On the use of water vapour radiometry for assessment of wet delay estimates from space geodetic techniques

Conference poster, 2023

During the development of the Mark III VLBI system in the seventies, water vapour radiometers (WVR) were envisaged to provide independent observations of the signal propagation delay due to water vapour along the line of sight. The standard design of the WVR is to measure the atmospheric emission at two frequencies, close to and further away from the centre of the water vapour emission line at 22.2 GHz. These measurements are used to
estimate two unknowns, the amount of water vapour, or the wet delay, and the amount of liquid water, along the line of sight. The main drawback of using a WVR is that the retrieval algorithm requires that any drops of liquid water in the sensed volume of air are much smaller than the wavelength observed by the WVR, i.e. approximately 1 cm. The algorithm therefore more or less breaks down during rain, meaning that the instrument cannot be relied on for 100 % of time, unless it never rains on, or close to, the site. The method generally used to avoid using WVR data with poor accuracy is to ignore observations obtained during rain and when the inferred liquid water content is above a specific threshold. However, there are a couple of difficulties with these procedures. (i) There may be rain drops in the sensed atmospheric volume in spite of the fact that no drops are detected at the ground on the site; (ii) there may still be drops of water on the WVR instrument, such as on the protective covers of the horn antennas and the mirrors many minutes after the rain has stopped; (iii) a low density of large drops may result in a smaller liquid water content than many small drops.
We have used WVR data from 2022 together, with rain observations, to study the retrieval accuracy by comparing them to wet delay estimates from the GNSS station ONSA. We search for general rules of thumb searching for periods when WVR and GNSS data offer the best agreement in the equivalent zenith wet delay, given the rain observations and the inferred liquid water content.