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The sporadic E (Es) layer is an ionospheric layer with a high electron density and thin altitudinal thickness and appears at altitudes around 100 km. The density is sometimes so high that even VHF radio waves from the ground can be reflected and propagate beyond the radio horizon. It has been shown that aeronautical VHF navigation signals in the 108–118 MHz band can propagate beyond the radio horizon to cause potential interference. However, there has been little technique to monitor the Es layer in wide area. The fine structures of the Es layers which reflects the dynamics of the thermosphere has not been well understood yet. This study shows that the ROTI (rate of TEC index) map by utilizing dense GNSS observations is effective in detecting the Es layer at least in the daytime. The Es layer studies has a well-defined frontal structure, when ROTI values are mapped at the typical Es layer height (100 km). A methodology of detecting and characterizing the Es layer frontal structure without manual operation is developed by utilizing the Hough transformation, and the front direction and velocity are successfully derived. Utilizing the characteristics of the frontal structure, fine structures of the Es layer can be studied, and a couple of interesting features are found. However, this technique would not always be applicable. Application to a homogeneous Es layer and nighttime Es layer are challenging problems. Nevertheless, it is still very useful, and is suitable for automated real-time Es layer monitoring system in a wide area.

Plasma bubbles can affect ground-based augmentation system (GBAS) performance by posing large spatial ionospheric gradients between the aircraft and the airport, which makes it important to detect plasma bubbles effectively. In this study, we investigate the use of VHF radar to detect plasma bubbles and mitigate their potential impact on GBAS through a simulation. When the line-of-sight of a certain GPS satellite is determined to be passing through an area of plasma bubbles as detected by EAR, such GPS satellite shall be removed from the positioning calculation on board the aircraft. We set up different simulation scenarios by varying the satellite geometry, and airport coordinate. Results are described in the form of vertical protection level (VPL) and vertical position error (VPE), which we then analyse as a function of zonal and meridional distance. The results show that such a radar-based monitoring system can be used to mitigate the effects of plasma bubbles on GBAS to cover surrounding airports within a certain range, especially in the meridional direction.

GNSS ground-based augmentation system (GBAS) is designed to support precision approach and landing of aircraft. Though GBAS approach service type (GAST) C is operational in the mid-latitude region in the world, ionospheric issues prevent GAST C from full availability. In this study, possible use of GAST D for CAT I operation is examined. The GAST D protection levels in all the analyzed cases were below the vertical alert limit of 10 m. However, the airborne satellite geometry screening degraded the CAT I availability for the cases with the E_ig parameter, which is the maximum range error assured by the ground subsystem, greater than 2.75m. Two potential approaches to relax the threshold value in the airborne satellite geometry screening (limitS_vert^2) are considered. One is to set a larger max E_v value, which is the maximum allowed vertical error, for CAT I operation in the airborne receiver in addition to that for CAT II/III operation, and the other is to deflate the broadcast E_ig value. While the first approach is straight forward but requires airborne receiver standard changes, the second approach requires changes in the broadcast parameter only. When the deflated E_ig is broadcast in the second approach, CAT II/III operations must be prohibited. Nevertheless, deflated E_ig for CAT I could be beneficial, because the period of CAT II/III conditions are much less than the CAT I condition. Deflated and non-deflated E_ig values could be used depending on the weather condition at the airport, if certain mechanism to notify aircraft on the supported approach operations.

This paper describes an alternative architecture candidate proposal for consideration for the future Dual Frequency MultiConstellation (DFMC) Ground Based Augmentation System (GBAS). The GBAS architecture candidate builds off the work done over the last 10 years by the Single European Sky Air Traffic Management Research (SESAR) program. However, the proposed alternative architecture is designed to enable more optimal airborne processing that is expected to enable better performance (e.g. higher accuracy and smaller alert limits), increased robustness to loss of civil GNSS signals on either L1/E1 or L5/E5a frequency as well as to the loss of a constellation, and higher availability for DFMC GBAS globally. The alternative architecture is envisioned to support one or more new levels of GBAS approach service. This GBAS Approach Service Type (GAST) will be referred to generically as GAST X in this paper. GAST X is envisioned as a natural extension of the currently defined GAST C and GAST D. The proposal borrows heavily in many respects from the SESAR proposal for DFMC GBAS documented in reference [1]. However, some of the processing has been reallocated to the airborne equipment in order to enable some very desirable capabilities. In the proposed GAST X DFMC GBAS concept, raw measurements from both L1/E1 and L5/E5a civil GNSS frequencies are uplinked in addition to corrections and other parameters that are already uplinked from ground stations that support the existing GAST C and D services. Uplinking raw measurements instead of corrections potentially enables the use of pure carrier phase position solutions such as Real Time Kinematic (RTK) Carrier Phase (CP) based solutions. Moreover, it allows for a low noise long baseline airborne iono gradient monitoring capability. This paper describes the basic architecture to support GAST X and gives some predicted performance results for accuracy, the size of protection levels and the availability based on a simulation study. In addition, the paper discusses a potential set of services and considers the transition between service levels that would result from loss of a frequency or total failure of a constellation.

This paper reports on the development of a real-time three-dimensional ionospheric tomography technique using GNSS measurements. GPS measurement data from 200 GPS receivers distributed throughout Japan are analyzed in real time to calculate ionospheric Total Electron Contents (TECs). The three-dimensional ionospheric electron density distribution was reconstructed using the tomography technique from the slant TEC data. The tomography results were found to be in good agreement with both the electron density profiles obtained by radio occultation measurements by the COSMIC satellite and the ionospheric peak densities obtained by vertical radio sounding measurements (ionosondes). The real-time ionospheric tomography system commenced operation in March 2016 and it continuously produces three-dimensional electron density distributions over Japan every 15 min with latency of about 6 min. This system is a very powerful tool for monitoring the ionosphere in real time, which could both improve the performance of GNSS-based systems and further the understanding of ionospheric variations.

GBAS (Ground-Based Augmentation System) is a system based on local GNSS differential correction technique to support aircraft precision approaches. It is one of the most important issues for GBAS safety design to evaluate spatial gradients of ionospheric delay especially with a horizontal scale of about 10km. In our previous study, we examined spatial gradients of ionospheric delay using experimental equipment associated with a prototype of CAT-III GBAS at New Ishigaki airport, which was located in southern part of Japan. As a result, large spatial gradients of ionospheric delay up to ±54 mm/km in slant direction were found under an event of nighttime medium-scale traveling ionospheric disturbances (MSTIDs). In addition, our latest result derived from the same equipment shows that a large tropospheric gradient of about 87mm/km in slant direction is observed. Because the facts related to the MSTIDs and the tropospheric disturbances are recently recognized, it is important to reevaluate effects of them on GBAS safety design. There are three key points for the revaluation, which are broadcast parameters of sigma_pr_ground and sigma_vertical_iono_gradient, and system parameters related to integrity monitors. The effects on the broadcast parameters might be potentially reflected in them because they are determined through safety assessment based on the actual field data. However, these effects have not been explicitly evaluated in view point of such large atmospheric “nominal” spatial gradients. As the first step of the revaluation, we used a data set of the prototype with 12-day duration and investigated the two broadcast parameters, which are sigma_pr_ground and sigma_vertical_iono_gradient. Regarding the former parameter, we analyzed probability density functions of B values, which are also GBAS broadcast parameters related to sigma_pr_ground, on each atmospheric disturbed and quiet days. For evaluating the latter parameter, RMS (root mean square) of vertical ionospheric gradients derived from an ionospheric spatial gradient monitor of the prototype are also investigated as the same manner. As results, we confirmed that the RMS of vertical ionospheric gradients tend to increase on high MSTIDs / troposphere delay activity day than quiet day. For future work, it is essential to examine the three key points of our purpose with long term data, distinguishing between ionospheric and tropospheric delay components.

GAST-D (GBAS Approach Service Type D) is a class of GBAS (Ground-based Augmentation System) based on a single-frequency signals of GNSS to support Category II/III approach and landing of aircraft. To satisfy the very high requirements for safety, integrity monitors are needed to be implemented. Among different threats to the integrity of GAST-D, ionospheric irregularities are one of the most serious ones. Especially, the low latitude ionosphere, where ionospheric disturbances frequently occur is a very challenging environment for GAST-D. In this study, effects of ionospheric amplitude scintillations on integrity monitors of GAST-D were investigated based on the data obtained by an experimental prototype of a GAST-D ground subsystem installed at New Ishigaki Airport located in a low magnetic latitude region (24.3oN, 124.2oE, 19.6oN magnetic) and scintillation data obtained near the GAST-D ground subsystem. Existence of plasma bubbles which are characteristic ionospheric disturbances in the low magnetic latitude region are monitored by an all-sky airglow imager located near the GAST-D ground subsystem. Impacts of ionospheric amplitude scintillations on four integrity monitors, namely cycle-slip (CS), code-carrier divergence (CCD), ionospheric spatial gradient monitor (ISGM), and DSIGMA range domain monitors were discussed. Occurrence of cycle-slips increased with increasing S4 index values. At an S4 index of 1.0, the occurrence rate of cycle-slip was more than 50%. The CCD test statistics as well as a general test statistics of ISGM, which is the double-differences of carrier-phase (DD carrier-phase) also increased as the S4 index increases. On the cause of the positive correlation of the CCD test statistics and DD carrier-phases to the S4 index, more investigation is necessary. For the DSIGMA range domain monitor, methodologies of investigation are presented. Detailed analysis including impacts of the phase scintillation on the GAST-D integrity monitors is left for the next steps of this study.

Since anomalous ionospheric gradient can result in differential range errors for Ground-Based Augmentation System (GBAS) of several meters, it is one of the most important integrity risks. Because GBAS Approach Service Type D (GAST-D) that can support Category III precision approach and landing requires extremely high safety, its international standards specify ground and airborne integrity monitors to detect ionospheric anomalies in real time operations. An ionospheric threat model specifies upper bounds for ionospheric anomalies, and it is important for GBAS safety design. In the validation of Standards and Recommended Practices (SARPs) for GAST-D which was developed by International Civil Aviation Organization (ICAO), an ionospheric threat model is developed mainly based on knowledge of mid-latitude ionosphere, while at lower latitude region where plasma bubbles occur often which is the cause of steep gradient of the ionospheric delays. Therefore, it is important to investigate characteristics of such steep gradients in the low magnetic latitude region. This paper presents preliminary results of analyses of the spatial scale of ionospheric gradient in the low latitude region based on data of GNSS Earth Observation NETwork (GEONET), Japan for the statistical analyses using long-term data. An automatic system to pick up candidates of steep gradients has been developed. With this result, impact on GBAS ionospheric threat model will be assessed.

The GAST D version of GBAS that supports Category II and III precision approach and landing operations adds additional monitoring to both the ground and airborne segments in order to minimize the differential errors that can be caused by anomalous ionospheric gradients. Extensive analysis and simulation work was conducted by a team within the International Civil Aviation Organization (ICAO) Navigation Systems Panel (NSP) to validate the performance requirements for ionospheric gradient mitigation. The analysis was specifically designed to confirm that the requirements could be met by the combination of ground and airborne monitors. This paper provides an overview of the validation activity. It includes a description of the design choices and monitors that make up the GBAS GAST D defense against anomalous ionospheric spatial gradients and the analysis and simulation results that demonstrate that the system successfully meets the performance requirements.

Reflected GNSS signal is used for various scientific applications using space and ground-based GNSS receivers. Using the ground-based GNSS receiver, methodology to continuously observe snow depth has been developed and demonstrated [1]. This novel technique is based on multipath analysis of fading patterns in receiving intensity (i.e. Signal-to-Noise Ratio; SNR) produced by a direct and a reflected signal from the snow surface. Namely, a vertical distance between GNSS antenna and the snow surface is estimated from such a fading pattern, which has a typical period depending on the height difference. Currently, it has been widely used over the world using GNSS stations which primary purposes are different from snow observation. In this study, we focus on reflectance at the snow surface, which depends on the snow surface conditions. Reflection coefficient is determined by permittivity of snow accumulation and it also changes with incidence angle and polarization. Therefore, simultaneous observation by two GNSS antennas which have different gain patterns has a potential to provide distinctive reflectance variations enough to monitor snow surface conditions. Based on this idea, we investigated characteristics of snow reflection signals using a choke-ring antenna and a multipath limiting antenna (MLA), which is designed to dramatically reject multipath signal with low elevation angle [2]. In general, low-elevation GNSS signal received by the choke-ring antenna is contaminated by multipath effects in contrast to the MLA. Therefore, an observational data set with the two antennas enables us to extract a component of snow reflection signals including its dependency on satellite elevation angle. To accomplish our purpose, we performed a winter experiment from December 2014 to May 2015 at meteorological observational filed of Snow and Ice Research Center, National Research Institute for Earth Science and Disaster Prevention (NIED). Our ultimate purpose is comprehensive evaluation of reflected GNSS signals and development of estimation method of the snow surface conditions. Toward the goal, we extracted components of the snow surface reflection signals from the observational data set and investigated their temporal changes over the entire snow-covered period of four months. Namely, we quantitatively analyzed temporal variations in receiving intensity and ranging errors due to interferences by snow reflection signals using observational parameters of sigma-C/N0 and sigma-Code-Carrier-Divergence (CCD), respectively. We investigated them with distinction between daytime and nighttime because meteorological conditions such as solar radiation and air temperature are different in local time. As results, we confirmed that general tendency was the same between daytime and nighttime and results for MLAs had more numbers of spike variations in nighttime. Meteorological observation is also important for the evaluation. Introducing meteorological data, another example analysis was performed with "common-satellite" single differences for both code and carrier measurements. For the analysis, a period of rapid temporal changes in snow properties from wet to dry was selected. Through the analysis, we successfully extracted a periodical variation of carrier phase measurement due to snow reflection and confirmed that its amplitude changed depending on snow surface conditions. On the other hand, it was difficult to extract a periodical variation pattern from code results. As future work, we are going to use permittivity from weekly snowpack observation at the experimental site. The permittivity enable us not only to calculate reflective coefficient at the snow surface but also to reconstruct multipath effects enough to compare real GNSS results, which are variations in receiving intensity and a common-satellite single difference for each code and carrier phase.