We have analyzed data from an experiment over the Madrid (Spain) area obtained from 5 GPS receivers and 3 Water Vapor Radiometers (WVR), in order to compare their retrievals of Tropospheric Slant Delays. For this purpose we have fitted a simple gradient model to both types of data, using a Kalman filter to account for the temporal variability of the zenith and gradient parameters. We show that the retrieved gradients with the two instruments are compatible, thus suggesting that the derived slant delays can be useful for tomographic analysis. We compare the estimates of ZWD obtained with the GIPSY and the Bernese software packages. Finally, we compare the estimated gradients with those obtained with HIRLAM, a Numerical Weather Prediction model.

Studies of atmospheric effects on Global Positioning System (GPS) signals have proven the possibility of deriving the total water vapor content from estimates of tropospheric path delays. The accuracy of GPS derived Precipitable Water (PW) depends (besides other parameters) on the quality of satellite orbits used in the analysis. High precision orbits provided by the International GPS Service (IGS) yield PW estimates with an accuracy of about 1 mm. While these orbits are provided with a delay of several days, weather forecasting requires near real‐time determination of PW. Therefore operational meteorological GPS analysis would have to rely on orbit predictions. We investigate the impact of introducing predicted orbit information on the accuracy of GPS water vapor retrievals. The presented data were acquired during a 14‐day field experiment carried out in the north‐west region of Madrid, Spain using GPS and a Water Vapor Radiometer (WVR). The comparison of WVR measurements with estimated time series of PW using both 24 and 48 hour predicted orbits and final precise IGS orbits shows that the accuracy of PW decreases by a factor of about 2 from precise to predicted orbit data.

Tropospheric water vapor is of central interest in a large variety of geoscientific fields, such as geodesy, geodynamics, climate research and meteorology. A new instrumental approach to ground-based mapping of tropospheric water vapor has been developed. It is based on high-resolution absorption measurements in the near infrared region by means of a solar spectrometer (SSM). To prove the feasability and accuracy potential of the new technique, a 30 day field experiment was carried out, performing SSM measurements simultaneously with two independent methods. One of them uses the Global Positioning System (GPS). This technique, called GPS meteorology, exploits the high sensitivity of the satellite signals to atmospheric delay for a determination of tropospheric parameters. As a third technique two ground-based microwave water vapor radiometers (WVR) were operated. A comparison of the three different techniques, exploiting absorption-, refraction-, and emission properties of water vapor, respectively, demonstrated the potential of solar spectrometry for precise and absolute determination of PW without meteorological a priori information.

Tropospheric water vapor is of central interest in a large variety of geoscientific fields, such as geodesy, geodynamics, climate research, and meteorology. A new instrumental approach to ground‐based mapping of tropospheric water vapor has been developed. It utilizes high‐resolution absorption measurements in the near‐infrared region by means of a solar spectrometer (SSM). The processing algorithm for retrieval of the precipitable water vapor (PW) is based on a line‐by‐line calculation of the observed solar spectrum in a narrow wavelength interval (1 nm) using a simple absorption model of the troposphere. To prove the feasibility and accuracy potential of the new technique, we carried out a 30‐day field experiment. Simultaneous measurements of colocated SSM, water vapor radiometers (WVR) and Global Positioning System (GPS) receivers were performed, exploiting absorption, emission and refraction properties of water vapor, respectively. A comparison of the three different techniques demonstrated the potential of solar spectrometry for precise and absolute determinaton of PW without meteorological a priori information. Apart from apparent systematic errors of the GPS measurements, a good agreement between the SSM and WVR results within their individual accuracy limits was observed. The PW standard deviations of the techniques were determined to 0.37 mm for the WVR, 0.75 mm for the SSM, and 1.40 mm for the GPS retrievals. The independence of SSM from external calibration by radiosondes and the high potential for further development may qualify this new technique to contribute to developing an error budget for other techniques, such as GPS meteorology.