Speaker
Dr
Ed Fomalont
(NRAO)
Description
The definition of a precise inertial reference frame is fundamental to
the determination of the location and motion of objects in the sky
(astrometry) and features on the earth (geodesy). Since most
observing instruments are on the earth surface or in earth orbit, the
earth rotation, motion and crustal distortions add considerable
complexity to the determination of astrometric and geodetic
parameters.
After the discovery in the 1960's of quasars---bright radio compact
objects in the center of giant galaxies that are less than
one-millarcsecond (mas) (5 nanoradian) in angular size and exceedingly
distant (>10**21 km)---astronomers realized that these objects could
form the backbone of the nearly quasi-reference system that was more
accurate than previous systems.
The majority of the radio data comes from Very Long Baseline
Interferometry (VLBI) that has mas-resolution, and has been
incorporated with Global Positioning Satellite (GPS) and Laser ranging
technologies. The ICRS is the name of the overall system dealing with
the data collection, its analysis and interpretation, and is described
here. Nearly forty years of data collecting and its analysis have
produced a quasi-inertial reference frame that is stable in direction
to about 0.02 mas, positions of thousands of radio sources that are
accurate to 0.05 mas, earth orientation accuracy to 0.1 mas, earth
rotation accuracy of 0.01 msec, polar motion accuracy of < 1cm, and
position/motion of radio telescopes to an accuracy of a about 3 mm per
year. The major limitations to the ICRF accuracy are the tropospheric
and ionospheric refraction, and the variable structure of the radio
emission from quasars.
Primary author
Dr
Ed Fomalont
(NRAO)