Near real-time GNSS meteorology: a preliminary feasibility demonstration based on the variometric approach&#160;
Authors: De Pace Alessandra Maria , Fratini Rachele , Mazzoni Augusto , Crespi Mattia 
Published: March 2024

The variometric approach has been demonstrated effective in GNSS seismology and GNSS ionospheric seismology to estimate ground shaking (VADASE) and earthquake/tsunami induced ionospheric disturbances (VARION) in real time for years. In this study the same variometric approach has been utilized to appraise the potential for real-time tracking of tropospheric delay (VARTROPO): this investigation holds significance for timely enhancements in weather forecasting, by incorporating this data into numerical weather models.In contrast to the prevailing method of tracking tropospheric delay, which relies on employing a mapping function and estimating a singular zenith tropospheric delay (ZTD) for all satellites within a specific time interval, the proposed approach is based on the estimation of single-epoch variation of the slant tropospheric delay (VSTD) for individual satellite. The low-pass filtering process and the integration of this variation over time, starting from a known initial value of the STD, allows to estimate the STD in near (due to low-pass filtering) real time for each satellite. It is noteworthy that the proposed approach allows to highlight the azimuthal anisotropy of the troposphere, valuable during periods of intense weather fronts.The preliminary research focuses on evaluating how the estimates derived from the proposed approach, in near real-time scenario, match with both the official ZTD estimates provided by CDDIS and those obtained through Precise Point Positioning (PPP) technique. In this respect, it has to be underlined that the assessment hereafter illustrated has been developed: (i) using 1-second rate GNSS data; (ii) in both terms of ZTDs and STDs, using a standard 1/sin(elevation) mapping function for conversion; (iii) with fixed position of the GNSS permanent station (only the receiver clock variation has been estimated in the variometric approach); (iv) without multipath mapping and removal. The first presented comparison is between VARTROPO and PPP (MATE station; satellite G03; 1st October 2023) (Figure 1).Fig. 1VARTROPO derived VSTD and VZTD exhibits higher noise level; therefore, to mitigate the highfrequency noise, a simple low-pass filter (moving median) with different moving windows (from 5 seconds to 2 minutes) has been applied. Then, the different low-pass filtered VZTDs have been integrated over time, starting from the ZTD at the initial epoch as derived from PPP, and compared to the ZTDs estimated by PPP (Figure 2).&#160;Fig2The differences between the reconstructed VARTROPO ZTDs trends and the PPP ZTDs have been represented (Figure 3). &#160; Fig.3The second comparison is between VARTROPO and CDDIS, to substantiate the aforementioned findings (Figures 4, 5).&#160; &#160; &#160;Fig4&#160;Fig5In conclusion, it has been understood that simple moving medians are able to effectively low-pass filter the VARTROPO ZTDs: with 2-minute moving window the agreement with PPP ZTDs and CDDIS ZTDs are at within 1-2 millimeters, what preliminarily demonstrate that near real-time track of the tropospheric delay is feasible. Next research steps will involve: (i) enhancing the VSTDs estimates (e.g. with multipath mitigation); (ii) investigating the possibility to estimate troposphere azimuthal anisotropy in presence of weather fronts.