We investigated characteristics of anomalous spatial gradients in ionospheric delay on GNSS signals in the Asia-Pacific (APAC) low-magnetic latitude region in the context of the ground-based augmentation system (GBAS). The ionospheric studies task force established under the Communications, Navigation, and Surveillance subgroup of International Civil Aviation Organization (ICAO) Asia-Pacific Air Navigation Planning and Implementation Regional Group, analyzed GNSS observation data from the Asia-Pacific region to establish a regionally specified ionospheric threat model for GBAS. The largest ionospheric delay gradient value in the analyzed data was 518 mm/km at the L1 frequency (1.57542 GHz), observed at Ishigaki, Japan in April 2008. The upper bound on the ionospheric delay gradient for a common ionospheric threat model for GBAS in the ICAO APAC region was determined to be 600 mm/km, irrespective of satellite elevation angle.

/pdf/ionospheric-delay-gradient-model-for-gbas-in-the-asia-4xf0xb2j3g.pdf

Ionospheric delay gradient model for GBAS in the Asia-Pacific region

The Global Navigation Satellite System (GNSS) observations with global coverage and high temporal and spatial resolution, provide abundant and high-quality Earth-ionosphere observations. By calculating the total electron content (TEC), estimations from GNSS observables global and regional ionosphere TEC morphology can be further investigated. For the multiple constellation case, the numbers of ionosphere pierce points (IPP) has increased tremendously, and it is worth studying the features of the GNSS derived TEC under geomagnetic storms to show the benefits of multiple constellation measurements. With the Multi-GNSS Experiment (MGEX) observation data, ionosphere TEC responses to the geomagnetic storm on the 22 June 2015 were well studied. TEC perturbations were discovered, accompanied by ionosphere irregularities concentrating in high and middle latitudes. Through analysis of multi-GNSS observations, the Rate of TEC Index (ROTI) perturbations were proved to be generated by the geomagnetic storm, with simultaneous behaviors at different local times around the world, also indicating ionosphere scintillation. The ionosphere spatial gradient was also discussed with two short baseline MGEX sites; the maximum ionosphere gradient of 247.2 mm/km was found, due to ionosphere irregularity produced by the storm. This research has discussed ionosphere responses to geomagnetic storms with multi-GNSS data provided and has analyzed the availability of multi-GNSS observations to investigate ionosphere irregularity climatology. The proposed work is valuable for further investigation of GNSS performances under geomagnetic storms.

Studying Ionosphere Responses to a Geomagnetic Storm in June 2015 with Multi-Constellation Observations

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JA025253

Physics‐Based Modeling of Earthquake‐Induced Ionospheric Disturbances

Extreme space weather events affect the stability and quality of the global navigation satellite systems (GNSS) of the second generation (GPS, GLONASS, Galileo, BeiDou/Compass) and GNSS augmentation. We review the theory about mechanisms behind the impact of geomagnetic storms, ionospheric irregularities, and powerful solar radio bursts on the GNSS user segment. We also summarize experimental observations of the space weather effects on GNSS performance in 2000–2020 to confirm the theory. We analyze the probability of failures in measurements of radio navigation parameters, decrease in positioning accuracy of GNSS users in dual-frequency mode and differential navigation mode (RTK), and in precise point positioning (PPP). Additionally, the review includes data on the occurrence of dangerous and extreme space weather phenomena and the possibility for predicting their im- pact on the GNSS user segment. The main conclusions of the review are as follows: 1) the positioning error in GNSS users may increase up to 10 times in various modes during extreme space weather events, as compared to the background level; 2) GNSS space and ground segments have been significantly modernized over the past decade, thus allowing a substantial in- crease in noise resistance of GNSS under powerful solar radio burst impacts; 3) there is a great possibility for increasing the tracking stability and accuracy of radio navigation parameters by introducing algorithms for adaptive lock loop tuning, taking into account the influence of space weather events; 4) at present, the urgent scientific and technical problem of modernizing GNSS by improving the scientific methodology, hardware and software for monitoring the system integrity and monitoring the availability of required navigation parameters, taking into account the impact of extreme space weather events, is still unresolved.

/pdf/space-weather-risk-factors-for-global-navigation-satellite-51y7jgg8ld.pdf

Space weather: risk factors for Global Navigation Satellite Systems

. The relation between the occurrence of ionospheric irregularities and the spatial gradient of total electron content (TEC) derived from two closely located
stations (ASAB: 4.34 ∘  N, 114.39 ∘  E and DEBK: 3.71 ∘  N, 109.34 ∘  E, geomagnetic), located within the equatorial region,
over Ethiopia, during the postsunset hours was investigated. In this study, the Global Positioning System (GPS)-derived TEC during the year 2014
obtained from the two stations were employed to investigate the relationship between the gradient of TEC and occurrence of ionospheric irregularities. The
spatial gradient of TEC ( ΔTEC∕Δlong ) and its standard deviation over 15 min, σ(ΔTEC∕Δlong ), were used in this study. The
rate of change of TEC-derived indices (ROTI, ROTI ave ) were also utilized. Our results revealed that most of the maximum enhancement and reduction values in
 ΔTEC∕Δlong are noticeable during the time period between 19:00 and 24:00 LT. In some cases, the peak values in the spatial gradient
of TEC are also observed during daytime and postmidnight hours. The intensity level of σ(ΔTEC∕Δlong) observed after postsunset show
similar trends with ROTI ave , and was stronger (weaker) during equinoctial (solstice) months. The observed enhancement of σ(ΔTEC∕Δlong) 
in the equinoctial season shows an equinoctial asymmetry where the March equinox was greater than the September equinox. During the postsunset period, the
relation between the spatial gradient of TEC obtained from two closely located Global Navigation Satellite System (GNSS) receivers and the equatorial electric
field (EEF) was observed. The variation in the gradient of TEC and ROTI ave observed during the evening time period show similar trends with EEF with a
delay of about 1–2 h between them. The relationship between σ(ΔTEC∕Δlong) and ROTI ave correlate linearly with correlation
coefficient of C=0.7975 and C=0.7915 over ASAB and DEBK, respectively. The majority of the maximum enhancement and reduction in the spatial gradient of TEC
observed during the evening time period may be associated with ionospheric irregularities or equatorial plasma bubbles. In addition to latitudinal gradients,
the longitudinal gradient of TEC has contributed significantly to the TEC fluctuations.

/pdf/investigation-of-the-relationship-between-the-spatial-41kp40dy3b.pdf

Investigation of the relationship between the spatial gradient of total electron content (TEC) between two nearby stations and the occurrence of ionospheric irregularities

Associated with plasma bubbles, extreme spatial gradients in ionospheric total electron content (TEC) were observed on 8 April 2008 at Ishigaki (24.3°N, 124.2°E, +19.6° magnetic latitude), Japan. The largest gradient was 3.38 TECU km
 −1
 (total electron content unit, 1 TECU = 10
 16
 el m
 −2
), which is equivalent to an ionospheric delay gradient of 540 mm km
 −1
 at the GPS L1 frequency (1.57542 GHz). This value is confirmed by using multiple estimating methods. The observed value exceeds the maximum ionospheric gradient that has ever been observed (412 mm km
 −1
 or 2.59 TECU km
 −1
) to be associated with a severe magnetic storm. It also exceeds the assumed maximum value (500 mm km
 −1
 or 3.08 TECU km
 −1
) which was used to validate the draft international standard for Global Navigation Satellite System (GNSS) Ground-Based Augmentation Systems (GBAS) to support Category II/III approaches and landings. The steepest part of this extreme gradient had a scale size of 5.3 km, and the front-normal velocities were estimated to be 71 m s
 −1
 with a wavefront-normal direction of east-northeastward. The total width of the transition region from outside to inside the plasma bubble was estimated to be 35.3 km. The gradient of relatively small spatial scale size may fall between an aircraft and a GBAS ground subsystem and may be undetectable by both aircraft and ground.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017RS006291

S. Saito

Papers

Evaluation of extreme ionospheric total electron content gradient associated with plasma bubbles for GNSS Ground-Based Augmentation System