The ngVLA Band-4 Water Vapor Radiometry Concept

Abstract

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.

Figures (6)

• Figure 1: Schematic representations of the approximate response regions and interrelationships of the standalone WVR (gray areas with dotted boundaries), Band-4 WVR (light blue areas with dashed line boundaries), and the Band 5 & 6 science feeds (open areas with solid boundaries) in the troposphere. The left panel with three sections depicts the cross sections of the response regions at three heights: the aperture, a representative near-field distance for the 1.5 m standalone antenna of $\sim$ 600 m, and a representative phase screen distance of $\sim$ 1200 m, approximately to scale. The right most panel is a schematic 3-D representation, not to scale. The Band-4 WVR response region is more representative the response regions of the science feeds in the lower layers of the troposphere.
• Figure 2: Preliminary ngVLA frontend layout (dimensions in mm) Sturgis2021FE_Layout_DWG_Draft. The distances in the current version of the layout are marked in yellow. The proposed locations for the feeds are labeled in green. Band-4 feed is already at the center.
• Figure 3: The Beam pattern of the Band 4 axially corrugated horn at 22 GHz on the ideal ngVLA antenna system. The pattern is shown over several azimuth angles for the horn offset by 0 mm, 200 mm, and 450 mm. The black and red lines are the co- and cross-polarized pattern cuts. The physical lengths at the tropospheric phase screen with an assumed indicative distance of 1.2 km are also marked (adapted from Lehmensiek2022WVROffsetBeamMemo).
• Figure 4: : 2-D beam pattern at 22 GHz for 200 mm Band-4 feed offset. The 17 m distance for the 0.8$^{\circ}$ beam offset is for a tropospheric phase screen height of 1.2 km. The 82% beam efficiency shown is for a circular region of 1$^{\circ}$ radius. The $+$ sign denotes the on-axis science beam direction. (adapted from Lehmensiek2022WVROffsetBeamMemo).
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