in a number of review papers. The interested reader is referred to Robertson (1991), Sovers et al (1998), or Ryan and Ma (1998) and further references therein.…”

Earth Rotation Variations – Long Period

“…The VLBI technique measures the difference in the arrival time of a radio signal at two or more radio telescopes that are simultaneously observing the same distant extragalactic radio source (Lambeck, 1988, Chapter 1.08;Robertson, 1991;Sovers et al, 1998). The VLBI technique measures the difference in the arrival time of a radio signal at two or more radio telescopes that are simultaneously observing the same distant extragalactic radio source (Lambeck, 1988, Chapter 1.08;Robertson, 1991;Sovers et al, 1998).…”

Atmosphere and Earth's rotation

“…Since the moment of inertia of the system mantle-crust is only slightly influenced by atmospheric pressure loading, this requires mainly a change of the angular velocity of the solid Earth, i.e., a change of the length of the day (LOD). Presently, LOD can be measured to a high accuracy with integration times of only a few hours (e.g., Robertson, 1991), and general circulation models of the atmosphere allow a determination of changes in the AAM (e.g., Barnes et al, 1983;Eubanks et al, 1985). A comparison between changes in AAM and LOD shows that both are indeed highly correlated (see Figure 5).…”