Background

Very Long Baseline Interferometry (VLBI) observations for geodesy and astrometry have been conducted since about mid--1979. These observations can be used to establish and define an astrometric quasi-inertial Celestial Reference Frame (CRF), positions and velocities of radio antennas that define a VLBI-based Terrestrial Reference Frame (TRF) and the Earth Orientation Parameters (EOP) that link the CRF and the TRF.

• VLBI observations of distant celestial sources, from many stations on different tectonic plates, are combined to estimate ‘Global’ parameters of precise celestial source positions, and station positions and velocities. Positions/velocities that are determined from each observing session independently are ‘local’ parameters. 
• The primary geodetic parameters -- the station positions --are estimated separately for each session in a solution.  Station motions within a day, from solid Earth tides and ocean loading, are derived from unadjusted a priori models. The adjusted "arc" parameters (i.e., parameters that depend only on the data from an individual experiment and are estimated separately for each epoch of observation) include: positions of sources with identified excessive apparent motion or random variation; celestial pole offsets in ecliptic longitude and obliquity to account for errors in the standard precession/nutation models; positions of the stations; the rate of UT1 relative to a good a priori time series; twenty-minute piecewise linear continuous troposphere parameters; tropospheric gradients in the east-west and north-south directions, linear in time, estimated once every 6 hours; quadratic clock polynomials for the gross clock behavior; sixty-minute piecewise linear continuous clock parameters; and necessary nuisance parameters such as clock jumps and baseline clock offsets. The remaining parameters, including source positions, are adjusted as invariant, or "global," quantities (i.e., parameters that are dependent on all data sets and are carried from step to step resulting in a single estimate derived from the combined data of all experiments in the solution).
• The resulting global solution, calculated with Batch CALC/SOLVE, includes most available VLBI observations at 8.4 GHz (X-band) and 2.2 GHz (S-band) from 1979 August 3 up to the current yearly quarter. This typically includes thousands of diurnal (24-hour) sessions encompassing millions of measurements of group delay and delay rate. The weighted root-mean-square residuals of the solution are typically tens of ps in delay and tens of fs/s in delay rate.