The ngVLA Band-4 Water Vapor Radiometry Concept

The ngVLA has adopted a standalone system with a $\sim$~1.5 m diameter antenna and associated ambient temperature receiver and electronics at each antenna as its baseline design for water vapor radiometer (WVR). The WVR is intended to decrease calibration overheads for tropospheric phase correction for the high frequency bands (5 \&6; 30.5-116 GHz) to levels lower than feasible with fast switched reference gain calibration. In this memo, we present an alternative concept utilizing the standard Band-4 science receivers and the main antennas. This would require the Band-4 feed to be placed between those of Bands 5 \&6 resulting in a beam offset of 0.8 deg, similar to the beam size of the standalone 1.5 m antenna. The main benefits of the Band-4 WVR concept are: (1) sensing water vapor in a region more representative of the Band 5 \&6 science beams in the lower layers of the troposphere, in the near-field of the 18 m antenna, than the 1.5 m antenna (2) significantly relaxing the stability requirements due to the lower receiver temperature of the standard Band-4 cryogenic receiver and (3) eliminating the procurement, deployment, and maintenance of a large number of standalone units. Given these advantages, it is recommended that the details and the pros and cons of Band-4 WVR implementation compared a standalone system be carefully considered.