Variance Component Estimation for Combination of Terrestrial Reference Frames

Combining Terrestrial Reference Frames (TRFs) to frames of superior quality (like the ITRF) usually involves homogenisation by an empirical weighting scheme. Different approaches on variance component estimation have been evaluated for this purpose. The statistically rigorous Helmert estimator has been compared with two other methods: the degree of freedom method and a simplified, approximate estimator. Tests have been performed, covering two elementary types of combinations.