• Journal of Astronomy and Space Sciences
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• 제37권1호
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• pp.61-68
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• 2020

Recent studies have suggested that detectable ionospheric disturbances precede earthquakes. In the present study, variations in the vertical total electron content (TEC) for eight earthquakes with magnitudes of M ≥ 5.5 in the western United States were investigated during the solar maximum of 2013-2015 using United States total electron content (US-TEC) data provided by the National Oceanic and Atmospheric Administration. Analyses of 12 earthquakes with magnitudes of 5.0 ≤ M < 5.5 in the same region were also performed. The TEC variations were examined for 40 days, including the times when the earthquakes occurred. The results indicated a correlation between earthquakes with magnitudes of M ≥ 5.0 and ionospheric TEC anomalies. TEC anomalies occurred before 60% of the earthquakes. Additionally, they were more frequently observed for large earthquakes (75%, M ≥ 5.5) than for small earthquakes (50%, 5.5 > M ≥ 5.0). Anomalous increases in the TEC occurred 2-18 days before the earthquakes as an ionospheric precursor, whereas solar and geomagnetic activities were low or moderate.

• 지구물리와물리탐사
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• 제19권4호
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• pp.228-235
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• 2016

대규모 지진 전/후에 이온층에서 발생하는 총 전자수(TEC)의 이상 변화를 재 확인하고, 국내에서 발생하는 지진에서도 이와 유사한 현상이 발견되는지를 조사하였다. 먼저 최근 지진관련 이온층 변화가 보고되었던 국외 대규모 지진들과, 근래에 발생한 규모 7.8 에콰도르 강진에 대해 TEC를 계산하고, 지진 발생 전 후의 특성을 확인하였다. TEC 계산 결과, 선행 연구들과 유사한 TEC 변화 시계열을 얻을 수 있었으나, 시계열들이 후처리되는 방법에 따라 전조현상 또는 후지진 현상으로 해석될 수 있다는 사실을 확인하였다. 최근 국내에서 발생한 지진들에 대하여 지진에 의한 TEC 변화 가능성을 조사하였으나, 2016년 경주지진을 포함한 가장 높은 규모의 국내지진들과 관련하여 지진 전/후에 TEC의 이상 변화를 찾아볼 수 없었다. 이는 판의 경계에서 발생하는 국외의 대규모 지진들에 비해, 국내 지진이 대기 중으로 방출하는 탄성파(음파) 에너지가 작기 때문이라고 여겨진다.

• Journal of Astronomy and Space Sciences
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• 제33권1호
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• pp.29-36
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• 2016

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.

• Journal of Astronomy and Space Sciences
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• 제26권2호
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• pp.221-228
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• 2009

한반도 상공의 전리층 총전자수(TEC, Total Electron Content)를 추정하는 방법을 통해 GPS 수신기와 위성의 코드 바이어스(DCB, Differential Code Bias)를 함께 추정하였다. 한국천문연구원에서 운영하고 있는 GPS 기준국망 데이터를 사용하였으며, 가중치 최소자승법을 이용하여 매 1시간 간격으로 DCB를 산출하였다. 총 3일간의 데이터를 처리한 결과 9개 GPS 수신기의 DCB는 ${\pm}2m$ 이내에서 변화하는 것으로 나타났으며, 3일 동안 크게 변하지 않았다. 또한 일일 평균값으로 산출된 위성의 DCB는 최대 약 4.09ns(nano-second), 최소 약 -6.28ns를 갖는 것으로 나타났다. 그리고 산출된 DCB를 전리층 총전자수 산출에 적용한 결과, 적용 전에 비해 특정시점에서 최대 약 9TECU 이상의 총전자수 변화가 검출됨을 확인 할 수 있었다.

• Journal of Astronomy and Space Sciences
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• 제27권4호
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• pp.359-366
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• 2010

In this study, we developed a global ionosphere model based on measurements from a worldwide network of global positioning system (GPS). The total number of the international GPS reference stations for development of ionospheric model is about 100 and the spherical harmonic expansion approach as a mathematical method was used. In order to produce the ionospheric total electron content (TEC) based on grid form, we defined spatial resolution of 2.0 degree and 5.0 degree in latitude and longitude, respectively. Two-dimensional TEC maps were constructed within the interval of one hour, and have a high temporal resolution compared to global ionosphere maps which are produced by several analysis centers. As a result, we could detect the sudden increase of TEC by processing GPS observables on 29 October, 2003 when the massive solar flare took place.

• Journal of Astronomy and Space Sciences
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• 제24권4호
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• pp.269-274
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• 2007

강한 지자기 폭풍이 발생하였을 때 한반도 상공의 전리층내 총전자수(TEC, Total Electron Contents) 변화를 분석하기 위해 GPS 기준국망을 데이터를 이용한 지역적인 전리층 감시모델을 개발하였다. 전리층 총전자수 감시모델 개발을 위해 한국천문연구원에서 운용중인 대전 IGS(International GNSS Service) 기준국을 포함한 전국에 고르게 분포하고 있는 9개의 GPS 기준국 데이터를 이용하였다. 또한 순간적인 전리층 변화 특성을 분석하기 위해 CSS(Cubic Spline Smoothing)기법을 적용하였고, 그 결과 2003년 11월 20일 강한 지자기 폭풍이 발생했을 때 한반도 상공에서 총전자수의 순간적인 변화를 검출할 수 있었다. 이때에는 평일과 비교했을 때 특정시각의 약 1.5배 이상 총전자수가 증가함을 보였다. 마지막으로 지구 자기장 활동 정도를 나타내는 Kp 지수, Dst 지수 그리고 천문연 GPS 기준국망 데이터를 이용해 산출된 총전자수 변화와의 연관성을 제시했다.

• Journal of Astronomy and Space Sciences
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• 제28권4호
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• pp.305-310
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• 2011

The total electron content (TEC) using global positioning system (GPS) is analyzed to see the characteristics of ionosphere over King Sejong station (KSJ, geographic latitude $62^{\circ}13'S$, longitude $58^{\circ}47'W$, corrected geomagnetic latitude $48^{\circ}S$) in Antarctic. The GPS operational ratio during the observational period between 2005 and 2009 is 90.1%. The annual variation of the daily mean TEC decreases from January 2005 to February 2009, but increase from the June 2009. In summer (December-February), the seasonal mean TEC values have the maximum of 26.2 ${\pm}$ 2.4 TEC unit (TECU) in 2005 and the minimum of 16.5 ${\pm}$ 2.8 TECU in 2009, and the annual differences decrease from 3.0 TECU (2005-2006) to 1.4 TECU (2008-2009). However, on November 2010, it significantly increases to 22.3 ${\pm}$ 2.8 TECU which is up to 5.8 TECU compared with 2009 in summer. In winter (June-August), the seasonal mean TEC slightly decreases from 13.7 ${\pm}$ 4.5 TECU in 2005 to 8.9 ${\pm}$ 0.6 TECU in 2008, and the annual difference is constantly about 1.6 TECU, and increases to 10.3 ${\pm}$ 1.8 TECU in 2009. The annual variations of diurnal amplitude show the seasonal features that are scattered in summer and the enhancements near equinoxes are apparent in the whole years. In contrast, the semidiurnal amplitudes show the disturbed annual peaks in winter and its enhancements near equinoxes are unapparent. The diurnal phases are not constant in winter and show near 12 local time (LT). The semidiurnal phases have a seasonal pattern between 00 LT and 06 LT. Consequently, the KSJ GPS TEC variations show the significant semidiurnal variation in summer from December to February under the solar minimum between 2005 and 2009. The feature is considered as the Weddell Sea anomaly of larger nighttime electron density than a daytime electron density that has been observed around the Antarctica peninsula.

• Journal of Astronomy and Space Sciences
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• 제34권1호
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• pp.7-17
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• 2017

In South Korea, there are about 80 Global Positioning System (GPS) monitoring stations providing total electron content (TEC) every 10 min, which can be accessed through Korea Astronomy and Space Science Institute (KASI) for scientific use. We applied the computerized ionospheric tomography (CIT) algorithm to the TEC dataset from this GPS network for monitoring the regional ionosphere over South Korea. The algorithm utilizes multiplicative algebraic reconstruction technique (MART) with an initial condition of the latest International Reference Ionosphere-2016 model (IRI-2016). In order to reduce the number of unknown variables, the vertical profiles of electron density are expressed with a linear combination of empirical orthonormal functions (EOFs) that were derived from the IRI empirical profiles. Although the number of receiver sites is much smaller than that of Japan, the CIT algorithm yielded reasonable structure of the ionosphere over South Korea. We verified the CIT results with NmF2 from ionosondes in Icheon and Jeju and also with GPS TEC at the center of South Korea. In addition, the total time required for CIT calculation was only about 5 min, enabling the exploration of the vertical ionospheric structure in near real time.

• Journal of Positioning, Navigation, and Timing
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• 제7권3호
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• pp.175-181
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• 2018

The use of dual-frequency measurements from the Global Navigation Satellite System (GNSS) enables us to observe precise ionospheric total electron content (TEC). Currently, many GNSS reference stations in South Korea provide both GPS and GLONASS data. In the present study, we estimated the grid-based TEC values and relative receiver differential code biases (DCB) from a GNSS network operated by the Korea Astronomy and Space Science Institute. In addition, we compared the diurnal variations in a TEC time series from solutions of the GPS only, the GLONASS only, and combined GPS/GLONASS processing. A significant difference between the GPS only TEC and combined GPS/GLONASS TEC at a specific grid point over South Korea appeared near the solar terminator. It is noted that GLONASS measurements can contribute to observing a variation in ionospheric TEC over high latitude regions.

• Journal of Positioning, Navigation, and Timing
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• 제11권3호
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• pp.157-161
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• 2022

The Nuri rocket developed by South Korea was launched at approximately 07:00 UT on June 21, 2022. We use GPS observations obtained from the Korean GNSS network to analyze ionospheric total electron content (TEC) disturbances induced by the 2022 Nuri rocket launch. TEC disturbances are observed south over South Korea 4-5 min after the rocket launch. In addition, the maximum depletion in the vertical TEC shows approximately 8 TEC units (TECU). We also compute a horizontal velocity from initial ionospheric disturbances triggered by the 2022 Nuri rocket launch. Its velocity is about 1.4 km/s. It may be related to the rocket's flight trajectory at the observation time of the ionospheric TEC disturbance.