Ionospheric Propagation Effects on GNSS Signals and New Correction Approaches

The ionosphere is the ionized part of the earth’s atmosphere lying between about 50 km and several earth radii (Davies, 1990) whereas the upper part above about 1000 km height up to the plasmapause is usually called the plasmasphere. Solar extreme ultraviolet (EUV) radiation at wave lengths < 130 nm significantly ionizes the earth’s neutral gas. In addition to photoionisation by electromagnetic radiation also energetic particles from the solar wind and cosmic rays contribute to the ionization. The ionized plasma can affect radio wave propagation in various ways modifying characteristic wave parameters such as amplitude, phase or polarization (Budden, 1985; Davies, 1990). The interaction of the radio wave with the ionospheric plasma is one of the main reasons for the limited accuracy and vulnerability in satellite based positioning or time estimation. A trans-ionospheric radio wave propagating through the plasma experiences a propagation delay / phase advance of the signal causing a travel distance or time larger / smaller than the real one. The reason of the propagation delay can be realized considering the nature of the refractive index which depends on the density of the ionospheric plasma. The refractive index (n ≠ 1) of the ionosphere is not equal to that of free space (n = 1). This causes the propagation speed of radio signals to differ from that in free space. Additionally, spatial gradients in the refractive index cause a curvature of the propagation path. Both effects lead in sum to a delay / phase advance of satellite navigation signals in comparison to a free space propagation. The variability of the ionospheric impact is much larger compared to that of the troposphere. The ionospheric range error varies from a few meters to many tens of meters at the zenith, whereas the tropospheric range error varies between two to three meters at the zenith (Klobuchar, 1996). The daily variation of the ionospheric range error can be up to one order of magnitude (Klobuchar, 1996). After removal of the Selective Availability (SA, i.e., dithering of the satellite clock to deny full system accuracy) in 2000, ionosphere becomes the single largest error source for Global Navigation Satellite Systems (GNSS) users, especially for high-accuracy (centimeter - millimeter) applications like the Precise Point Positioning (PPP) and Real Time Kinematic (RTK) positioning. Fortunately, the ionosphere is a dispersive medium with respect to the radio wave; therefore, the magnitude of the ionospheric delay depends on the signal frequency. The advantage is that an elimination of the major part of the ionospheric refraction through a linear combination of dual-frequency observables is possible. However, inhomogeneous plasma distribution and anisotropy cause higher order nonlinear effects which are not removed in this linear approach. Mainly the second and third order ionospheric terms (in the expansion of the refractive index) and errors due to bending of the signal remain uncorrected. They can be several tens of centimeters of range error at low elevation angles and during high solar activity conditions. Brunner & Gu (1991) were pioneers to compute higher order ionospheric effects and developing correction for them. Since then higher order ionospheric effects have been studied by different authors during last decades, e.g., Bassiri & Hajj (1993), Jakowski et al. (1994), Strangeways & Ioannides (2002), Kedar et al. (2003), Fritsche et al. (2005), Hawarey et al. (2005), Hoque & Jakowski (2006, 2007, 2008, 2010b), Hernández-Pajares et al. (2007), Kim & Tinin (2007, 2011), Datta-Barua et al. (2008), Morton et al. (2009), Moore & Morton (2011). The above literature review shows that higher order ionospheric terms are less than 1% of the first order term at GNSS frequencies. Hernández-Pajares et al. (2007) found sub-millimeter level shifting in receiver positions along southward direction for low latitude receivers and northward direction for high latitude receivers due to the second order term correction. Fritsche et al. (2005) found centimeter level correction in GPS satellite positions considering higher order ionospheric terms. Elizabeth et al. (2010) investigated the impacts of the bending terms described by Hoque & Jakowski (2008) on a Global Positioning System (GPS) network of ground receivers. They found the bending correction for the dual-frequency linear GPS L1-L2 combination to exceed the 3 mm level in the equatorial region. Kim & Tinin (2011) found that the systematic residual ionospheric errors can be significantly reduced (under certain ionospheric conditions) through triple frequency combinations. All these studies were conducted to compute higher order ionospheric effects on GNSS signals for ground-based reception. Recently Hoque & Jakowski (2010b, 2011) investigated the ionospheric impact on GPS occultation signals received onboard Low Earth Orbiting (LEO) CHAMP (CHAllenging Minisatellite Payload) satellite. In this chapter, the first and higher order ionospheric propagation effects on GNSS signals are described and their estimates are given at different level of ionospheric ionization. Multi-frequency ionosphere-free and geometry-free solutions are studied and residual terms in the ionosphere-free solutions are computed. Different correction approaches are discussed for the second and third order terms, and ray path bending correction. Additionally, we have proposed new approaches for correcting straight line of sight (LoS) propagation assumption error, i.e., ray path bending error for ground based GNSS positioning. We have modelled the excess path length of the signal in addition to the LoS path length and the total electron content (TEC) difference between a curved and LoS paths as functions of signal frequency, ionospheric parameters such as TEC and TEC derivative with respect to the elevation angle. We have found that using the TEC derivative in addition to the TEC information we can improve the existing correction results

Recent activities of IAG working group “Ionosphere Prediction”

Ionospheric disturbances pose, for instance, an increasing risk on economy, national security, satellite and airline operations, communications networks and the navigation systems. Constructing forecasted ionospheric products with a reliable accuracy is still an ongoing challenge. In this sense, a Working Group (WG) with the title “Ionosphere Prediction” within the International Association of Geodesy (IAG) under Sub-Commission 4.3 “Atmosphere Remote Sensing” of the Commission 4 “Positioning and Applications” has been created and is actively working since 2015 to encourage scientific collaborations on developing models and discussing challenges of the ionosphere prediction problem. Different centers contribute to the WG such as the German Aerospace Center (DLR), Universitat Politècnica de Catalunya (UPC), Technical University of Munich (TUM) and GMV. One of the main focus of the WG is to evaluate different ionosphere prediction approaches and products which are highly depending on solar and geomagnetic conditions as well as on data from different measurement techniques (e.g. GNSS) with varying spatial-temporal resolution, sensitivity and latency. In this contribution, the recent progress of the WG on ionosphere prediction studies including individual and cooperated activities will be presented.Postprint (published version

Evaluation of optimizing Monteggia fracture-dislocation care: surgical innovations, radiological insights, and functional rehabilitation in adult patients

Background: Monteggia fractures, rare in adults, involve proximal ulna fracture and radial head dislocation. Managing these injuries poses challenges, fueling historical debates and driving advancements in internal fixation. Watson Jones' frustration highlights the ongoing pursuit of effective surgical approaches for optimal outcomes and functional limb restoration. his study aims to evaluate Monteggia fracture-dislocation treatment by analyzing radiological outcomes for structural insights and alignment post-surgery. Methods: This prospective observational study, conducted at Swapno general hospital, Mirpur-2, Dhaka, Bangladesh from 1st January 2021 to 31 January 2024, enrolled 30 patients with radiologically confirmed Monteggia fracture-dislocation. Surgical procedures involved creating an interval, anatomical reduction, and fixation, with regular follow-ups assessing outcomes, including range of motion, X-rays, and VAS scores, while statistical analysis utilized SPSS version 23. Results: The highest frequency percentage in the age distribution was observed among individuals aged 41-45, constituting 20% of the total sample, while the lowest frequencies were recorded in the 31-35 and &gt;51 age groups, each representing 10% of the sample. Physical assault emerged as the leading cause of injury, accounting for 40% of cases, followed by road traffic accidents at 36.66% and falls at 23.33%. In terms of final outcomes, the majority of patients (43.33%) achieved a good outcome, while the lowest percentage (10%) resulted in poor outcomes.  Conclusions: In conclusion, addressing Monteggia fracture-dislocation in adults requires navigating inherent complexities. Modern internal fixation methods prove impactful, emphasizing the need for precise classification and stable anatomical reduction

Inhibition of HIV-1 gene expression by Ciclopirox and Deferiprone, drugs that prevent hypusination of eukaryotic initiation factor 5A

<p>Abstract</p> <p>Background</p> <p>Eukaryotic translation initiation factor eIF5A has been implicated in HIV-1 replication. This protein contains the apparently unique amino acid hypusine that is formed by the post-translational modification of a lysine residue catalyzed by deoxyhypusine synthase and deoxyhypusine hydroxylase (DOHH). DOHH activity is inhibited by two clinically used drugs, the topical fungicide ciclopirox and the systemic medicinal iron chelator deferiprone. Deferiprone has been reported to inhibit HIV-1 replication in tissue culture.</p> <p>Results</p> <p>Ciclopirox and deferiprone blocked HIV-1 replication in PBMCs. To examine the underlying mechanisms, we investigated the action of the drugs on eIF5A modification and HIV-1 gene expression in model systems. At early times after drug exposure, both drugs inhibited substrate binding to DOHH and prevented the formation of mature eIF5A. Viral gene expression from HIV-1 molecular clones was suppressed at the RNA level independently of all viral genes. The inhibition was specific for the viral promoter and occurred at the level of HIV-1 transcription initiation. Partial knockdown of eIF5A-1 by siRNA led to inhibition of HIV-1 gene expression that was non-additive with drug action. These data support the importance of eIF5A and hypusine formation in HIV-1 gene expression.</p> <p>Conclusion</p> <p>At clinically relevant concentrations, two widely used drugs blocked HIV-1 replication <it>ex vivo</it>. They specifically inhibited expression from the HIV-1 promoter at the level of transcription initiation. Both drugs interfered with the hydroxylation step in the hypusine modification of eIF5A. These results have profound implications for the potential therapeutic use of these drugs as antiretrovirals and for the development of optimized analogs.</p

A New Method of Electron Density Retrieval from MetOp-A&rsquo;s Truncated Radio Occultation Measurements

The radio occultation (RO) measurements of the Global Navigation Satellite System&rsquo;s (GNSS&rsquo;s) signals onboard a Low Earth Orbiting (LEO) satellite enable the computation of the vertical electron density profile from the LEO satellite&rsquo;s orbit height down to the Earth&rsquo;s surface. The ionospheric extension experiment performed by the GNSS Receiver for Atmospheric Sounding (GRAS) receiver on board MetOp-A provides opportunities for ionospheric sounding but with the RO measurements only taken with an impact parameter height below 600 and 300 km within two different experiments, although MetOp-A was flying at an orbit height of about 800 km. Here, we present a model-assisted RO inversion technique for electron density retrieval from such kind of truncated data. The topside ionosphere and plasmasphere above the LEO orbit height are modelled by a Chapman layer function superposed with an exponential decay function representing the plasmasphere. Our investigation shows that the model-assisted technique is stable and robust and can successfully be used to retrieve the electron density values up to the LEO height from the truncated MetOp-A data, in particular when observations are available until 600 km. Moreover, this model-assisted technique is also successful with the availability of a small number of observations of the topside above the peak density height. For observations available only up to 300 km, the accuracy of the retrieved profile is comparable to the one obtained by the data truncated at a 600 km height only when the peak electron density lies below the 250 km altitude level

Mitigation of Ionospheric Mapping Function Error

Ionospheric delay is currently considered as a major error source for satellite based navigation systems. The operational standards set by the International Civil Aviation Organization (ICAO) for Wide Area Differential GPS (WADGPS) ensure Space Based Augmentation System (SBAS) users an ionospheric delay correction in addition to satellite clock and orbit corrections. In the current standard, the SBAS broadcasts vertical ionospheric delays in meters at the ionospheric grid points (IGP) located at every five degrees of latitude and longitude. The SBAS user receivers convert them to the slant delay using the so called obliquity factor derived under the assumption of a thin-shell ionosphere fixed at 350 km height. Although the ionospheric correction is available for all broadcast IGPs, the user does bilinear interpolation between IGPs those surround the ionospheric piercing point (IPP) at the thin-shell height. In this paper we present a new mapping function approach for vertical to slant delay conversion. Considering broadcast delays not only at a single IPP rather at different geographic locations along the ray path projected on the thin-shell at 350 km height, our algorithm incorporates the horizontal gradients in the slant delay computation. Instead of collapsing the vertical structure of the ionosphere into a thin-shell we consider the Chapman layer assumption for describing the vertical electron density distribution of the ionosphere. Our investigation shows that using the proposed algorithm instead of the thin-shell algorithm we can reduce the mapping function error by about 50% under high as well as low solar activity conditions