Tong Ning

Retrieving of atmospheric parameters from multi‐GNSS in real time: Validation with water vapor radiometer and numerical weather model

The multiconstellation Global Navigation Satellite Systems (GNSS) (e.g., GPS, GLObal NAvigation Satellite System (GLONASS), Galileo, and BeiDou) offers great opportunities for real-time retrieval of atmospheric parameters for supporting numerical weather prediction nowcasting or severe weather event monitoring. In this study, the observations from different GNSS are combined to retrieve atmospheric parameters based on the real-time precise point positioning technique. The atmospheric parameters, retrieved from multi-GNSS observations of a 180 day period from about 100 globally distributed stations, including zenith total delay, integrated water vapor, horizontal gradient, and slant total delay (STD), are analyzed and evaluated. The water vapor radiometer data and a numerical weather model, the operational analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), are used to independently validate the performance of individual GNSS and also demonstrate the benefits of multiconstellation GNSS for real-time atmospheric monitoring. Our results show that the GLONASS and BeiDou have the potential capability for real-time atmospheric parameter retrieval for time-critical meteorological applications as GPS does, and the combination of multi-GNSS observations can improve the performance of a single-system solution in meteorological applications with higher accuracy and robustness. The multi-GNSS processing greatly increases the number of STDs. The mean and standard deviation of STDs between each GNSS and ECMWF exhibit a good stability as function of the elevation angle, the azimuth angle, and time, in general. An obvious latitude dependence is confirmed by a map of station specific mean and standard deviations. Such real-time atmospheric products, provided by multi-GNSS processing with higher accuracy, stronger reliability, and better distribution, might be highly valuable for atmospheric sounding systems, especially for nowcasting of extreme weather.

Real-time retrieval of precipitable water vapor from GPS and BeiDou observations

The rapid development of the Chinese BeiDou Navigation Satellite System (BDS) brings a promising prospect for the real-time retrieval of zenith tropospheric delays (ZTD) and precipitable water vapor (PWV), which is of great benefit for supporting the time-critical meteorological applications such as nowcasting or severe weather event monitoring. In this study, we develop a real-time ZTD/PWV processing method based on Global Positioning System (GPS) and BDS observations. The performance of ZTD and PWV derived from BDS observations using real-time precise point positioning (PPP) technique is carefully investigated. The contribution of combining BDS and GPS for ZTD/PWV retrieving is evaluated as well. GPS and BDS observations of a half-year period for 40 globally distributed stations from the International GNSS Service Multi-GNSS Experiment and BeiDou Experiment Tracking Network are processed. The results show that the real-time BDS-only ZTD series agree well with the GPS-only ZTD series in general: the RMS values are about 11–16 mm (about 2–3 mm in PWV). Furthermore, the real-time ZTD derived from GPS-only, BDS-only, and GPS/BDS combined solutions are compared with those derived from the Very Long Baseline Interferometry. The comparisons show that the BDS can contribute to real-time meteorological applications, slightly less accurately than GPS. More accurate and reliable water vapor estimates, about 1.3–1.8 mm in PWV, can be obtained if the BDS observations are combined with the GPS observations in the real-time PPP data processing. The PWV comparisons with radiosondes further confirm the performance of BDS-derived real-time PWV and the benefit of adding BDS to standard GPS processing.

Multi-GNSS Meteorology: Real-Time Retrieving of Atmospheric Water Vapor From BeiDou, Galileo, GLONASS, and GPS Observations

The rapid development of multi-Global Navigation Satellite Systems (GNSSs, e.g., BeiDou, Galileo, GLONASS, and GPS) and the International GNSS Service (IGS) Multi-GNSS Experiment (MGEX) brings great opportunities and challenges for real-time determination of tropospheric zenith total delays (ZTDs) and integrated water vapor (IWV) to improve numerical weather prediction, particularly for nowcasting or severe weather event monitoring. In this paper, we develop a multi-GNSS model to fully exploit the potential of observations from all currently available GNSSs for enhancing real-time ZTD/IWV processing. A prototype multi-GNSS real-time ZTD/IWV monitoring system is also designed and realized at the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences (GFZ) based on the precise point positioning technique. The ZTD and IWV derived from multi-GNSS stations are carefully analyzed and compared with those from collocated Very Long Baseline Interferometry and radiosonde stations. The performance of individual GNSS is assessed, and the significant benefit of multi-GNSS for real-time water vapor retrieval is also evaluated. The statistical results show that accuracy of several millimeters with high reliability is achievable for the multi-GNSS-based real-time ZTD estimates, which corresponds to about 1- to 1.5-mm accuracy for the IWV. The ZTD/IWV with improved accuracy and reliability would be beneficial for atmospheric sounding systems, particularly for time-critical geodetic and meteorological applications.

Evaluation of the atmospheric water vapor content in a regional climate model using ground-based GPS measurements

Ground-based GPS measurements can provide independent data for the assessment of climate models. We use the atmospheric integrated water vapor (IWV) obtained from GPS measurements at 99 European sites to evaluate the regional Rossby Centre Atmospheric climate model (RCA) driven at the boundaries by the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data (ERA Interim). The GPS data were compared to the RCA simulation and the ERA Interim data. The comparison was first made using the monthly mean values. Averaged over the domain and the 14 years covered by the GPS data, IWV differences of about 0.47 kg/m^2 and 0.39 kg/m^2 are obtained for RCA-GPS and ECMWF-GPS, respectively. The RCA-GPS standard deviation is 0.98 kg/m^2 whereas it is 0.35 kg/m^2 for the ECMWF-GPS comparison. The IWV differences for RCA are positively correlated to the differences for ECMWF. However, this is not the case for two sites in Italy where a wet bias is seen for ECMWF, while a dry bias is seen for RCA, the latter being consistent with a cold temperature bias found for RCA in that region by other authors. Comparisons of the estimated diurnal cycle and the spatial structure function of the IWV were made between the GPS data and the RCA simulation. The RCA captures the geographical variation of the diurnal peak in the summer. Averaged over all sites, a peak at 17 local solar time is obtained from the GPS data while it appears later, at 18, in the RCA simulation. The spatial variation of the IWV obtained for an RCA run with a resolution of 11 km gives a better agreement with the GPS results than does the spatial variation from a 50 km resolution run.

Retrieving high‐resolution tropospheric gradients from multiconstellation GNSS observations

The developing multi-Global Navigation Satellite Systems (GNSS) constellations have the potential to provide accurate high-resolution tropospheric gradients. Such data, closely linked to strong humidity gradients accompanying severe weather phenomena, are considered a new important data source for meteorological studies, e.g., nowcasting of severe rainfall events. Here we describe the development of a multi-GNSS processing system for the precise retrieval of high-resolution tropospheric gradients. The retrieved products were validated by using independent water vapor radiometer (WVR) observations and numerical weather model (NWM) data. The multi-GNSS high-resolution gradients agree well with those, derived from NWM and WVR, especially for the fast-changing peaks which were mostly associated with synoptic fronts. Compared to GPS-only gradients, the correlations with the validation data are significantly improved up to 20–35%. The new data product has significant potential to improve numerical weather prediction and to advance meteorological studies.

Trends in the Atmospheric Water Vapor Content From Ground-Based GPS: The Impact of the Elevation Cutoff Angle

We used 14 years of data from 12 GPS sites in Sweden and Finland to estimate trends in the atmospheric integrated water vapor (IWV) for 8 different elevation cutoff angles, from 5° to 40°, for the observations used in the analyses. These trends were compared to the corresponding trends obtained from radiosonde data at 7 nearby (<;120 km) sites. The results show a variation in the correlation of the trends between the two techniques for different elevation cutoff angles. The highest correlation coefficient of 0.88 is obtained for the 25^ solution, whereas the smallest root-mean-square (RMS) differences between the IWV estimates themselves are obtained mainly for elevation cutoff angles of 10° and 15°. The results show that elevation-angle-dependent systematic errors vary with time. Therefore the elevation cutoff angle giving the best agreement between radiosonde and GPS for individual IWV estimates is not necessarily the optimum when estimating linear trends. The correlation between the trends from the two completely independent techniques is strong evidence that the two techniques provide information on the IWV trends although the true individual values are too small to be uniquely detected. In addition, we found that the choice of mapping functions is not critical for the IWV trend estimation.

High-rate GNSS techniques for the detection of large seismic displacements

Measurement of co-seismic strong-motion displacements at sub-second temporal resolution is of great importance for earthquake studies. We have investigated the usage of highrate sampled Global Navigation Satellite System (GNSS) data to measure seismic motion by implementing an industrial robot simulating the displacements close to an earthquake epicenter. The robot arm is tracked by GNSS signals. Two baselines-400 m and 60 km-from the robot to reference stations are used to process the observed GPS data. Both methods give similar (within 0.5 mm) Root Mean Square (RMS) differences between the estimated and the commanded coordinates. The RMS differences are 3.5 mm in the east component, 5.6 mm in the north component, and 8.1 mm in the vertical component.

Observation of long term trends in the amount of atmospheric water vapor by space geodesy and remote sensing techniques

We present long term trends in the amount of atmospheric water vapor at the Swedish West Coast. These trends are derived from geodetic Very Long Baseline Interferometry (VLBI), ground based microwave radiometry, and radiosonde observations. The time span of observations covers 25 years and the data were collected at the Onsala Space Observatory (VLBI and microwave radiometry) and the Gothenburg-Landvetter Airport (radiosondes). The three techniques detect positive trends in the integrated precipitable water vapor (IPWV) on the order of 0.4 to 0.6 kg/m2 per decade. The IPWV data derived from the three techniques have correlation coefficients on the order of 0.95 and better. However, there is no perfect agreement between the IPWV trends derived by the three techniques. This might partly be explained by different temporal sampling and data gaps.

On the information content in linear horizontal delay gradients estimated from space geodesy observations

We have studied linear horizontal gradients in the atmospheric propagation delay above ground-based stations receiving signals from the Global Positioning System (GPS). Gradients were estimated from 11 years of observations from five sites in Sweden. Comparing these gradients with the corresponding ones from the European Centre for MediumRange Weather Forecasts (ECMWF) analyses shows that GPS gradients detect effects over different timescales caused by the hydrostatic and the wet components. The two stations equipped with microwave-absorbing material below the antenna, in general, show higher correlation coefficients with the ECMWF gradients compared to the other three stations. We also estimated gradients using 4 years of GPS data from two co-located antenna installations at the Onsala Space Observatory. Correlation coefficients for the east and the north wet gradients, estimated with a temporal resolution of 15 min from GPS data, can reach up to 0.8 for specific months when compared to simultaneously estimated wet gradients from microwave radiometry. The best agreement is obtained when an elevation cut-off angle of 3 is applied in the GPS data processing, in spite of the fact that the radiometer does not observe below 20. We also note a strong seasonal dependence in the correlation coefficients, from 0.3 during months with smaller gradients to 0.8 during months with larger gradients, typically during the warmer and more humid part of the year. Finally, a case study using a 15 d long continuous verylong-baseline interferometry (VLBI) campaign was carried out. The comparison of the gradients estimated from VLBI and GPS data indicates that a homogeneous and frequent sampling of the sky is a critical parameter.