An array of GPS receivers can map the variations in the atmosphere. VLBI contributes its long history of measurements, several times better accuracy from a single day's measurement, stable terrestrial reference frame, and ability to measure the celestial reference frame and long-term UT1, fundamental for all satellite tracking. GPS contributes a lower operational cost, dense networks for deformation zone monitoring, flexibility to go where measurements are needed, and continuous temporal coverage. VLBI provides Earth orientation data which are an integrated measurement of the Earth's response to numerous forces. Geophysicists use VLBI data to extract characteristics of the oceans, atmosphere, and solid Earth.
Numerous phenomena affect the orientation of the Earth.
Looking at everything possible with a sharply improved capability is a good prescription for making important contributions.