First frequency phase transfer from the 3 mm to the 1 mm band on an Earth-sized baseline

TL;DR

This paper demonstrates the first successful frequency phase transfer from 3 mm to 1 mm wavelengths on an Earth-sized baseline, enhancing VLBI coherence at millimeter wavelengths and enabling improved high-frequency radio interferometry.

Contribution

It provides the first empirical validation of FPT at ~1 millimeter wavelength using simultaneous dual-frequency VLBI observations, including paired-antenna FPT with co-located telescopes.

Findings

Strong correlation between phases at 86 and 215 GHz confirming atmospheric fluctuation models.

FPT systematically improves coherence at 215 GHz across all timescales.

Demonstrates feasibility of paired-antenna FPT for future VLBI arrays.

Abstract

Frequency Phase Transfer (FPT) is a technique designed to increase coherence and sensitivity in radio interferometry by making use of the non-dispersive nature of the troposphere to calibrate high-frequency data using solutions derived at a lower frequency. While the Korean VLBI Network has pioneered the use of simultaneous multi-band systems for routine FPT up to an observing frequency of 130 GHz, this technique remains largely untested in the (sub)millimeter regime. A recent effort has been made to outfit dual-band systems at (sub)millimeter observatories participating in the Event Horizon Telescope (EHT) and to test the feasibility and performance of FPT up to the observing frequencies of the EHT. We present the results of simultaneous dual-frequency observations conducted in January 2024 on an Earth-sized baseline between the IRAM 30-m in Spain and the JCMT and SMA in Hawai`i. We performed simultaneous observations at 86 and 215 GHz on the bright sources J0958+6533 and OJ287, with strong detections obtained at both frequencies. We observe a strong correlation between the interferometric phases at the two frequencies, matching the trend expected for atmospheric fluctuations and demonstrating for the first time the viability of FPT for VLBI at a wavelength of $\sim$1 millimeter. We show that the application of FPT systematically increases the 215 GHz coherence on all averaging timescales. In addition, the use of the co-located JCMT and SMA as a single dual-frequency station demonstrates the feasibility of paired-antenna FPT for VLBI for the first time, with implications for future array capabilities (e.g., ALMA sub-arraying and ngVLA calibration strategies).