Measuring Precipitable Water Vapor through Dual-Band GPS at Rubin Observatory
Precipitable water vapor (PWV) in Earth's atmosphere is a known source of systematic error in ground-based astronomical measurements, particularly at wavelengths greater than 7,000 angstroms. PWV produces wavelength-dependent extinction in the spectral energy distributions of observed sources. For high-precision applications, such as supernova cosmology, the transmission function must be measured accurately and precisely for each observation. The Vera C. Rubin Observatory is located in the Chilean Andes, a region known for dry conditions. However, even at this location, time-variable absorption by water vapor is a factor. Signal delays from Global Navigation Satellite Systems (GNSS) offer a robust, easy, 24-hour monitoring of PWV. In this paper we present measurements from the first six months of GNSS-PWV at Rubin Observatory. We verified our analysis by replicating results from the International GNSS Service (IGS), demonstrating high consistency. We then installed and commissioned a Trimble Net R9 receiver at Allegheny Observatory for a commissioning run before it was relocated to Rubin Observatory. At Rubin Observatory we have been continuously measuring PWV since 2025 September 5. We find average PWV levels of 3.0 kg/m$^2$ in Southern Hemisphere spring (September to November) and 5.4 kg/m$^2$ in summer (December to February). The PWV at Rubin also has a clear daily variation with an amplitude of 1.2-3.8 kg/m$^2$ that is highly correlated with the east-west wind strength. We conclude that the proposed GNSS-based processing method achieves an accuracy of 0.9 kg/m$^2$ with 0.36 kg/m$^2$ precision, confirming the capability for continuous, high-temporal-resolution PWV monitoring to support demanding LSST science cases.