Remote sensing of the atmosphere using GNSS
Continuous observations from GNSS receivers provide an excellent tool for studying the earth atmosphere. Nowadays, GNSS is routinely used to
- Observe the Total Electron Content in the ionosphere and above (50-20000km)
- Observe Integrated Water Vapour in the troposphere
Both observations are a must in order to reduce errors in geodetic and geophysical applications; however, both observations are also valuable for atmospheric sciences. The GNSS signals are now routinely used in numerical weather forecasting, atmospheric research, and space weather applications.
Near real-time estimation of water vapour (GNSS Meteorology)
The last five years significant progress has been made in the use of GNSS data for numerical weather prediction and weather forecasting in collaboration with KNMI.
As the GNSS signals travel through the atmosphere they are delayed by roughly 2.5-10 meters, depending on the elevation of the satellite and atmospheric pressure, temperature and humidity. The hydrostatic delay can be predicted quite accurately knowing the surface pressure. Together with a known mapping function, which gives the relation between the vertical and slant delay, the zenith wet delay (ZWD) is computed. The zenith wet delay is related to the Integrated Water Vapour (IWV) or Precipital Water Vapour: the amount of water vapour in a column of the atmosphere, expressed in kg/m2 or mm precipital water vapour.
The Zenith Total Delay, Zenith Wet Delay, or Integrated Water Vapour, can be assimilated into numerical weather prediction models to improve weather forecasting Within the European COST action 716 “Exploitation of ground-based GPS for operational numerical weather prediction and climate applications” it was demonstrated that GNSS ZTD observations has a positive impact on the forecast of precipitation. The 2-3 month demonstration experiment for COST 716 actually developed in a three year long experiment, with at the end of the action in 2004 consisted of over 400 GPS stations delivering data in near real-time for weather forecasting.
More information can be found in the final report of COST-716, available through the COST-716 website http://www.oso.chalmers.se/~kge/cost716.html/.(http://www.oso.chalmers.se/~kge/cost716.html/COST716_FR_Oct27.pdf )
The very successful demonstration experimented continued to be run after the COST action 716, first within the framework of the EU TOUGH project, and later in the framework of the EUMETNET E-GVAP program. The network presently consists of more than 500 stations, including dense networks in the Netherlands, Denmark, UK, Scandinavia, and Germany, and the results are now assimilated operationally by two Meteorological institutes (UK and France), with others to follow suit. The status of the network is monitored by KNMI, who is also one of the GNSS analysis centers. For more information see the EUMETNET GPS Water Vapour Programme (E-GVAP) validation site at knmi http://www.knmi.nl/samenw/egvap/validation/ztd_iwv.html.
Within the European 5th framework project “Targeting Optimal Use of GPS Humidty Measurements in Meteorology” (TOUGH) http://web.dmi.dk/pub/tough/ and a SRON GO-2 project “Resolving spatial and temporal atmospheric water vapour structures using a ground based GPS receiver network” a novel method for the estimation of slant water vapour in the line-of-sight direction was developed, as opposed to estimating water vapour in the zenith direction only. The estimates of slant water vapour were successfully assimilated by KNMI and the Finish Meteorological Institute, showing a positive impact. During these projects we also developed a method to study site-dependent carrier phase multipath and antenna characterization. The application of this technique goes beyond GPS meteorology as it is also very useful in improving the stability of position time-series. The example below shows effects of carrier phase multipath at one of the GNSS stations
Two dimensional maps of real-time integrated water vapour can be found at the KNMI web site http://www.knmi.nl/research/groundbased_observations/gps/real_time_IWV.html
Cabauw Experimental Site for Atmospheric Research (CESAR)
In collaboration with meteorologists, integrated water vapour estimations from GNSS are used for meteorological system studies, in the framework of the Cabauw Experimental Site for Atmospheric Research (CESAR)http://www.cesar-observatory.nl/ . DEOS is operating three GNSS stations at Cabauw. The GNSS receivers are collocated with other in-situ sensors (radiometers, lidars, radar, radiosondes) for cross-calibration purposes and to improve understanding of GNSS signal propagation. One of the receivers is located on the 213m high Cabauw tower, giving unique opportunities to sense vertical differences in water vapour. This activity is also partly funded by the BSIK programme “Klimaat voor Ruimte”.
Ionosphere delay modeling for Network D-GPS
The development of accurate ionosphere delay models is important for single frequency GPS users and Network D-GPS users.
Our research has focused on a number of topics:
· Use of IGS global ionosphere maps for single-frequency precise point positioning
· Evaluation of the NeQuick model (Galileo) for single-frequency users, including the development of methods for the estimation of NeQuick model parameters from IGS Global Ionosphere Maps.
Development of dynamical models for ionosphere delay interpolation for Network-DGPS. Recent research has shown that the data driven models can be improved by incorporating dynamical models based on ionospheric physics.