Astrometry and geodesy with radio interferometry: experiments, models, results

Abstract

Interferometry at radio frequencies between Earth-based receivers separated by intercontinental distances has made significant contributions to astrometry and geophysics during the past three decades. Analyses of such very long baseline interferometric (VLBI) experiments now permit measurements of relative positions of points on the Earth's surface and of angles between celestial objects at levels of better than one cm and one nanoradian, respectively. The relative angular positions of extragalactic radio sources inferred from this technique presently form the best realization of an inertial reference frame. This review summarizes the current status of radio interferometric measurements for astrometric and geodetic applications. It emphasizes the theoretical models that are required to extract results from the VLBI observables at present accuracy levels. An unusually broad cross section of physics contributes to the required modeling. Both special and general relativity need to be considered in properly formulating the geometric part of the propagation delay. While high-altitude atmospheric charged-particle (ionospheric) effects are easily calibrated for measurements employing two well-separated frequencies, the contribution of the neutral atmosphere at lower altitudes is more difficult to remove. In fact, mismodeling of the troposphere remains the dominant error source. Plate tectonic motions of the observing stations need to be taken into account, as well as the nonpointlike intensity distributions of many sources. Numerous small periodic and quasiperiodic tidal effects also make important contributions to space geodetic observables at the centimeter level, and some of these are just beginning to be characterized. Another area of current rapid advances is the specification of the orientation of the Earth's spin axis in inertial space: nutation and precession. Highlights of the achievements of very long baseline interferometry are presented in four areas: reference frames, Earth orientation, atmospheric effects on microwave propagation, and relativity. The order-of-magnitude improvement of accuracy that was achieved during the last decade has provided essential input to geophysical models of the Earth's internal structure. Most aspects of VLBI modeling are also directly applicable to interpretation of other space geodetic measurements, such as active and passive ranging to Earth-orbiting satellites, interplanetary spacecraft, and the Moon. [S0034-6861(98)

On the estimation of a celestial reference frame in the presence of source structure

“…In geodetic/astrometric analysis (e.g. Sovers et al 1998;Schuh & Böhm 2013), the group delay τ is the main observable, defined as the difference in signal arrival time at the two antennas of a baseline. This delay is determined in the correlation process, as a combination of the measurements recorded in multiple frequency channels.…”

“…Besides contributing to the overall error budget of VLBI measurements as a systematic delay depending on the relative orientation of the source and the VLBI baseline (e.g. Sovers, Fanselow & Jacobs 1998), a temporal change in the brightness distribution of a source might also result in an apparent motion of this source (Ma et al 1998). Fey & Charlot (1997) showed that the size of the source structure (as, for example, determined by the SI) correlates with both the ICRF position accuracy and also with the magnitude of observed time variations in the source coordinates.…”

Testing impact of the strategy of VLBI data analysis on the estimation of Earth Orientation Parameters and station coordinates

“…VLBI can also be used to create and improve ionosphere models and validate tropospheric parameters derived from other space geodetic techniques (Schuh & Behrend, 2012). It also furnish information about relativistic bending (Sovers et al, 1998), gravitational deflection of radio waves, rudimentary details of radio-source structure, Earth's long term motion in inertial space (galactic rotation) or Love numbers (Krasna, 2013). Therefore it is required to maintain all technical and computational aspects of the technique at the highest possible level (Petrachenko et al, 2013).…”

Terrestrial monitoring of a radio telescope reference point using comprehensive uncertainty budgeting

“…During a geodetic VLBI session, a network of several radio telescopes observes a number of extragalactic radio sources to achieve interferometric measurements [e.g. Sovers et al (1998)]. The radio telescopes are located far away from each other, often on different tectonic plates, and thus form very long baselines.…”