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.
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.
Author
Gunnar Elgered
Chalmers, Space, Earth and Environment, Geoscience and Remote Sensing
Tong Ning
The Swedish Mapping, Cadastral and Land Registration Authority
The 26th Meeting of the European VLBI Group for Geodesy and Astrometry (EVGA)
Kötzting, ,
Kötzting, ,
Subject Categories
Other Earth and Related Environmental Sciences
Roots
Basic sciences
Infrastructure
Onsala Space Observatory
DOI
10.5281/zenodo.8064170