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Besides the total electron content (TEC) and the peak electron density of the F2 layer (NmF2) also the equivalent slab thickness (τ) is considered as a key parameter of the ionosphere. The equivalent slab thicknessτ is a shape parameter of the vertical electron density profile defined by the ratio TEC/NmF2. Because the equivalent slab thickness is closely related to the neutral gas scale height under steady-state conditions at daytime, τ is correlated with thermospheric temperature changes. Based on earlier studies that have revealed a continuous decrease of the slab thickness over European ionosonde stations since 1996, the presentation analyses trends of τ in relation to TEC and NmF2 for an extended data base up to the end of 2022. A continuous TEC data base is available from DLR and the International GNSS Service (IGS). The peak electron density NmF2 is obtained from vertical sounding stations located over Europe covering solar cycles 23 and 24. In addition to the ground-based observations of NmF2 also radio occultation measurements are used to derive the peak electron density for equivalent slab thickness computations. It is found that the slab thickness behavior continues decreasing, e.g. by 50-60 km over the Juliusruh ionosonde station since 1996. The geophysical and solar activity background will be considered to explain the significant contraction of the equivalent slab thickness associated with a long-term cooling of the thermosphere. It is concluded that the slab thickness analysis has a high potential for estimating long-term changes in the thermosphere/ionosphere systems., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

High-resolution 3D ionosphere modelling is crucial to monitor and understand space weather. Prol et al. (2021) is one of the most recent developments in advanced global-scale tomography that can represent the ionospheric electron density in a relatively high spatial and temporal resolution. The study was applied to the March 17, 2015 geomagnetic storm, showing that global-scale tomography could be useful to reproduce the system dynamics during a severe geomagnetic storm. Our results show good agreement with several ground- and space-based measurements of the ionosphere. The dataset used as reference is based on electron density observations from worldwide ionosondes, Millstone Hill incoherent scatter radar and in-situ measurements from the DMSP, GRACE and SWARM missions. The obtained results presented more accurate electron density profiles when compared to the Neustrelitz Electron Density Model, which is the model used as background, and physics-based thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM). Here we will present the main results in Prol et al. (2021), as well as provide guidelines and recommendations for future improvements in the development of 3D ionospheric models. Reference: Prol, F. S., Kodikara, T., Hoque, M. M., & Borries, C. (2021). Global-scale ionospheric tomography during the March 17, 2015 geomagnetic storm. Space Weather, 19, e2021SW002889. https://doi.org/10.1029/2021SW002889

GNSS single-frequency applications are affected by the interaction of the radio signals with the free electrons of the ionosphere, introducing range errors of up to 100 m in the L-band. Besides the GPS Klobuchar and the Galileo NeQuick ionospheric models, also the Neustrelitz Total Electron Content Model (NTCM) has been proposed as a practicable solution to mitigate such propagation errors.In this investigation we present a global statistical validation of the NTCM version driven by Galileo ionization coefficients (NTCM-GlAzpar) by comparing its performance with the performance achieved by the NeQuickG and Klobuchar models in the position domain. For this aim, we used the ESA analysis tool “gLAB” in the Standard Point Positioning (SPP) mode and GNSS data over two different time periods. The first period covers one month of perturbed solar activity (December 2014) and the second period corresponds to a month of quiet conditions (December 2019). We achieved a worldwide coverage with data from up to 73 receivers of the International GNSS Service (IGS).Our statistical analysis allows us to conclude that the NTCM-GlAzpar model clearly outperforms the results achieved with the Klobuchar model and is slightly better, or at least comparable, to the performance shown by NeQuickG. Indeed, the root mean squared (RMS) values of the hourly mean 3D position errors obtained for the global dataset are 4.36, 4.61 and 6.71 meters for perturbed conditions and 2.32, 2.35 and 2.75 meters for the quiet period, respectively for the NTCM-GlAzpar, NeQuickG and Klobuchar models. Nevertheless, through a geographic- and diurnal-specific analysis, we identify also that the performance of NTCM-GlAzpar slightly decreases for conditions of reduced solar activity – at night time, higher latitudes and low perturbations.We further discuss the applicability of the NTCM-GlAzpar ionospheric model in GNSS single-frequency applications motivated by its simple software adaptations and low computational cost.

High-precision GNSS (global navigation satellite e system) measurements can be used for remote sensing and nowadays play a significant role in atmospheric sounding (station data, radio occultation observations) and sea surface altimetry based on reflectometry. A limiting factor of high-precision reflectometry is the loss of coherent phase information due to sea-state-induced surface roughness. This work studies airborne reflectometry observations recorded over coastal waters to examine the sea-state influence on Doppler distribution and the coherent residual phase retrieval. From coherent observations, the possibility of zenith total delay inversion is also investigated, considering the hydrostatic mapping factor from the Vienna mapping function and an exponential vertical decay factor depending on height receiver changes. The experiment consists of multiple flights performed along the coast between the cities of Calais and Boulogne-sur-Mer, France, in July 2019. Reflected signals acquired in a right-handed circular polarization are processed through a model-aided software receiver and passed through a retracking module to obtain the Doppler and phase-corrected signal. Results from grazing angle observations (elevation < 15°) show a high sensitivity of Doppler spread with respect to sea state with correlations of 0.75 and 0.88 with significant wave height and wind speed, respectively. An empirical Doppler spread threshold of 0.5 Hz is established for coherent reflections supported by the residual phase observations obtained. Phase coherence occurs in 15% of the observations; however, the estimated zenith total delay for the best event corresponds to 2.44 m, which differs from the typical zenith total delay (2.3 m) of 5%.