Impact of Ground Based GPS Data on Numerical Weather Prediction

Journal of geosciences and geomatics, 2021

The total zenith tropospheric delay (ZTD) is an important parameter of the atmosphere which directly or indirectly give reflection of the atmospheric condition in a local GPS network. The use of the global tropospheric models such as the Saastamoinen model, Hopfield model, Neil model etc in estimating the tropospheric effects at the local level leaves much to be desired. These models are derived using data from available radiosonde obtained from Europe and North America continents. The global atmospheric condition used as constants in these models provides a broad approximation of the tropospheric conditions, but ignores the actual atmospheric conditions on a given location i.e. do not take into account the latitudinal and seasonal variations in the atmosphere. It is necessary to assess the effects of the troposphere on geodetic measurements based on ground meteorological measurements including temperature, pressure, and relative humidity. Daily RINEX GPS data from eight (8) Malaysian Real Time Kinematic GPS Network (MyRTKnet) stations in Southern Malaysia from year 2006 to June 2008 thus; covering 2½ years were processed. A computer program for modelling 2½ years of local meteorology within the network was developed and called MetMOD, similarly a program, call SaastroMOD was developed to estimate the local zenith hydrostatic delay. The results showed that, the estimated local tropospheric delay considered the temporal and spatial variation of the network thus; giving true reflection of the tropospheric delay in a local GPS network as against the standard atmosphere which ignores the actual local condition with a standard deviation of 0.18 m having a maximum zenith tropospheric delay (ZTD) of about 2.6 m. The best zenith hydrostatic delay (ZHD) comes from station JHJY with a standard deviation of 0.353 cm, while the best zenith wet delay (ZWD) comes from station TGRH with a standard deviation of 5.943 cm. The models show that, while parameters evolved for ZHD are constant at different locations, however, those parameters evolved for ZWD show significant variations from one location to the other. Hence then use of the local meteorological parameter in modelling tropospheric delay improves GPS positional accuracy.

Accurate GPS-derived ZTD (zenith tropospheric delay) plays a key role in near real-time weather forecasting, especially in improving the precision of Numerical Weather Prediction (NWP) models. However, ZTD is usually estimated under the assumption that all the GPS measurements, carrier phases or pseudo-ranges, have the same accuracy. These assumptions are unrealistic, which will inevitably degrade the precision of ZTD estimation. This communication aims to further improve the precision of ZTD estimation by stochastic modelling. The results show that the precision of GPS-derived ZTD can be obviously improved using a suitable stochastic model for GPS measurements. The stochastic model using satellite elevation angle-based cosine function is better than other investigated stochastic models. This improvement of ZTD estimation is certainly critical for reliable NWP and other tropospheric delay corrections.

The troposphere is one of the most significant sources of error in precise GPS (Global Positioning System) positioning solutions nowadays. Uncertainties in troposphere estimation distort the height component especially. We have implemented the troposphere slant delays derived from a numerical weather prediction model to GPS processing. In this case study we calculated 24 vectors for 61 days, the length of the vectors varying between 110 and 1100 km. When the slant delay is used instead of a standard troposphere model the variation in the positions solution's up component is reduced considerably. At Sodankylä station (67.42ºN, 26.39ºE), for example, the standard deviation decreases from 5.9 cm to 1.3 cm for this 61-day period if no site-specific troposphere parameters are estimated. We also found that the length of the vector did not greatly affect the variance in the slant delay based positioning solutions.

Advances in Space Research, 2014

The Geodetic Observatory Pecný (GOP) routinely estimates near real-time zenith total delays (ZTD) from GPS permanent stations for assimilation in numerical weather prediction (NWP) models more than 12 years. Besides European regional, global and GPS and GLONASS solutions, we have recently developed real-time estimates aimed at supporting NWP nowcasting or severe weather event monitoring. While all previous solutions are based on data batch processing in a network mode, the real-time solution exploits real-time global orbits and clocks from the International GNSS Service (IGS) and Precise Point Positioning (PPP) processing strategy. New application G-Nut/Tefnut has been developed and real-time ZTDs have been continuously processed in the nine-month demonstration campaign (February-October, 2013) for selected 36 European and global stations. Resulting ZTDs can be characterized by mean standard deviations of 6-10 mm, but still remaining large biases up to 20 mm due to missing precise models in the software. These results fulfilled threshold requirements for the operational NWP nowcasting (i.e. 30 mm in ZTD). Since remaining ZTD biases can be effectively eliminated using the bias-reduction procedure prior to the assimilation, results are approaching the target requirements in terms of relative accuracy (i.e. 6 mm in ZTD). Real-time strategy and software are under the development and we foresee further improvements in reducing biases and in optimizing the accuracy within required timeliness. The real-time products from the International GNSS Service were found accurate and stable for supporting PPP-based tropospheric estimates for the NWP nowcasting.

Troposphere zenith path delays derived from the Global Data Assimilation System (GDAS) numerical weather model (NWM) are compared with those of the International GNSS Service (IGS) solutions over a 1.5-year period at 18 globally distributed IGS stations. Meteorological parameters can be interpolated from the NWM model at any location and at any time after December 2004. The meteorological parameters extracted from the NWM model agree with in situ direct measurements at some IGS stations within 1 mbar for pressure, 3° for temperature and 13% for relative humidity. The hydrostatic and wet components of the zenith path delay (ZPD) are computed using the meteorological parameters extracted from the NWM model. The total ZPDs derived from the GDAS NWM agree with the IGS ZPD solutions at 3.0 cm RMS level with biases of up to 4.5 cm, which can be attributed to the wet ZPDs estimates from the NWM model, considering the less accurate interpolated relative humidity parameter. Based on this study, it is suggested that the availability and the precision of the GDAS NWM ZPD should be sufficient for nearly all GPS navigation solutions.

Geophysical Research Letters, 1998

Differences between long term precipitable water (PW) time series derived from radiosondes, microwave water vapor radiometers, and GPS stations reveal offsets that are often as much as 1-2 mm PW. All three techniques are thought to suffer from systematic errors of order 1 mm PW. Standard GPS processing algorithms are known to be sensitive to the choice of elevation cutoff angle at this level. We present a processing protocol that is shown to reduce elevation angle dependence in geodetic estimates of zenith delay and, hence, reduce the systematic errors in derived precipitable water. This protocol includes use of the Niell zenith delay mapping functions and International GPS Service phase center models.

Journal of the Meteorological Society of Japan, 2004

We characterize the temporal and spatial variation of the Zenith Wet Delay (ZWD) and the Zenith Total Delay (ZTD), estimated using a network of Global Positioning System (GPS) receivers. This characterization is important for the improvement and validation of atmospheric water vapor models, applicable in GPS meteorology, as well as in the navigation.

International Association of Geodesy Symposia, 2018

As a pivotal infrastructure for the socioeconomic development of Nigeria, the Nigerian Global Navigation Satellite Systems (GNSS) Reference Network-NIGNET-can serve as a tool for weather and climate monitoring, by obtaining and analyzing the neutral atmospheric Zenith Total Delays (ZTD) from processed GNSS data. With the use of surface meteorological measurements, the ZTD can be transformed to the integrated water vapor content in the neutral atmosphere, which is an essential parameter in weather forecasting, and climate change and variability analysis. The focus of this research is to assess the adaptability of the NIGNET for meteorological applications using the global positioning system precise point positioning (PPP) derived ZTD at the stations. ZTD estimates are derived from daily data obtained from the NIGNET and International GNSS Service (IGS) stations spanning the years 2011-2016. These estimates are compared with ray-traced delay estimates from the National Centre for Environmental Prediction Reanalysis II (NCEP II) global Numerical Weather Model (NWM) and the IGS zenith path delay products. A comprehensive analysis is performed to assess the level of agreement of the different ZTD estimates and to identify possible systematic effects from the different sources. Comparisons between the PPP and NCEP II NWM ZTD estimates show a range of mean offsets from 6.4 to 23.9 mm, and standard deviations from 33.1 to 44.9 mm. With the PPP and IGS ZTD estimates, mean offsets of 2.4 and 0.1 mm, and standard deviations of 9.9 and 13.8 mm are obtained.

Journal of Physics: Conference Series

The tropospheric delay is one of the main error sources in Global Positioning Systems (GPS) and its impact plays a crucial role in near real-time weather forecasting. Accessibility and accurate estimation of this parameter are essential for weather and climate research. Advances in GPS application has allowed the measurements of zenith tropospheric delay (ZTD) in all weather conditions and on a global scale with fine temporal and spatial resolution. In addition to the rapid advancement of GPS technology and informatics and the development of research in the field of Earth and Planetary Sciences, the GPS data has been available free of charge. Now only required sophisticated processing techniques but user friendly. On the other hand, the ZTD parameter obtained from the models or measurements needs to be converted into precipitable water vapor (PWV) to make it more useful as a component of weather forecasting and analysis atmospheric hazards such as tropical storms, flash floods, landslide, pollution, and earthquake as well as for climate change studies. This paper addresses the determination of ZTD as a signal error or delay source during the propagation from the satellite to a receiver on the ground and is a key driving force behind the atmospheric events. Some results in terms of ZTD and PWV will be highlighted in this paper.

Geophysical Research Letters, 2002

1] We have developed a GPS data processing software and demonstrate that by using receivers in an area of approximately 800 Â 400 km we can produce estimates of the atmospheric zenith total delay (ZTD) with a latency of a few seconds. It is based on a Kalman filter which simultaneously estimates clock offsets and satellite coordinates. The ZTD quality is typically 15 mm in terms of rms difference to post-processed ZTD time series. This level of uncertainty is useful in many navigation applications and if an additional reduction in the uncertainty can be obtained the usefulness for short term weather forecasting could be significant.