Quantification of Tropospheric and Ionospheric Activity according to Double Differences Analysis of GNSS Phases Records
Brenot, H.; Lejeune, S.; Wautelet, G.; Bavier, M.; Spitz, J.; Warnant, R.
Royal Meteorological Institute of Belgium

In the framework of the GALOCAD project (development of a GAlileo LOcal Component for the nowcasting and forecasting of Atmospheric Disturbances affecting the integrity of high precision Galileo applications) we focus on Double Differences (DD) of GNSS (Global Navigation Satellites System) phases records. Our objective is to assign a level of accuracy to the Real-Time-Kinematics (RTK) positioning solution according to small-scale tropospheric activity. The use of DD analysis allows to determine activity index of the atmosphere which can be linked to level of accuracy of RTK positions.
Based on L1- and L2-phases ambiguities resolutions of GNSS signals (with the l-method), tropospheric and ionospheric activities can be isolated considering linear DD-combinations IF and GF (“Ionospheric-Free” and “Geometric-Free”).

As you would expect it is necessary to correct phase’s records of cycle’s slips and daily multipass. IF-combination is able to evict the first order of ionospheric influence and to detect the tropospheric activity defined by double differences of micro-wave GNSS signals propagations (according to a couple of station and a couple of satellites). IF-DD analysis index of activity have been validated by neutral atmosphere Zenith Total path Delays (ZTD) and horizontal gradients of delays, specifically in reference to a baseline defined by a couple of stations which allows the identification of water vapour bubble. But mostly, in the case study of the Belgian Dense Network (BDN) possessing since 2003 about 70 GNSS stations (distant from 5 km to 30 km), high level of real time DD-analysis detection are associated with the presence of hydrometeors observed by radar and infrared imaging. In fact, passages of small-scale stormy structures and of meso-scale weather fronts have been diagnosed from couples of stations (located close to these structures) and from couples of satellites (in direction of these tropospheric activities).

On the other side, the study of ionospheric activity by GF-DD analysis - which is harder to validate by the lake of observations - allows to isolate the geometric component of carrier phase’s records. The comparison with the Total Electron Contents (TEC) at each stations shows systematically that GF-DD activity observations with index greater than a quarter of carrier phase (misclosure about 5 cm) are the results of the ionosphere. Two types of structures are clearly diagnosed by DD misclosures. The first one is characterized by geomagnetic storm, which can affect a big area located far from the couple of stations (up to more than 1000 km). According to different geographic directions defined by baselines (in respect to the same couple of satellites concerned by ionospheric activity) a quantification of the activity can be observed referring to the wave front propagation. The second type, distinctly different, is the result of Traveling Ionospheric Disturbances (TID) which are small-structures. Contrary to geomagnetic storm, TID can affect very quickly an area of just a few kilometers. Such ionospheric activity can change in few minutes. The GF-DD analysis clearly allows to identify such event by the shape of time series.

The interest of tropospheric and ionospheric activity detections consist of the fact that it is not necessary to resolve positions in addition to atmospheric parameters (like for geodetic software). At the same time, in the framework of Real-Time Kinematics architecture (RTK), atmospheric activities identified is a pertinent information to quantify the accuracy of real-time positioning solution of a user (considering like a rover) in reference to permanent stations (for which positions are well known).
The real-time informations of DD analysis affect several domains of applications. The tropopheric small-structures observed (stormy cells or water vapour bubbles) can be used in hydrometeorological processes (nowcasting and system of alert). In addition, tropospheric activity index can be useful for several systems of telecommunications because meteorological events represent significant disruptions. On the other side, small-scale ionospheric structures (TID) or identified geomagnetic storms can induce a consequent uncertainty source for aviation navigation. To conclude, our establishments of atmospheric activity index are means to characterize level of accuracy of real-time positions, in the same time DD analysis of phase’s delays records is a way to allow real-time atmospheric observations.