Optimal VLBI baseline geometry for UT1-UTC Intensive observations

Area of Science:

• Geodesy and Geophysics
• Astronomy and Astrophysics
• Earth Rotation and Dynamics

Background:

• Very Long Baseline Interferometry (VLBI) is crucial for determining Earth's rotation, specifically the difference between Universal Time UT1 and Coordinated Universal Time UTC (dUT1).
• Daily "Intensives" sessions, lasting one hour on single baselines, are conducted for rapid dUT1 estimation.
• Previous understanding favored long east-west baselines for maximizing dUT1 accuracy.

Purpose of the Study:

• To re-evaluate and challenge the conventional understanding of optimal baseline geometry for VLBI Intensives.
• To determine the ideal length and orientation of baselines for accurate dUT1 estimation.
• To develop a more robust metric for assessing Intensive baseline suitability.

Main Methods:

• A comprehensive simulation study involving nearly 3000 artificial baselines across a regular grid.
• Analysis of partial derivatives of observed group delays with respect to dUT1.
• Investigation of baseline length, orientation, and inclination relative to the equatorial plane.

Main Results:

• Long east-west baselines are not universally optimal; optimal orientation depends on specific lengths and may require inclination.
• Equatorial baselines are less suitable due to limited source visibility and derivative variety.
• North-south baselines can yield accurate dUT1 if oriented away from the Earth's rotation axis, requiring precise polar motion data.

Conclusions:

• The optimal geometry for VLBI Intensives is more complex than previously assumed, requiring consideration of baseline inclination.
• A new metric based on the effective spread of partial derivatives provides a better assessment of baseline suitability.
• These findings will refine strategies for Earth rotation monitoring using VLBI.