Search

In this paper, we propose a method for the generation of real-time global ionospheric map (RT-GIM) of vertical total electron content (VTEC) from GNSS measurements. The need for interpolation arises from the fact that the ionospheric pierce point (IPP) measurements from satellites to stations are not distributed uniformly over the ionosphere, leaving unfilled gaps at oceans or poles. The method we propose is based on using a high-quality historical database of post-processed GIMs that comprises more than two solar cycles, calculates the GIM by weighted superposition on a subset of the database with the compatible solar condition. The linear combination of GIMs in the database was obtained by minimizing a ℓ distance between VTEC measurements at the IPPs and the VTECs from the database, adding a ℓ penalization on the weights to assure a sparse solution. The process uses a Sun-fixed geomagnetic reference frame. This method uses the atomic decomposition/least absolute shrinkage and selection operator (LASSO), which will be denoted as atomic decomposition interpolator of GIMs (ADIGIM). As the computation is done in milliseconds, the interpolation is performed in real time. In this work, two products were developed, denoted as UADG and UARG, the UADG in real time and UARG with a latency of 24 h to benefit from the availability of a greater number of stations. The altimeter JASON3 VTEC measurements were used as reference. The quality of interpolated RT-GIMs from day 258 of 2019 to 155 of the year 2020 is compared with other RT/non-RT GIM products such as those from International GNSS Service (IGS), Centre National d’Etudes Spatiales (CNES), Chinese Academy of Sciences (CAS), Polytechnic University of Catalonia (UPC) and others. The RT ADIGIM performance proved to be better, nearly as good as the rapid or final GIMs computed retrospectively with delays of hours to days. Besides, the non-RT ADIGIM quality is as good or better than most GIM products. The oceanic regions have been included in the assessment which showed that ADIGIM interpolation gives the best estimation (referred to JASON3). The developed method, UADG, will constitute the next-generation UPC RT-GIM, and also UARG will improve the current product UQRG (the current UPC rapid GIM product computed retrospectively) due to its complementary information., This work has been partially supported by the Project PID2019-107579RB-I00 (MICINN). Also was partially supported by the 2017 SGR-0851 Grant of the Generalitat de Catalunya and by the EU Project 101007599 - PITHIA-NRF.

Ionospheric delay is an important factor affecting the high precision satellite positioning. Under short baseline conditions, it is possible to directly eliminate most of the ionospheric delay by using double-difference (DD) observation, while under long baseline conditions, it is necessary to use the ionospheric information from the base station network to perform interpolation. A DD ionospheric interpolation method under long baseline conditions based on Gaussian process regression is proposed in this paper. In our theory, we introduced the difference between the geographic longitude of the subsolar point and the ionospheric pierce point, and use it together with geographic coordinates as the input of kernel function. Kernel of the zenith ionospheric delay at the pierce point is reasonable assumed, and kernel of the DD ionosphere delay is derived based on it. Experimental results show that using the kernel proposed in this paper to perform Gaussian process regression interpolation, its effect can be better than the traditional Kriging and linear interpolation algorithms, and the accuracy is generally within 1 cm.

When signals from satellites propagate through the ionosphere, a delay is introduced due to the presence of Total Electron Content (TEC) between transmitter and receiver. The generation of TEC in the ionosphere is primarily dependent on solar activity (Diurnal and Seasonal). The ionospheric delay can cause a major degradation in the positional accuracy of the satellite navigation system. For the estimation of ionospheric delay, slant TEC (STEC) along the path between satellite and receiver is needed. For a single-frequency user, a modeled ionospheric vertical TEC (VTEC) at Ionospheric Pierce Point (IPP) is converted into STEC for delay estimation. However, the behavior of TEC is highly dynamic in low-latitude and equatorial regions (Indian region), and thus conventional ionospheric model introduces additional error in positioning. The NavIC (Navigation with Indian Constellation) system geostationary satellite constellation is uniquely capable of the investigation of ionospheric TEC, and it can facilitate for ionospheric modeling applications. This paper deals with estimating of accurate STEC and VTEC using dual-frequency NavIC code and carrier measurements, and investigation of its temporal variation for modeling applications.

Vertical total electron content (VTEC) global ionospheric maps (GIM) are commonly used to correct the ionospheric delay of global navigation satellite system (GNSS) signals for single-frequency positioning and other ionospheric studies. The measurements observed by inhomogeneously distributed ground reference stations are the only data used to generate the GIMs. Thus the accuracy of the GIMs over ocean and polar regions is relatively poor due to the lack of measurements over these regions. In this study, space-borne VTECs obtained from ocean-altimetry and GNSS radio occultation measurements are incorporated into the modelling process. Since the three types of VTEC data have different qualities, the weight for each type of data is determined using the Helmert-variance component estimation (Helmert-VCE) method. In addition, unlike the traditional weighted least squares (WLS) estimation method in which the design matrix of observation equations is fixed, in this study, the design matrix, especially those elements in design matrix that are derived from the coordinates of either tangent point or ionospheric pierce point, are considered to be inaccurate. Thus they are adjusted together with the unknown coefficient parameters of the fitting model using the fast-weighted total least squares (fast-WTLS) technique. The proposed approach, named Helmert-WTLS, was tested using the data in the period of day of year (DOY) 217–224, 2016 and validated using GIMs produced by the research team for ionosphere and precise positioning based on BDS/GNSS (GIPP) at the Academy of Opto-Electronics, Chinese Academy of Sciences (CAS). Comparison results showed that the GIMs (with a 2 h temporal resolution) generated using the new approach can improve the determination of ionospheric TEC by 0.28 TEC units (TECU) over those from the Helmert-VCE-aided WLS approach (w.r.t CAS references, respectively) and by 1.61 TECU better than those from WLS, in terms of the mean of all root-mean-squares errors of all 2 h time slots in the 8-day testing period. In addition, in comparison with out-of-sample Jason-3 observations, results from the proposed method also outperformed Helmert-VCE-aided WLS, CAS and CODE models by 1.5, 2.4 and 2.4 TECU, respectively.

This paper presents a method of deriving the instrumental differential code biases (DCBs) of GPS satellites and dual frequency receivers. Considering that the total electron content (TEC) varies smoothly over a small area, one ionospheric pierce point (IPP) and four more nearby IPPs were selected to build an equation with a convolution algorithm. In addition, unknown DCB parameters were arranged into a set of equations with GPS observations in a day unit by assuming that DCBs do not vary within a day. Then, the DCBs of satellites and receivers were determined by solving the equation set with the least-squares fitting technique. The performance of this method is examined by applying it to 361 days in 2014 using the observation data from 1311 GPS Earth Observation Network (GEONET) receivers. The result was crosswise-compared with the DCB estimated by the mesh method and the IONEX products from the Center for Orbit Determination in Europe (CODE). The DCB values derived by this method agree with those of the mesh method and the CODE products, with biases of 0.091 ns and 0.321 ns, respectively. The convolution method's accuracy and stability were quite good and showed improvements over the mesh method.

An ionospheric spatial decorrelation is one of the most dominant error factors that affects the availability of safety-critical differential global navigation satellite systems (DGNSS). This is because systems apply significant conservatism on the error source when ensuring navigation safety due to its unpredictable error characteristic. This paper investigates a correlation between GNSS-derived ionospheric spatial decorrelation and space weather intensity. The understanding of the correlation has significant advantages when modeling residual ionospheric errors without being overly pessimistic by exploiting external sources of space weather information. An ionospheric spatial decorrelation is quantified with a parameter of spatial gradient, which is an ionosphere total electron content (TEC) difference per unit distance of ionospheric pierce point (IPP). We used all pairs of stations from dense GNSS networks in the conterminous United States (CONUS) that provide an IPP separation distance of less than 100 km to obtain spatial gradient measurements under both ionospherically quiet and active conditions. Since the correlation results would be applied to safety-critical navigation applications, special attention was paid by taking into consideration all non-Gaussian tails of a spatial gradient distribution when determining spatial gradient statistics. The statistics were compared with space weather indices which are disturbance storm time (Dst) index and interplanetary magnetic field (IMF) Bz index. As a result, the ionospheric spatial decorrelation showed a significant positive correlation with both indices, especially under active ionospheric conditions. Under quiet conditions, it showed positive correlation slightly weaker than those under active conditions, and the IMF Bz showed preceding response to the spatial gradient statistics revealing the potential applicability for predicting the spatial decorrelation conditions.

The nightglow observations of OI 630.0 nm emission carried out from low latitude station Kolhapur using All Sky Imager (ASI) with 140° field of view (FOV) for the month of April 2011 are used. The images were processed to study the field aligned irregularities often called as equatorial plasma bubbles (EPBs). The present study focuses on the occurrence of scintillation during the traversal of EPBs over ionospheric pierce point (IPP). Here we dealt with the depletion level (depth) of the EPB structures and itseffect on VHF signals. We compared VHF scintillation data with airglow intensities at Ionospheric pierce point (IPP)from the same location. The determination of IPP can be done by drawing its geometry as shown in Figure 1.

Total electron content (TEC) is an important parameter in studying the characteristics of the ionosphere of the Earth. By measuring the phase and group delays of the signals produced by Global Navigation Satellite System (GNSS) satellites, the slant TEC along the ray path can be measured. Assuming that the ionosphere is a thin shell, the point of intersection between the shell and the ray path is the ionospheric pierce point (IPP). The slant TEC at this point can be converted to the vertical TEC (VTEC). To produce continuous 2-dimensional TEC maps, a mapping method must be used based on the data that has been measured. While there are numerous methods to map TEC that are in used today, this study investigates the performance of Kriging interpolation over the Philippine region using the data from the Philippine Active Geodetic Network (PAGENet). To test the performance, the maps produced were compared to the Global Ionosphere Maps (GIMs) which are provided by the International GPS Service (IGS). The initial results show that the PAGENet maps have lower TEC values compared to GIMS with a mean difference of ~6 TECU with 12 UT having the highest difference.

The work presents results of the diagnostics of travelling ionospheric disturbances (TIDs) over the Antarctic Peninsula obtained by using two different techniques. The first method is based on ground-based multi- positional GNSS TEC measurements, and the second one is bistatic coherent ionospheric HF radio sounding between “Akademik Vernadsky” and “Palmer” Antarctic stations. A comparison of Doppler frequency shifts of the probe HF signal received at “Palmer” station with the dynamics of TEC variations has been made. Quasi-periodic variations associated with the propagation of TIDs were registered simultaneously in both types of the data. A case study performed in this paper has shown that the parameters of the ionospheric disturbance estimated by two methods are in a good agreement, given a specific choice of ionospheric pierce point (IPP) trajectories of GNSS links near the reflection point of the HF probing signal and TID motion direction collinear to the radiopath and perpendicular to IPP trajectories.

The East African ionosphere (3°S-18°N, 32°E-50°E) was mapped using Total Electron Content (TEC) measurements from ground-based GPS receivers situated at Asmara, Mekelle, Bahir Dar, Robe, Arbaminch, and Nairobi. Assuming a thin shell ionosphere at 350 km altitude, we project the Ionospheric Pierce Point (IPP) of a slant TEC measurement with an elevation angle of >10° to its corresponding location on the map. We then infer the estimated values at any point of interest from the vertical TEC values at the projected locations by means of interpolation. The total number of projected IPPs is in the range of 24-66 at any one time. Since the distribution of the projected IPPs is irregularly spaced, we have used an inverse distance weighted interpolation method to obtain a spatial grid resolution of 1°×1° latitude and longitude, respectively. The TEC maps were generated for the year 2008, with a 2 hr temporal resolution. We note that TEC varies diurnally, with a peak in the late afternoon (at 1700 LT), due to the equatorial ionospheric anomaly. We have observed higher TEC values at low latitudes in both hemispheres compared to the magnetic equatorial region, capturing the ionospheric distribution of the equatorial anomaly. We have also confirmed the equatorial seasonal variation in the ionosphere, characterized by minimum TEC values during the solstices and maximum values during the equinoxes. We evaluate the reliability of the map, demonstrating a mean error (difference between the measured and interpolated values) range of 0.04-0.2 TECU (Total Electron Content Unit). As more measured TEC values become available in this region, the TEC map will be more reliable, thereby allowing us to study in detail the equatorial ionosphere of the African sector, where ionospheric measurements are currently very few.

The nightglow observations of OI 630.0 nm emission carried out from low latitude station Kolhapur using All Sky Imager (ASI) with $140^{\circ}$ field of view (FOV) for the month of April 2011 are used. The images were processed to study the field aligned irregularities often called as equatorial plasma bubbles (EPBs). The present study focuses on the occurrence of scintillation during the traversal of EPBs over ionospheric pierce point (IPP). Here we dealt with the depletion level (depth) of the EPB structures and its effect on VHF signals. We compared VHF scintillation data with airglow intensities at Ionospheric pierce point (IPP) from the same location and found that the largely depleted EPBs make stronger scintillation. From previous literature, it is believed that the small scale structures are present near the steeper walls of EPBs which often degrades the communication, the analysis presented in this paper confirms this belief.

The current constellation of the BeiDou Navigation Satellite System (BDS) consists of five geostationary earth orbit (GEO) satellites, five inclined geosynchronous satellite orbit (IGSO) satellites, and four medium earth orbit (MEO) satellites. The advantage of using GEO satellites to monitor the ionosphereis the almost motionless ionospheric pierce point (IPP), which is analyzed in comparison with the MEO and IGSO satellites. The results from the analysis of the observations using eight tracking sites indicate that the ionospheric total electron content (TEC) sequence derived from each GEO satellite at their respective fixed IPPs is always continuous. The precision of calculated vertical TEC (VTEC) using BDS B1/B2, B1/B3, and B2/B3 dual-frequency combinationsis compared and analyzed. The VTEC12 precision based on the B1/B2 dual-frequency measurements using the smoothed code and the raw code combination is 0.69 and 5.54 TECU, respectively, which is slightly higher than VTEC13 and much higher than VTEC23. Furthermore, the ionospheric monitoring results of site JFNG in the northern hemisphere, and CUT0 in the southern hemisphere during the period from 1 January to 31 December 2015 are presented and discussed briefly.

Ionospheric delay error is one of the most significant error sources for single frequency SBAS (Satellite Based Augmentation System) users and its error bound is the key point for PL (protection level) calculation. To satisfy the integrity specifications for users anywhere in the service region, the error bound need to be inflated according to various threat. Among them, the undersampled threat plays an important role in the inflation. The fundamental reason for the undersampled threat lies in the disagreement of the linear planar model to the actual ionosphere characteristics, which cannot be accurately described by the discrete IPP (Ionospheric Pierce Point) samples. Ionosphere threat model is established to handle this problem. It uses historical irregular ionosphere data to generate the two-dimensional lookup table which illustrates the relationship between the IPP distribution and the maximum estimation error. How to abstract the metrics to describe the discrete two-dimensional IPP distribution is the key to the establishment of ionosphere threat model. In this paper, we conclude the properties of a ‘good’ metric from a physical point of view and propose the Relative COVerage (RCOV) metric. Based on the measured data from IGS (International GNSS Service) stations, taking the regions of Europe (20°N~70°N, −30°W~60°E) as an example, the ionosphere threat model based on RCOV metric is established and compared with the RCM (Relative Centroid Metric) model used in WAAS (Wide Area Augmentation System) and MSAS (MTSAT Satellite based Augmentation System). At the same time, the integrity of the ionosphere GIVE (Grid Ionosphere Vertical Error) results is verified. The experimental results show that under the experimental configuration given in this paper, the RCOV metric proposed can improve the system availability on the premise of meeting the integrity requirements.

Total electron content (TEC) is an important parameter for revealing latitudinal ionospheric structures, such as the equatorial ionization anomaly (EIA) in Southeast Asia. Understanding the EIA is beneficial for studying equatorial spread F. To reveal the structures, the absolute TEC as a function of latitude must be accurately determined. In early 2012, we expanded a GNU Radio Beacon Receiver (GRBR) network to provide latitudinal coverage in the Thailand-Indonesia sector. We employed the GRBR network to receive VHF and UHF signals from polar low-Earth-orbit satellites. The TEC offset is an unknown parameter in the absolute TEC estimation process. We propose a new technique based on the two-station method to estimate the offset for the latitudinal TEC estimation, and it works better than the original method for a sparse network. The TEC estimation system requires two iterations to minimize the root-mean-square error (RMSE). Once the RMSE reaches the global minimum, the absolute TECs are estimated simultaneously over five GRBR stations. GPS-TECs from local stations are used as the initial guess of the offset estimation. The height of the ionospheric pierce point is determined from the ionosonde hmF2. As a result, the latitudinal GRBR-TEC was successfully estimated from the polar orbit satellites. The two EIA humps were clearly captured by the GRBR-TEC. The result was well verified with the TEC reconstructed from the C/NOFS density data and the ionosonde bottomside data. This is a significant step showing that the GRBR is a useful tool for the study of low-latitude ionospheric features.

Global Positioning System (GPS) has been widely used to sense crustal deformation and ionospheric anomalies, particularly seismic ionospheric disturbances. In March 2011, the earthquakes with magnitude of up to Mw = 9 occurred in Tohoku near the east coast of Honshu, Japan. The GPS Earth Observation Network (GEONET) in Japan with more than 1200 continuously operating stations provides a unique opportunity to study the detailed seismic ionospheric disturbances. In this paper, the pattern and evolution of seismic ionospheric disturbances following the Tohoku earthquakes are investigated by dense GEONET data, including amplitude, propagation pattern, direction, speed, and evolution. Maximal coseismic ionospheric disturbances are found with up to more than 4 TECU, and the disturbance period is around 10–20 min. The seismic ionospheric effects following the aftershocks are attenuated with the increase of the time and distance between the ionospheric pierce point and the epicenter of the main event, which last more than 2 h. Seismic ionospheric disturbance detected by GPS measurement is not only related to the main shock but also the giant aftershocks. Propagation velocities of the total electron content (TEC) disturbance show a decrease when it spreads 400–600 km away from the epicenter in the north-western direction, where it is just near the west coast of Japan. Furthermore, the TEC disturbance also has obvious directional features. In the first half hour, the TEC disturbance in the southeast direction has the biggest amplitude, while the dominant direction is changed to northwest tens of minutes later. In addition, signals with higher frequencies are existed in seismic TEC variation at the epicenter region but do not appear in the far field.

Firstly, this paper introduces the common methods and accounts for the advantage of ionosphere grid products for correcting ionosphere errors with single-frequency GPS, according the analysis that ionosphere grid product can effectively ameliorate errors of single-frequency GPS navigation and positioning. In detail, this article gives the establishment, forecast and operation methods of ionosphere grid products, and introduces the method that calculates the VTEC (Vertical Total Electronic Contents) values of the geocentric latitude of the ionospheric pierce point, the VTEC values of four-point interpolation models and the single-layer mapping function. It’s also compares different pairs of ionosphere correction data and satellite ephemeris which use to analyze the impact of real-time positioning accuracy of single-frequency GPS. The experiments prove that the predicted ionosphere grid products can obviously improve the precision for single-frequency users, and have some practical values for single-frequency positioning and orbit determination.

A modified ionospheric correction method and the corresponding approximate algorithm for spaceborne single-frequency Global Positioning System (GPS) users are proposed in this study. Single Layer Model (SLM) mapping function for spaceborne GPS was analyzed. SLM mapping functions at different altitudes were calculated. Ionospheric Pierce Point (IPP) trajectories of the dlft station (An IGS station located at the longitude of 4°23′15.22′′E and the latitude of 51°59′9.63′′N, in the TU Delft University, The Netherlands.) and the GRACE satellite were computed with the corresponding single layer height of 350 and 500 km, respectively. The Klobuchar model was used to compute ionospheric delays for the dlft station, and modified Klobuchar model, together with scale factors, was used to compute the fractional ionospheric corrections above the GRACE altitudes. Calculation results were validated using dual-frequency observations. The study shows that the single layer height needs to be changed from 350 to 500 km according to the altitude of GRACE. Approximate forms of Earth angle and slant factor developed for modified Klobuchar model are applicable to GRACE, with accuracy adequate to preserve the essential elements required to compute ionospheric delays. Results show that the Klobuchar model is effective for ground GPS, and the modified Klobuchar model corrects more than 80% on average of the ionospheric delays for spaceborne single-frequency GPS.

Medium Scale Travelling Ionospheric Disturbance (MSTID), thought to be manifestation of atmospheric gravity wave (AGW) in the ionospheric altitude that propagates horizontally and effects on in the electron density structure of ionosphere. These atmospheric gravity waves sourced from lower atmospheric activities such as typhoons, volcanic eruptions and tsunamis. Wave energy by its coupling induction process can travel to the ionosphere region. It has been understood that the TID's wave structure have an impact on the propagation of radio waves in the ionosphere so that it will affect the performance of navigation satellite-based positioning measurements. Based on Aceh tsunami in December 2004, this study aimed to investigation of the induction of atmospheric gravity waves in the ionosphere using total electron content (TEC) data from the Sumatra GPS network (Sumatra GPS Array, SUGAR). The detection technique of TEC changes due to AGW induction with a filter to separate medium scale disturbance at the ionospheric pierce point at an altitude of 350 km (IPP, Ionospheric Pierce Point). The results show the horizontal wavelength of a medium-scale TID around 180 ± 12 Km with a velocities of around 376 ± 9 ms-1. Based on two-dimensional map, the TID moves to the southeast.

Solar, geomagnetic, gravitational and seismic activities cause disturbances in the ionospheric region of upper atmosphere that may disrupt or lower the quality of space based communication, navigation and positioning system signals. These disturbances can be categorized with respect to their amplitude, duration and frequency. Typically in the literature, ionospheric disturbances are investigated with gradient based methods on Total Electron Content (TEC) data estimated from ground based dual frequency Global Positioning System (GPS) receivers. In this study, a fast algorithm is developed for the automatic detection of the variability in Slant TEC (STEC) data. STEC is defined as the total number of electrons on the ray path between the ground based receiver and GPS satellite in the orbital height of 20,000 km. The developed method, namely, Differential Rate of TEC (DROT), is based on Rate of Tec (ROT) method. ROT is widely used in the literature and it is usually applied to Vertical TEC (VTEC) that corresponds to the projection of STEC to the vertical direction along the ray path at the Ionospheric Pierce Point (IPP) using a mapping function. The developed DROT method can be defined as the normalized metric norm between the ROT and its baseband trend structure. In this study, the performance of DROT is determined using synthetic data with variable bounds on the parameter set of amplitude, frequency and duration of disturbance. It is observed that DROT method can detect disturbances in three categories. For DROT values less than 50%, there is no significant disturbance in STEC data. For DROT values between 50% and 70%, a medium scale disturbance can be observed. For DROT values over 70%, severe disturbances such as Large Scale Traveling Ionospheric Disturbances (LSTIDs) can be observed. DROT method is highly sensitive to the amplitude of the wave-like oscillations. For a disturbance amplitude as low as 1.01 TECU, the disturbances that have durations longer than or equal to 20 min and frequencies higher than 1.095 mHz; or frequencies higher than 0.511 mHz and durations longer than 55 min can be detected automatically with DROT values of 50%. When DROT method is applied to midlatitude STEC values for disturbed days, it is observed that DROT method can detect the disturbances in near-real time, even if the GPS data from the station is uploaded with 15 min intervals. (C) 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

This chapter deals with the ionospheric impairments and is a logical extension of the previous chapter. In this chapter, the methods of measuring the impairment parameters including the respective suitable instruments, corresponding models and methods of mitigating the impairments are explained. Both the direct and derived methods of estimating the ionospheric delay and scintillation are described. Two important models, viz. the Nequick and the Klobuchar model, for the delay and WBMOD for the scintillation are illustrated. The basic approach of mitigating these impairments is also elucidated in this chapter. Finally, the applications of these impairment data have been mentioned and the technique of tomographic estimation has been elaborated.

Large spatial variations in ionospheric delay of Global Navigation Satellite System signals observed during severe ionospheric storms pose potential threats to the integrity of the Ground-Based Augmentation System, which supports aircraft precision approaches and landing. Range-domain monitoring within the Ground-Based Augmentation System ground facility cannot completely eliminate all possible ionospheric threats, because ionospheric gradients are not observable to the ground monitor if they impact the satellite-to-ground lines of sight with the worst-possible geometry and velocity. This paper proposes an algorithm called position-domain geometry screening to remove potentially hazardous satellite geometries under worst-case ionospheric conditions. This is done by inflating one ormore integrity parameters broadcast by the ground facility. Hence, the integrity of the system can be guaranteedwithout anymodification of existing avionics. This paper develops an algorithm that allows the ground station to conservatively estimate the worst-case ionospheric errors for Ground-Based Augmentation System users. The results of this algorithm determine which potential aircraft satellite geometries are safe and which are unsafe, and inflation of the broadcast vig parameter is used to make all unsafe geometries unusable for the Ground-Based Augmentation System. Although the elimination of unsafe geometries reduces system availability, this paper shows that acceptable availability for category I precision approaches is attainable at Memphis International Airport and Newark Liberty International Airport while guaranteeing system integrity under anomalous ionospheric gradients.

Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the U.S. local area augmentation system are shown. During extreme ionospheric activity, the gradient suffered by a global navigation satellite system user a few kilometers away from a ground reference station may reach as high as 425 mm of delay (at the GPS L1frequency) per km of user separation. The method of data analysis that produced these results is described, and a threat space that parameterizes these possible threats to user integrity is defined. Certain configurations of user, reference station, global navigation satellite system satellite, and ionospheric storm-enhanced density may inhibit detection of the anomalous ionosphere by the reference station.

In recent years, with the development of wide area differential technology, Satellite-Based Augmentation System (SBAS) has gotten extensive application in every field, such as aviation, marine, ground transportation and so on. However, with the increase of satellites serving time, the capability of satellites may degenerate. For another, as more and more satellites have been used, the probability of double satellite ephemeris fault will be increased, making the navigation system unreliable to supply Safety of Life (SOL) Service. Therefore, the availability of SBAS has been growing more and more important. It is necessary to predict the availability of SBAS in advance so that users could choose a safe and efficient navigation system. Unfortunately, existing availability algorithm based on broadcast information doesn't work in this situation, where two real-time integrity parameters are required. One is User Difference Range Error Indicator (UDREI), which is used to describe integrity related to satellite clock/ephemeris error correction for each satellite. The other is Grid Ionospheric Vertical Error Indicator (GIVEI), which is used to describe integrity related to ionospheric error correction for each Ionospheric Grid Point (IGP). In this paper, the relationship between UDREI and the number of visible Ranging and Integrity Monitoring Stations (RIMS), as well as the relationship between GIVEI and the number of visible Ionospheric Pierce Point (IPP), is analyzed through statistical analysis. With the relationship, a simplified method of predicting the availability of SBAS is proposed to solve the heavy computation and weak timeliness of the traditional algorithms. Finally, experiments are performed to evaluate the effectiveness of our proposed method. Compared with the actual performance, the prediction is encouraging and this model may be a good contender for predicting the availability of SBAS.

The GPS data provides an effective way to estimate the total electron content (TEC) from the differential time delay of L1 and L2 transmissions from the GPS. The spacing of the constellation of GPS satellites in orbits are such that a minimum of four GPS satellites are observed at any given point in time from any location on the ground. Since these satellites are in different parts of the sky and the electron content in the ionosphere varies both spatially and temporally, the ionospheric pierce point (IPP) altitude or the assumed altitude of the centroid of mass of the ionosphere plays an important role in converting the vertical TEC from the measured slant TEC and vice versa. In this paper efforts are made to examine the validity of the IPP altitude of 350 km in the Indian zone comprising of the ever-changing and dynamic ionosphere from the equator to the ionization anomaly crest region and beyond, using the simultaneous ionosonde data from four different locations in India. From this data it is found that the peak electron density height (hpF2) varies from about 275 to 575 km at the equatorial region, and varies marginally from 300 to 350 km at and beyond the anomaly crest regions. Determination of the effective altitude of the IPP employing the inverse method suggested by Birch et al. (2002) did not yield any consistent altitude in particular for low elevation angles, but varied from a few hundred to one thousand kilometers and beyond in the Indian region. However, the vertical TEC computed from the measured GPS slant TEC for different IPP altitudes ranging from 250 to 750 km in the Indian region has revealed that the TEC does not change significantly with the IPP altitude, as long as the elevation angle of the satellite is greater than 50 degrees. However, in the case of satellites with lower elevation angles (

Orbit determination results are presented for a low-Earth-orbiter (LEO) satellite carrying a single-frequency global-positioning-system (GPS) receiver. Various techniques to correct for errors induced by ionospheric refraction are compared. These include the Jet Propulsion Laboratory’s (JPL) global ionosphere maps (GIM), which provide global maps of total electron content. The direct-calibration method, differenced range versus integrated Doppler (DRVID), which uses range differences of group delay and phase advance to compensate for the first-order ionospheric error, was also tested. The fidelity of the orbit solutions was compared using orbit overlaps and orbit differences from dual-frequency truth orbits computed using MicroCosm and JPL’s Gipsy-Oasis-II program. Analyses showed that, with a single-frequency GPS receiver on the LEO satellite, the DRVID-corrected carrier phase can determine the orbit with accuracy below the meter level. JPL’s GIM correction to pseudorange data also provided improved results over no ionospheric error correction.

The Total Electron Content (TEC) measured from a station situated near the northern crest of the equatorial anomaly is compared with that obtained from models such as the Parameterized Ionospheric Model (PIM 1.6) and International Reference Ionosphere (IRI-95) for well over one solar cycle (1977–1990). The limitations of conversion from vertical to slant TEC and vice versa as required for GPS ionospheric corrections in the equatorial region are discussed. It is found that the correspondence among the vertical TEC at the ionospheric pierce point, geometrically (sec χ) converted slant TEC, and slant TEC along a GPS signal propagation path becomes poor for elevation angles of less than 80 deg. Based on this finding, an optimum grid size for reliable operation of the Satellite-Based Augmentation System (SBAS) in the Indian subcontinent (GAGAN, or GPS and Geo Augmented Navigation) is estimated. The suggested grid size is much smaller than the standard 5 × 5 deg.

By establishing the ionospheric eclipse factor (IEF) λ of the ionospheric pierce point (IPP) and its ionospheric influence factor (IFF) λ, and combining λ, λ with t of IPP, a new method of modeling high-precision ionospheric delay using GPS data—ionospheric eclipse factor method (IEFM)—is presented in this paper. The IEFM can effectively select the proper ionospheric models to model the total electron content (TEC) with different changes corresponding to annual, seasonal and diurnal variations. Initial experimental results show that the correction precision of ionospheric delay modeled by the IEFM seems to be close to that of using L3 GPS observations to directly correct the corresponding ionospheric delay.

Chapter 8 describes how to reduce the effects of the errors in positioning using differential positioning techniques, in which a measurement done at a reference receiver at a precisely known position is used along with some of its derivatives. In this chapter, first the basic idea of differential positioning is elucidated and then the different options of the correcting ranges differentially are briefly mentioned. This is followed by a brief review of the types of errors, which is very important to understand the differential techniques. This also forms the basis of different possible classifications of differential positioning, the general classes of which are also described subsequently. The different techniques of differential positioning and their related intricacies are subsequently described in detail, including both the code-based and carrier-based procedures. Finally, we provide some overview of the typical implementations of these methods for different applications and systems that have been discussed qualitatively.

Theglobalpositioningsystem will be used for awide variety of applications.However, aircraft use of any satellitebased navigation system raises signiŽ cant concern with respect to accuracy and integrity. The wide-area augmentation system (WAAS) is a navigation system that adds an independent ground network and increases navigation accuracy and integrity. The WAAS will be a supplemental navigation aid for all phases of  ight down to category I precision approach in 1997. Eventually, it will be a primary navigation aid. Stanford University installed three wide-area reference stations in the Western United States and has been conducting independent  ight tests of WAAS since 1994. The comparison of the two different WAAS algorithms and the results from the Stanford UniversityWAAS static and  ight tests are presented. The results showed that 3.5m ofWAAS user vertical positioning accuracy, compared to 80m for stand-alone,can be achieved using bothWAAS algorithms.That accuracy is within the § 4:1-m 95% error limits for a category I instrument landing system. An improvement on the existing grid ionosphere estimation algorithm is also included as well as the introduction of a weighted least-square solution in WAAS user position computation.

Several countries are involved in developing satellite-based augmentation systems (SBAS) for improving the positional accuracy of GPS. India is also developing one such system, popularly known as GPS-aided geo-augmented navigation (GAGAN), to cater to civil aviation applications. The ionospheric effect is the major source of error in GAGAN. An appropriate efficient and accurate ionospheric time model for GAGAN is necessary. To develop such a model, data from 17 GPS stations of the GAGAN network spread across India are used in modelling. The prominent model, known as bi-linear interpolation technique, is investigated for user IPP (UIPP) delay estimation. User IPP delays for quiet, moderate and disturbed days are estimated. It is evident that measured mean UIPP delays closely follow estimated mean UIPP delays. First published online: 01 Jul 2013

GPS ray propagates at two carrier frequencies; L 1 at 1575.42 MHz and L 2 at 1227.6 MHz. In this project, Jones 3D Ray Tracing program being used to determine the characteristics of the GPS ray as it propagates through the ionosphere. A numerical mathematical model which is continuous and contains no singularities has been used to represent the actual formation of the ionosphere in 3 dimensional (latitude, longitude and altitude). By using GPS ray tracing, it has been proved that the path of propagation for both L 1 and L 2 in the ionosphere is different. The point (latitude and longitude) where L 1 intersects at IPP is different than the point (latitude and longitude) of intersection of L 2 . IPP or Ionospheric Pierce Point is the altitude in the ionosphere where the composition of electron density is greatest. It is also shown that the distance between the L 1 and L 2 at IPP over the equatorial region is greater than in the mid-latitude region. This is due to the difference in the composition of electron density in both these regions and also due to the presence of ionospheric horizontal gradient. In GPS, the presence of ionospheric horizontal gradient can effect the final GPS positioning if its' being ignored, especially the GPS positioning over the equatorial region.

Ionosphere layer is very important to the communication system. In Malaysia the ionosphere layer is unique because the location is near the equator where it exposed more ultraviolet radiation (UV) compared to other region. This project focuses on the determination optimized ionospheric layers for TEC measurements at equatorial region. The main parameter in this project is Total Electron Content (TEC) with elevation angle of satellite between 50 to 85 degrees. Since these satellites are in different parts of the sky and the electron content in the ionosphere varies both spatially (track) and temporally (time), the Ionospheric Pierce Point (IPP) altitude plays an important role in converting the vertical TEC from the measured slant TEC. The TEC computed from the Global Positioning System (GPS) data for different altitudes ranging from 50 km to 750km. The data from GPS dual frequency which are in Receiver Independent Exchange Format (RINEX) are supplied by Department of Survey and Mapping Malaysia (JUPEM). The data taken from two different stations located at Universiti Sains Malaysia Pulau Pinang (USMP) and Universiti Teknologi Malaysia Johor (UTMJ). The results show a variation of TEC does not significantly changes when the elevation angle is greater than 50 degrees. Therefore, the IPP altitude of 450 km to 750 km may consider as effective ionospheric layer in the equatorial region.

Current dual-frequency methods of ionospheric correction eliminate the first order ionospheric term. However, the second and third order ionospheric terms in the refractive index, and errors due to bending of the signal are not fully compensated in this approach. The total electron content is assumed to be the same along two GPS signals, whereas in real cases, due to the dispersive nature of the ionosphere, the TEC along the GPS-L1 path will be different from that along the GPS-L2 path. Although the second order ionospheric term is considered in recent investigations, a common practice is to compute the magnetic field vector at a single point called the ionospheric pierce point assuming the ionosphere as a single layer at a certain altitude. All these approximations lead to erroneous estimations and corrections of the ionospheric errors. In this work, higher order ionospheric errors are rigorously treated and computed for a large number of ionosphere electron density profiles reconstructed from the CHAMP-GPS radio occultation measurements. Different higher order correction formulas have been evaluated using the CHAMP based profiles.

A new method for modeling the ionospheric delay using global positioning system (GPS) data is proposed, called the ionospheric eclipse factor method (IEFM). It is based on establishing a concept referred to as the ionospheric eclipse factor (IEF) λ of the ionospheric pierce point (IPP) and the IEF’s influence factor (IFF) $$\bar{\lambda}$$ . The IEF can be used to make a relatively precise distinction between ionospheric daytime and nighttime, whereas the IFF is advantageous for describing the IEF’s variations with day, month, season and year, associated with seasonal variations of total electron content (TEC) of the ionosphere. By combining λ and $$\bar{\lambda}$$ with the local time t of IPP, the IEFM has the ability to precisely distinguish between ionospheric daytime and nighttime, as well as efficiently combine them during different seasons or months over a year at the IPP. The IEFM-based ionospheric delay estimates are validated by combining an absolute positioning mode with several ionospheric delay correction models or algorithms, using GPS data at an international Global Navigation Satellite System (GNSS) service (IGS) station (WTZR). Our results indicate that the IEFM may further improve ionospheric delay modeling using GPS data.

[1] Global Navigation Satellite Systems like the Global Positioning System and the Russian counterpart GLONASS permit the estimation of ionosphere parameters in the form of vertical total electron content (vTEC) values. Although the GNSS reference systems are rotation-symmetric ellipsoids, simple approximations of gravitational equipotential surfaces, the modeling of global vTEC maps is carried out in spherical systems, since this simplifies mathematical expressions. Due to the Earth's oblateness this approximation is in slight disagreement to the geometric situation of the observations. Thus it is expected that estimated vTEC values will change when the WGS84 ellipsoid is taken as a reference for all computations. We will discuss both, the direct effect (i.e due to a shift of the ionospheric pierce point) and the indirect effect, caused by a change of the mapping function, on global vTEC values using a single layer model of the ionosphere. We show that the estimated global TEC maps change slightly by up to ±0.4 TECU when refined mathematical models are considered and we discuss how differential code biases are affected by this change.

[1] The equatorial and low-latitude ionosphere is characterized by dynamic changes in the spatial distribution of F region electron density. The in situ density data from Indian SROSS C2 satellite was used in conjunction with the international reference ionosphere (IRI) to demonstrate how the median models have limitations to represent the spatial gradients in the equatorial and subtropical latitudes in the Indian sector. The ground-based ionosonde data was used to derive the bottomside electron density profiles which were matched with IRI to derive the complete F region electron density profile for different times to show the variability in the centroid of ionization mass distribution due to the equatorial plasma dynamics associated with Appleton ionization anomaly. The study brings out that a uniform ionospheric pierce point altitude cannot be assumed for GPS applications in this sector. However, during low sunspot activity periods, the GPS-derived total electron content (TEC) using 2 × 2 grid point data indicates that spatial gradients in TEC are not too significant in meridian transects and that the assumption of homogeneous distribution is valid for the conversion of slant to vertical TEC even in the equatorial latitudes during solar minimum. Keeping this in view, a qualitative study has been carried out on the possible variability in ionospheric pierce point altitude at representative subtropical latitude in the Indian sector and the results are discussed in light of diurnal and seasonal features of the equatorial ionosphere.

[1] The relationship between total electron content (TEC) and the solar soft X-ray irradiances is presented. Three bands (2–7 nm, 6–19 nm, and 17–20 nm) of solar soft X-ray measurements from the Student Nitric Oxide Explorer (SNOE) satellite are examined and all show a similar relationship with TEC. The TEC data are from a GPS receiver near Ancon, Peru (� 11.78 degrees latitude, � 77.15 degrees longitude) from 11 March 1998 to 23 August 1999 and 2 October 1999 to 10 June 2000. During these periods the average TEC measurement was calculated from all observations whose ionospheric pierce point occurred within � 12 ± 2 degrees latitude and � 77 ± 2 degrees longitude and within the hour selected. TEC shows a more significant correlation with soft X-ray irradiances than with F10.7. The X rays lead the TEC by approximately 0.8–1.3 days, which is consistent with the neutral density affecting the TEC. The magnitude of these short term (27 days or less) changes is approximately 0.18 of the total TEC. During the period examined geomagnetic activity, as represented by Ap, could account for half as much variation in TEC (0.1 of the total TEC) as the solar irradiance.