The lower part of the atmosphere (troposphere) induces delays on GNSS signals that can be estimated during the positioning process. Since most of the atmospheric water vapor is contained in the tropospheric layers, generally within the first 10 km of altitude from the ground, it is possible to estimate the amount of precipitable water vapor (PWV) from the analysis of GNSS tropospheric delays: the estimated signal delay due to the tropospheric refractivity along each receiver-satellite line-of-sight, or slant total delay (STD), is mapped to the zenith direction to retrieve the zenith total delay (ZTD), which is in turn divided in its hydrostatic and wet components, in order to estimate the PWV over a GNSS station antenna. The water vapor distribution and its variability can therefore be monitored by employing a network of continuously operating stations.



Concept of Zenith Total Delay (ZTD) estimated from several Slant Total Delays (STD) in order to retrieve PWV (image source: Ivan Reguzzoni, Master’s degree thesis, Politecnico di Milano)

Monitoring the temporal and spatial variability of PWV, especially on a local scale, is deemed to be a fundamental step in improving the predictions made by mesoscale numerical weather prediction models, for example to improve the forecasting of local torrential rain. GNSS provides a way to monitor PWV that is continuous in time (contrary to radiosondes) and not adversely affected by meteorological conditions (contrary to microwave radiometers). Even the densest regional GPS networks, however, have inter-station distances of the order of tens of kilometers, which makes them unsuitable for the accurate detection of local fluctuations of water vapor. A densification of existing GPS networks, at least in the vicinity of urban areas, is necessary for the provision of reliable and continuous water vapor monitoring infrastructures with a sufficiently high horizontal resolution.



Example of localized heavy rainfall

GReD studies and implements solutions for retrieving PWV from (dense) GNSS receiver networks, in order to support the forecast of severe local rain events.

Relevant publications (with contributions by current members of GReD):

  • An observation campaign of precipitable water vapor with multiple GPS receivers in western Java, Indonesia (2014, Progress in Earth and Planetary Science – link)
  • Numerical Simulation on Retrieval of Meso-γ Scale Precipitable Water Vapor Distribution with the Quasi-Zenith Satellite System (QZSS) (2013, Journal of the Meteorological Society of Japan – link)
  • A High-Resolution, Precipitable Water Vapor Monitoring System Using a Dense Network of GNSS Receivers (2013, Journal of Disaster Research – link)
  • Analysis of the Temporal and Spatial Variability of the Wet Troposphere at a Local Scale by High-rate PPP Using a Dense GNSS Network (2012, Proceedings of ION GNSS 2012 – link)