The Impact of Frequency Standards on Coherence in VLBI at the Highest Frequencies

TL;DR

This study uses simulations to evaluate how different frequency standards, especially cryocooled sapphire oscillators versus traditional H-masers, affect coherence and sensitivity in high-frequency VLBI observations, highlighting significant potential improvements.

Contribution

It demonstrates the advantages of replacing H-masers with cryocooled sapphire oscillators for VLBI at frequencies above 175 GHz, especially with atmospheric correction techniques.

Findings

Cryocooled sapphire oscillators improve sensitivity at >175 GHz.

Replacing H-masers can increase sensitivity by 20-40% at 350 GHz.

Atmospheric correction with Water Vapour Radiometers further enhances sensitivity by 60-120%.

Abstract

We have carried out full imaging simulation studies to explore the impact of frequency standards in millimeter and sub-millimeter Very Long Baseline Interferometry (VLBI), focusing on the coherence time and sensitivity. In particular, we compare the performance of the H-maser, traditionally used in VLBI, to that of ultra-stable cryocooled sapphire oscillators over a range of observing frequencies, weather conditions and analysis strategies. Our simulations show that at the highest frequencies, the losses induced by H-maser instabilities are comparable to those from high quality tropospheric conditions. We find significant benefits in replacing H-masers with cryocooled sapphire oscillator based frequency references in VLBI observations at frequencies above 175 GHz in sites which have the best weather conditions; at 350 GHz we estimate a 20-40% increase in sensitivity, over that obtained when the sites have H-masers, for coherence losses of 20-10%, respectively. Maximum benefits are to be expected by using colocated Water Vapour Radiometers for atmospheric correction. In this case, we estimate a 60-120% increase in sensitivity over the H-maser at 350 GHz.