Beacon Satellite Symposium: Session 5B - June 30th 2016: Radio Occultation Techniques and Measurements

During the Beacon Satellite Symposium, held in Trieste, Italy, between June 26 and July 1 2016, the JRC chaired the session 5B: Radio Occultation Techniques and Measurements. The corresponding abstract of the session is provided as follows: Since the mid-1960s, the GNSS based radio occultation technique has been used to study the structure and properties of the atmospheres of not only Earth but also other planets, such as Venus, Mars, some other outer planets, and many of their moons. By measuring the phase delay of radio waves from GNSS satellites as they are occulted by the Earth’s atmosphere, the vertical density profiles of the bending angles of radio wave trajectories can be estimated using measurements onboard LEO satellites. The success of the GPS/MET mission in 1995 inspired a number of follow‐on missions that include radio occultation experiment, including the CHAMP, GRACE, SAC-C, COSMIC, Metop-A/B, C/NOFS, and upcoming COSMIC-2 satellites. The combined profiles from these different LEO satellites provide excellent opportunities to explore the dynamics and structure of the ionosphere, especially in the regions that have been devoid of ground-based instruments, allowing for investigation of the longitudinal variability of the ionospheric density structure. This session seeks contributions that advance the application of RO technique for space weather studies. In addition, we welcome presentations exploring innovative methodologies that address the current problem on RO inversion technique at the equatorial region where ionospheric irregularity, such as sporadic E and spread F, present and degrade the linear combination technique that affect the quality of density profile extracted in the region. The session was organized among Endawoke Yizengaw (Institute for Scientific Research, Boston College), Jann-Yenq Liu (National Space Organization –NSPO- Chief Scientist), and Angela Aragon-Angel (Joint Research Centre). The session consisted of both oral and poster presentation parts. This document presents the process of the session preparation within the Beacon Satellite Symposium organization. Moreover, the abstracts of the different contributions accepted to the session are also included for completeness.JRC.E.2-Technology Innovation in Securit

Long-term comparison of the ionospheric F2 layer electron density peak derived from ionosonde data and Formosat-3/COSMIC occultations

Electron density profiles (EDPs) derived from GNSS radio occultation (RO) measurements provide valuable information on the vertical electron density structure of the ionosphere and, among others, allow the extraction of key parameters such as the maximum electron density NmF2 and the corresponding peak height hmF2 of the F2 layer. An efficient electron density retrieval method, developed at the UPC (Barcelona, Spain), has been applied in this work to assess the accuracy of NmF2and hmF2 as determined from Formosat-3/COSMIC (F-3/C) radio occultation measurements for a period of more than half a solar cycle between 2006 and 2014. Ionosonde measurements of the Space Physics Interactive Data Resource (SPIDR) network serve as a reference. Investigations on the global trend as well as comparisons of the F2 layer electron density peaks derived from both occultations and ionosonde measurements are carried out. The studies are performed in the global domain and with the distinction of different latitude sectors around the magnetic equator ±[0°, 20°], ±]20°, 60°] and ±]60°, 90°]) and local times (LT) accounting for different ionospheric conditions at night (02:00 LT ± 2 h), dawn (08:00 LT ± 2 h), and day (14:00 LT ± 2 h). The mean differences of F2 layer electron density peaks observed by F-3/C and ionosondes are found to be insignificant. Relative variations of the peak differences are determined in the range of 22%–30% for NmF2 and 10%–15% for hmF2. The consistency of observations is generally high for the equatorial and mid-latitude sectors at daytime and dawn whereas degradations have been detected in the polar regions and during night. It is shown, that the global averages of NmF2 and hmF2 derived from F-3/C occultations appear as excellent indicators for the solar activity.JRC.G.5-Security technology assessmen

Galileo Ionospheric Correction Algorithm: An Optimization Study of NeQuick-G

At present most low-cost GNSS receivers operate one frequency in the L band. For them one of the largest error contributions is the delay of radio signals in the Ionosphere. NeQuick-G is the official ionospheric correction algorithm (ICA), which has been adopted for Galileo, the European GNSS Programme. The NeQuick-G implementation is complex when compared with other ICAs. It is also demanding in terms of computational resources. The Joint Research Centre completed a reference implementation of NeQuick-G based on the official document “Ionospheric Correction Algorithm for Galileo Single Frequency Users” provided by the European Global Navigation Satellite Systems Agency. The rationale behind the JRC implementation of NeQuick-G was the intent to write an independent source code from scratch, without using the pseudo-codes from the reference document and solely relying on the physics descriptions. Using such implementation as baseline, this paper describes an optimization attempt of the official pseudo code from an algorithmic perspective. The objective was to reduce the computational load while not sacrificing the performance. The new proposed integration method is able to speed up calculations to 21% and 49% with respect the two official integration algorithms. The overall computational burden depends on the number of operations, which is eventually closely correlated to the number of calls of the ionospheric model. This underlines the quest to find an integration method reducing this number of calls. Moreover, based on the findings of this study, the authors strongly recommend revisiting the convergence control of the integration routines introduced in (Galileo OS-ICA, 2016).JRC.E.2-Technology Innovation in Securit

A linear scale height Chapman model supported by GNSS occultation measurements

Global Navigation Satellite Systems (GNSS) radio occultations allow the vertical sounding of the Earth’s atmosphere, in particular, the ionosphere. The physical observables estimated with this technique permit to test theoretical models of the electron density such as, for example, the Chapman and the Vary-Chap models. The former is characterized by a constant scale height, whereas the latter considers a more general function of the scale height with respect to height. We propose to investigate the feasibility of the Vary-Chap model where the scale height varies linearly with respect to height. In order to test this hypothesis, the scale height data provided by radio occultations from a receiver on board a low Earth orbit (LEO) satellite, obtained by iterating with a local Chapman model at every point of the topside F2 layer provided by the GNSS satellite occultation, are fitted to height data by means of a linear least squares fit (LLS). Results, based on FORMOSAT-3/COSMIC GPS occultation data inverted by means of the Improved Abel transform inversion technique (which takes into account the horizontal electron content gradients) show that the scale height presents a more clear linear trend above the F2 layer peak height, hm, which is in good agreement with the expected linear temperature dependence. Moreover, the parameters of the linear fit obtained during four representative days for all seasons, depend significantly on local time and latitude, strongly suggesting that this approach can significantly contribute to build realistic models of the electron density directly derived from GNSS occultation data.JRC.E.2-Technology Innovation in Securit

Modelling and assessing ionospheric higher order terms for GNSS signals

High precision positioning and time transfer are required by a large number of scientific applications: seismic ground deformations, sea level monitoring or land survey applications require sub-centimeter precision in kinematic position; monitoring of stable atomic frequency standards requires an increasing sub –nanosecond precision. Differential GNSS is presently the best tool to reach such precisions, as it removes the majority of the errors affecting the GNSS signals. However, the associated need for dense GNSS observation networks is not fulfilled for many locations (e.g. Pacific, Africa). An alternative is to use Precise Point Positioning (PPP), but this technique requires correcting signal delays at the highest level of precision, including high order ionospheric effects. It is thus essential to accurately characterize the higher order ionospheric terms (I2+), i.e. I2, I3, I4, geometric bending and differential STEC bending, which is the goal of this paper. For that, we used a network of well-distributed GPS stations, and the Bernese v5.0 software. We have focused our attention in the I2+ terms, studying two approaches: A) Combining independent and simultaneous measurements of the same transmitter-receiver pair at three (or more) different frequencies, in order to remove the I2 term: it is theoretically possible to cancel out both I1 and I2 similarly as it is done typically in precise dual-frequency GNSS measurements for I1. It is shown that, as expected, due to the proximity of the corresponding frequencies in L-band, the high noise of the combinations makes this approach unpractical to either isolate or remove I2. B) Modelling the I2+ terms, in function of estimates of electron content, geomagnetic field and electron density values. Their characterization has been done in a realistic and full-control environment, by using the last version of the International Reference Ionosphere model (IRI2012) and International Geomagnetic Reference Model in its 11th version (IGRF11). Two metrics have been considered to assess the importance of the different higher order ionospheric corrections and their approximations: a) At the signal level, or range level, directly provided by the corresponding slant delays. b) At the geodetic domain level, provided by the impact of such values in the different geodetic parameters estimated consistently (i.e. simultaneously) from a global GNSS network.JRC.G.5-Security technology assessmen

A method for scintillation characterization using geodetic receivers operating at 1 Hz

Ionospheric scintillation produces strong disruptive ffects on Global Navigation Satellite System (GNSS) signals, ranging from degrading performances to rendering these signals useless for accurate navigation. The current paper presents a novel approach to detect scintillation on the GNSS signals based on its ffect on the ionospheric-free combination of carrier-phases, i.e. the standard combination of measurements used in Precise Point Positioning (PPP). The method is implemented using actual data, thereby having both its feasibility and its usefulness assessed at the same time. The results identify the main effects of scintillation, which consist of an increased level of noise in the ionospheric-free combination of measurements and the introduction of cycle-slips into the signals. Also discussed is how mis-detected cycle-slips contaminate the Rate of Change of the Total Electron Content Index (ROTI) values, which is especially important for low-latitude receivers. By considering the ffect of single jumps in the individual frequencies, the proposed method is able to isolate, over the combined signal, the frequency experiencing the cycle-slip. Moreover, because of the use of the ionospheric-free combination, the method captures the diffractive nature of the scintillation phenomena that, in the end, is the relevant ffect on PPP. Finally, a new scintillation index is introduced that is associated with the degradation of the performance in navigation.JRC.E.2-Technology Innovation in Securit

Ionospheric and Plasmaspheric Contribution to the Total Electron Content Inferred from Ground Data and Radio-Occultation-Derived Electron Density

The performance of a new method allowing the determination of the separate contributions from the ionosphere and the plasmasphere to ground measurements of the vertical total electron content (VTEC) is demonstrated. After introducing and testing the proposed method, a preliminary global-scale analysis of the ionosphere-plasmasphere system during a period of low solar activity is presented. The results show some regular features in the seasonal, geomagnetic latitude and local time variations of the fractional ionospheric electron content. These features are modeled by means of a simple fitting function allowing also the characterization of the fractional plasmaspheric contribution to the VTEC.JRC.G.5-Security technology assessmen

Neural Network Based Model for Global Total Electron Content Forecasting

We introduce a novel empirical model to forecast, 24 hours in advance, the Total Electron Content (TEC) at a planetary scale. The technique leverages on the Global Ionospheric Map (GIM), provided by the International GNSS Service (IGS), and applies a nonlinear autoregressive neural network with external input (NARX) to selected GIM grid points for the 24 hours single-point TEC forecasting, taking into account the actual and forecasted geomagnetic conditions. To extend the forecasting at a planetary scale, the technique makes use of the NeQuick2 Model fed by an effective sunspot number R12 (R12eff), estimated by minimizing the root mean square error (RMSE) between NARX output and NeQuick2 applied at the same GIM grid points. The novel approach is able to reproduce the features of the planetary ionosphere revealing, for example, its peculiarity at low-equatorial latitudes due to the Equatorial Ionosphere Anomaly (EIA). The performance of the forecasting approach is extensively tested under different geospatial conditions, against both TEC maps products by UPC (Universitat Politècnica de Catalunya) and independent TEC data from the dual frequency altimeter on board of Jason-3 spacecraft. The testing results are very satisfactory in terms of root mean square errors that ranges between 3 and 5 TECu. RMSE depend on the latitude sectors, time of the day, geomagnetic conditions, and provide a statistical estimation of the accuracy of the 24-hours forecasting technique even over the oceans. The validation of the forecasting during 5 geomagnetic storms reveals the very satisfactory results of the technique even during disturbed periods. This 24-hours empirical approach is currently implemented on the Ionosphere Prediction Service (IPS), a prototype platform to monitor and forecast the ionospheric effects on the performance of GNSS systems at service level for several classes of users.JRC.E.2-Technology Innovation in Securit

Polar Electron Content from GPS-data-based Global Ionospheric Maps: assessment, case studies and climatology

The electron content distribution of the north and south polar ionosphere from 2001 to the beginning of 2019 is analyzed by using the UQRG global ionospheric map (GIM) of vertical total electron content (VTEC), computed every 15 min by UPC-IonSAT with a tomographic-kriging combined technique. We first show that the accuracy of UQRG GIM is slightly better than that of the GIMs of other analysis centers on the whole and also over both poles. Second, we show examples of polar VTEC features in UQRG GIM, previously reported by different authors and with higher-resolution techniques. Third, by means of an unsupervised clustering algorithm, learning vector quantization, we characterize the main features of the ionospheric electron content climatology, separately for the north and south polar regions.JRC.E.2-Technology Innovation in Securit

Electron density extrapolation above F2 peak by the linear Vary-Chap model supporting new Global Navigation Satellite Systems-LEO occultation missions

The new radio-occultation (RO) instrument on-board of the future EUMETSAT Polar System 2nd Generation (EPS-SG) satellites, flying at 820 km height, is primarily focusing on neutral atmospheric profiling. It will provide an opportunity for ionospheric sounding as well, but only below impact heights of 500 km. This will leave a gap of 320 km without occultation measurements, which impedes the application of the direct inversion techniques to retrieve the electron density profile. The challenge of looking for new ways (accurate and simple) of extrapolating the electron density (also applicable to other future and past Low-Earth Orbiting, (LEO), radio-occultation missions like CHAMP) has motivated this research. In this paper we propose a new Vary-Chap Extrapolation Technique (VCET) of the electron density. VCET is based on the scale height behavior, linearly dependent on the altitude above hmF2. This allows the electron density profile extrapolation for impact heights above its peak height (this is the case for EPS-SG), up to the satellite orbital height. VCET has been assessed with more than 3700 complete electron density profiles obtained in 4 representative scenarios of FORMOSAT-3/COSMIC occultations, in solar maximum and minimum conditions, and geomagnetically disturbed conditions, by applying an updated Improved Abel Transform Inversion technique to dual-frequency GPS measurements. It is shown that VCET performs much better than other classical Chapman models, with 60% of occultations showing relative extrapolation errors below 20%, in contrast with conventional Chapman model extrapolation approaches with 10% or less of the profiles with relative error below 20%.JRC.E.2-Technology Innovation in Securit