A Study of the Compact Water Vapor Radiometer for Phase Calibration of the Karl G. Janksy Very Large Array

TL;DR

This paper reports laboratory test results of a five-channel Compact Water Vapor Radiometer prototype designed for phase calibration of the VLA, demonstrating it meets key specifications for stability, isolation, and dynamic range, with future testing planned on antennas.

Contribution

The study presents the design and laboratory validation of a new five-channel CWVR prototype that meets the VLA's phase calibration requirements, advancing water vapor radiometry technology.

Findings

Channel isolation requirement of < -20 dB met

Gain stability corrected for temperature fluctuations, < 2 x 10^-4

Prototype meets specifications for dynamic range and stability

Abstract

We report on laboratory test results of the Compact Water Vapor Radiometer (CWVR) prototype for the NSF's Karl G. Jansky Very Large Array (VLA), a five-channel design centered around the 22 GHz water vapor line. Fluctuations in precipitable water vapor cause fluctuations in atmospheric brightness emission, which are assumed to be proportional to phase fluctuations of the astronomical signal seen by an antenna. Water vapor radiometry consists of using a radiometer to measure variations in the atmospheric brightness emission to correct for the phase fluctuations. The CWVR channel isolation requirement of < -20 dB is met, indicating < 1% power leakage between any two channels. Gain stability tests indicate that Channel 1 needs repair, and that the fluctuations in output counts for Channel 2 to 5 are negatively correlated to the CWVR enclosure ambient temperature, with a change of ~ 405 counts per 1 degree C change in temperature. With temperature correction, the single channel and channel difference gain stability is < 2 x 10^-4, and the observable gain stability is < 2.5 x 10^-4 over t = 2.5 - 10^3 sec, all of which meet the requirements. Overall, the test results indicate that the CWVR meets specifications for dynamic range, channel isolation, and gain stability to be tested on an antenna. Future work consists of building more CWVRs and testing the phase correlations on the VLA antennas to evaluate the use of WVR for not only the VLA, but also the Next Generation Very Large Array (ngVLA).