Ground-based Global Positioning System Receivers Research Articles

Impact of incorporating Spire CubeSat GPS observations in a global GPS network solution

Commonly, Global Positioning System (GPS)-based precise orbit determination (POD) of a low Earth orbiting (LEO) satellite is conducted by introducing fixed GPS orbits and clock corrections that were derived in a previous independent network solution using ground-based GPS receivers only. There have been a number of studies showing that the integration of LEO GPS observations can be advantageous in global network solutions, particularly when it comes to estimating geodetic parameters, such as the Earth’s center-of-mass. There has been an increase in GPS data available from LEO CubeSats over the past few years, such as from the Spire satellites, which are equipped with dual-frequency GPS receivers. The goal of the present study is to determine how GPS observations collected by specific Spire satellites affect global network solutions when combined with GPS observations from ground stations of the International GNSS Service. To obtain a combined GPS-LEO solution, GPS observations collected from receivers of selected Spire satellites and additional scientific LEOs are used to obtain combined GPS-LEO solutions by performing one joint least-squares adjustment process. Using this method, LEO satellite orbit parameters are determined along with GPS orbit parameters and geodetic parameters, namely GNSS station coordinates, Earth rotation parameters (ERPs), and the Earth’s center-of-mass. The influence of the Spire LEOs is assessed by analyzing different solutions that use GPS observations from different Spire and LEO satellites. Besides comparing the different global network solutions with one another, a comparison to a solution using only terrestrial GPS data is performed as well. As part of the quality assessment of the GPS-LEO solution, quality characteristics are analyzed, which are the geodetic parameters, orbit overlaps at arc boundaries, and formal errors determined for the estimated parameters. The results show that the estimation of the Earth’s center-of-mass coordinates benefits from including GPS-LEO observations. The solution is more improved by integrating GPS-LEO observations from scientific LEOs than from GPS-Spire-included solutions. The analysis reveals that GPS orbits are improved using the LEO-integrated approach, while orbit misclosures of LEO and Spire trajectories currently indicate a decrease in LEO orbit precision due to LEO orbit modeling deficiencies.

A comparison of Jason-2 plasmasphere electron content measurements with ground-based measurements

Abstract. Previous studies utilizing the Global Positioning System (GPS) receivers aboard Jason satellites have performed measurements of plasmasphere electron content (PEC) by determining the total electron content (TEC) above these satellites, which are at altitudes of about 1340 km. This study uses similar methods to determine PEC for the Jason-2 receiver for 24 July 2011. These PEC values are compared to previous determinations of PEC from a chain of ground-based GPS receivers in Africa using the SCORPION method, with a nominal ionosphere–plasmasphere boundary at 1000 km. The Jason-2 PECs with elevations greater than 60∘ were converted to equivalent vertical PEC and compared to SCORPION vertical PEC determinations. In addition, slant (off-vertical) PECs from Jason-2 were compared to a small set of nearly co-aligned ground-based slant PECs. The latter comparison avoids any conversion of Jason-2 slant PEC to equivalent vertical PEC, and it can be considered a more representative comparison. The mean difference between the vertical PEC (ground-based minus Jason-2 measurements) values is 0.82 ± 0.28 TEC units (1 TEC unit=1016 electrons m−2). Similarly, the mean difference between slant PEC values is 0.168 ± 0.924 TEC units. The Jason-2 slant PEC comparison method may provide a reliable determination for the plasmasphere baseline value for the ground-based receivers, especially if the ground stations are confined to only midlatitude or low-latitude regions, which can be affected by a non-negligible PEC baseline.

Application of Classical Kalman filtering technique in assimilation of multiple data types to NeQuick model

This study attempts to improve the estimation of ionospheric electron density profiles over Korea and adjacent areas by employing the classical Kalman filtering technique to assimilate Total Electron Content (TEC) data from various sources into the NeQuick model. Successive corrections method was applied to spread the effect of TEC data assimilation at a given location to others that lacked TEC observations. In order to reveal that the assimilation results emulate the complex ionospheric changes during geomagnetic storms, the selected study days included both quiet (Kp ≤ 3) and disturbed geomagnetic conditions in the year 2015. The results showed that assimilation of TEC data derived from ground-based Global Positioning System (GPS) receivers could improve the root mean squared error (RMSE) associated with the NeQuick model estimation of ionospheric parameters by ≥56%. The improvement of RMSE achieved by assimilating TEC data measured using ionosondes was ~50%. The assimilation of TEC observations made by the COSMIC radio occultation technique yielded results that depicted RMSE improvement of >10%. The assimilation of TEC data measured by GPS receiver onboard Low Earth Orbiting satellites yielded results that revealed deterioration of RMSE. This outcome might be due to either the fact that the receivers are on moving platforms, and these dynamics might not have been accounted for during TEC computation or the limitation of the assimilation process. Validation of our assimilation results with global ionosphere TEC data maps as processed at the Center for Orbit Determination in Europe (CODE) revealed that both depicted similar TEC changes, showing response to a geomagnetic storm.

Tropospheric Refractivity Profile Estimation by GNSS Measurement at China Big-Triangle Points

Atmospheric radio refractivity has an obvious influence on the signal transmission path and communication group delay effect. The uncertainty of water vapor distribution is the main reason for the large error of tropospheric refractive index modeling. According to the distribution and characteristics of water vapor pressure, temperature, and pressure, which are the basic components of the refractive index, a method for retrieving atmospheric refractivity profile based on GNSS (Global Navigation Satellite System) and meteorological sensor measurement is introduced and investigated in this study. The variation of the correlation between zenith wet delay and water vapor pressure is investigated and analyzed in detail. The partial pressure profiles of water vapor are retrieved with relevance vector machine method based on tropospheric zenith wet delay calculated by single ground-based GPS (Global Positioning System) receiver. The atmospheric temperature and pressure is calculated with the least square method, which is used to fit the coefficients of the polynomial model based on a large number of historical meteorological radiosonde data of local stations. By combining the water vapor pressure profile retrieving from single ground-based GPS and temperature and pressure profile from reference model, the refractivity profile can be obtained, which is compared to radiosonde measurements. The comparison results show that results of the proposed method are consistent with the results of radiosonde. By using over ten years’ (through 2008 to 2017) historical radiosonde meteorological data of different months at China Big-Triangle Points, i.e., Qingdao, Sanya, Kashi, and Jiamusi radiosonde stations, tropospheric radio refractivity profiles are retrieved and modeled. The comparison results present that the accuracies of refractivity profile of the proposed method at Qingdao, Sanya, Kashi, and Jiamusi are about 5.48, 5.63, 3.58, and 3.78 N-unit, respectively, and the annual average relative RMSE of refractivity at these stations are about 1.66, 1.53, 1.49, and 1.23%, respectively.

Variability of GPS-derived ionospheric TEC over Nigeria during a year of low solar activity

An investigation of the diurnal and seasonal variability of ionospheric total electron content (TEC) over Nigeria is carried out in this study using the global positioning system (GPS) observable. Nigeria coordinates fall within the trough of the equatorial ionisation anomaly region of the African sector. The TEC data used were obtained from the ground-based GPS receiver stations of the Nigerian GNSS network of stations (NIGNET). The stations with their respective geomagnetic latitudes are: Abuja (–1.64°), Yola (–1.32°), Zaria (–0.13°), Kebbi (0.72°). The results of the diurnal analysis of the relative variability index (VD) revealed higher nighttime values than daytime values. The diurnal variation of VD also showed two conspicuous peaks: post-midnight and post-sunset. The diurnal–seasonal variation does not reveal any consistent pattern (no particular season leads the others throughout). On average, considering all the seasons together, maximum TEC variability occurred in Zaria (62%) and minimum in Yola (54%). Seasonally, maximum VD was recorded during the March equinox and the lowest was recorded during the December equinox.

Ionosphere delay remote sensing during geomagnetic storms over Egypt using GPS phase observations

Various factors can increase ionosphere activity, such as the geomagnetic latitude, altitude, and geomagnetic storms. These storms can result in a significant disruption of the Earth’s atmosphere due to the interchange of solar wind energy into the milieu encompassing the Earth. The principal reason for this research is investigating the geomagnetic activity effect on ionospheric delays over Egypt using GPS (Global Positioning System) multi-frequency (L2 and L1) measurements. In this contribution, a GPS network spread over Egypt was utilized to figure ionosphere errors over Egypt, utilizing two models that rely upon GPS observable linear combination and smoothed phase observables. An algorithm was coded in MATLAB® environment and was called the Ionosphere Error Estimation (IEE) program. GPS phase observables were considered in this investigation to avoid blunders from pseudo-range measurements. Data from six ground-based multi-frequency GPS receivers located over Egypt have been chosen to study the impact of geomagnetic storms on ionospheric blunders. This paper presents the consequences of ionospheric blunders during disturbed and quiet days throughout the years of 2013 and 2014. Results clarify that the applied models using unsmoothed and smoothed phase observables show a good agreement in estimating ionospheric blunders, especially in quiet days. Ionospheric blunder standard deviation of mean (SDM) results from using smoothed phase observables that ranges from 16 to 3 cm in quiet conditions and ranges from 21 to 8 cm in stormy conditions. While ionospheric blunder SDM ranges from 17 to 5 cm in quiet days and from 23 to 8 cm in stormy days using unsmoothed phase observables. In The maximum ionosphere delay estimated over stormy days using the unsmoothed phase observables, its magnitude was 13.19 m at ASWN while the highest ionosphere error at ASWN station in quiet days was 6.83 m. In the maximum ionosphere delay estimated over stormy days using the smoothed phase observables, its magnitude was 13.34 m at ALAM while the highest ionosphere error at ALAM station in quiet days was 4.94 m. Finally, geomagnetic storms represent a real problem in equatorial and high-latitude zones, which causes a significant influence on the ionosphere blunder, and they have the capability of upsetting the results.

Multi-Instrument Observations of the Atmospheric and Ionospheric Response to the 2013 Sudden Stratospheric Warming Over Eastern Asia Region

We investigate the atmospheric and ionospheric response to the 2013 sudden stratospheric warming (SSW) by using multiple instruments located in Eastern Asia. Three meteor radars and five ionosondes are used to investigate the mesospheric zonal wind fields and ionospheric parameters of F-layer virtual height (h'F), F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -layer peak height (h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), and critical frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) at midand low-latitudes (10.7°N to 40.3°N). The vertical total electron content (TEC) data derived from the groundbased global positioning system receiver network are analyzed to study the ionospheric perturbations in the equatorial ionization anomaly (EIA) region. The changes in equatorial electrojet (EEJ) are observed by using the magnetometer data from stations on and off the magnetic equator. The variations of the h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at Sanya and EEJ strength presented the semidiurnal pattern with increase/decrease and eastward/westward currents in the morning/afternoon hours. In addition, the EIA crest moved poleward/equatorward in the morning/afternoon. The f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> showed the most significant enhancements during daytime at Wuhan and Shaoyang but the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at Sanya and Chumphon reduced mildly. Most importantly, based on the time-period wavelet analysis, the diurnal tidal components in the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> over Beijing, Wuhan, and Sanya seemed similar those in zonal winds and the semidiurnal tides in the low-latitude h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> showed the similar temporal variations as those in EEJ strength during the later phase of SSW. Therefore, apart from the local tides propagating from lower atmosphere having influence on the mid- and low-latitude ionosphere directly during the early phase, the equatorial fountain effect modulated by the enhanced tides also disturbed the EIA region.

Ionospheric Scintillation Impacts on L-band Geosynchronous D-InSAR System: Models and Analysis

The upcoming L -band geosynchronous differential interferometric synthetic aperture radar (GEO D-InSAR) system has the capability to monitor rapid deformations due to its excellent revisit capability. However, because of its low working frequency, the random ionospheric scintillation signal will degrade the deformation retrieval accuracy by giving rise to extra interferometric phase errors and obvious decorrelations in GEO D-InSAR interferograms. In this paper, aiming at impacts of ionospheric scintillations on GEO D-InSAR system, we theoretically establish its interferometric phase error and decorrelation models by using the scintillation statistical parameters directly. Simulations based on the scintillation sampling model, the Cornell university scintillation model, the phase screen mode, and the ionospheric scintillation signal acquired by the ground-based global positioning system receiver are carried out to verify the proposed model. Moreover, quantitative analyses of the ionospheric scintillation interferometric phase error and decorrelation impacts under different scintillation cases are obtained. The results verify that the proposed models and the analyses are effective. Meanwhile, they also suggest that the generated defocusing decorrelation dominates the ionospheric scintillation impacts on GEO D-InSAR, which can induce a coherence loss of more than 0.1 in the interferogram when only one image of the interferometric pair suffers the weak ionospheric scintillation.

Polar traveling ionospheric disturbances inferred with the B-spline method and associated scintillations in the Southern Hemisphere

A new method for analyzing travelling ionospheric disturbances (TIDs) is developed by using two B-spline basis functions of degree 4 on the Total Electron Content (TEC) data from the ground-based Global Positioning System (GPS) receivers. This method enhances the spatial resolution to about 0.1° (geographic latitude) × 0.1° (geographic longitude), which is useful in studying all scales (small, medium and large) TIDs. Using this method, we investigated TIDs and their associated scintillation on 18–19 July 2013 at Southern Hemisphere and found phase scintillation is more sensitive than amplitude scintillation to the TIDs at South Pole. To see the full impact of TIDs on scintillation, we have used a proxy phase scintillation index, calculated using geodetic GPS receivers over Antarctica. We have verified the presence of TIDs during these two days by using a Global Navigation Satellite System (GNSS)-TEC single station approach and SuperDARN slant range signals. Our results show the TEC fluctuations are associated with ionospheric scintillation. The shape of TIDs, their elongation and flattening along/across the geographic latitude/longitude, seems to be related to the magnitude and occurrence of ionospheric scintillations. Magnetospheric particle precipitation boost TEC gradients and generate stronger amplitude scintillation, however, large-scale plasma irregularities cause overall enhancement in magnitude of the phase scintillation index. Due to the high turbulence in the polar ionosphere, TIDs change their shapes quite quickly and/or may disappear in the background ionosphere. B-spline TIDs analysis method is very useful in identifying the visible as well as hidden TIDs parts in the polar ionosphere. For the first time, ionospheric scintillation has been investigated in the vicinity of TIDs at high- latitude in the southern hemisphere. Further, the presented B-spline TIDs analysis method is unique and simple in itself as it uses GPS receiver processed TEC data as the primary input. Our results show that at polar latitude it is not necessary that TIDs always appear near the high TEC regions. Usefulness of the B-spline TIDs detection method has been demonstrated in analyzing TIDs at all geographic locations and different solar activity conditions by comparing B-spline TIDs method produced results with the previous case studies.

Using GNSS TEC technique to observe compression of the plasmasphere by an interplanetary shock

An interplanetary shock impinged on the Earth’s magnetosphere on 17 March 2015, and caused instant change of total electron content (TEC) measured by ground-based global positioning system (GPS) receivers. Hao et al. (2017) attributed the sudden TEC variation to magnetospheric compression by the shock, and in this paper we follow to examine the process by incorporating high time-resolution (1 s) TEC data from some GPS receivers as well as TEC measurements from BeiDou Navigation Satellite System. These observations add new understandings to the reaction of magnetosphere to a strong shock. The compression drives plasma in dayside magnetosphere to move earthward, and the contraction of the Earth’s magnetic field will make plasma concentrate near the equatorial plane. The scenario involves dynamics of the plasmasphere where the plasma is dense enough to contribute to the variations of both GPS and Beidou TEC. In addition, we also inspect TEC measured onboard several low Earth orbit satellites, but no significant change is identified possibly due to inappropriate satellite position. The above TEC measurements, known collectively as global navigation satellite system (GNSS) TEC techniques, exhibit an unprecedented capability of remote sensing and will hopefully provide new insight into the dynamics of magnetosphere.

A new approach for cycle slip detection and fix using single GPS receiver\u2019s single satellite dual frequency data containing arbitrarily large pseudorange errors

This paper presents a new cycle slip detection and repair method using Total Electron Content Rate (TECR) information derived from individual satellite dual-frequency data of a single Global Positioning System (GPS) receiver while pseudorange measurements are subject to arbitrarily large range errors. Sudden Increase of Pseudorange Error (SIPE), similar to cycle slips in nature, is quite common in various data acquisition scenarios. The basic principle of this method is to take advantage of the fact that the ionospheric TECR does not exceed certain threshold, which is set as 0.35 TECU/s in this study. Analytic expressions to evaluate the effect of SIPE on cycle slip detections have been developed. The search spaces for cycle slip candidate pairs are defined, given a predefined (sufficiently large) SIPE value. Two cycle slip validation rules are proposed to validate the cycle slip candidates. Over 99.9% of candidates can be rejected with the application of two validation rules. The theoretically maximal number of remaining cycle slip candidate pairs (NRCP) can be exactly calculated based on the magnitude of SIPE, TECR threshold, and the data sampling interval. After applying validation rules, the correct cycle slip pairs can be identified using a modified low-order polynomial fitting method. This method is tested on 13 high rate (1-Hz) dual-frequency datasets recorded by both ground-based static and satellite-borne high dynamic GPS receivers under various levels of ionospheric activities. Simulated cycle slips in 12 different possible cases and varying SIPE magnitudes are introduced into the data sets. In each test scheme, averagely 600–750 pairs of cycle slips are simulated. The SIPE magnitudes are set to vary from 50.0 m to 1000.0 m. Test results show that all the cycle slips in all the test schemes and all the datasets have been successfully detected and fixed even with a maximum SIPE of 1000.0 m in pseudoranges. A distinct advantage of this method is that it works in real-time with individual satellite’s data from a single dual-frequency receiver, even if the carrier phases have virtually any size of cycle slips and the pseudoranges have virtually arbitrarily large errors.

Near real-time tropospheric water vapour profiling using a ground-based GPS receiver

ABSTRACTThis article presents a remote-sensing tool employing an artificial neural networks algorithm for near real-time determination of the relative humidity (RH) profile above a site using global positioning system (GPS) data recorded by a ground-based GPS receiver. GPS data were processed to obtain Integrated Water Vapour. The integrated water vapour in conjunction with ground level information for temperature, pressure, and RH was fed as inputs to the developed neural network which in turn generated the instantaneous RH profile, at six standard pressure levels of 100, 150, 200, 300, 500, and 700 hPa, as output. GPS and radiosonde data for the years 2009 and 2010 were used to train the system while the same data for 2011 were used to validate the system. The relative humidity profile results for 2011 generated using GPS data and the neural network, upon comparison with recorded in situ radiosonde RH profile measurements for the same days and times in the year 2011, had root mean square error of less than 4%, which falls within the margin of error of the Vaisala RS92 Radiosonde’s humidity measurement regime.

Global equatorial plasma bubble growth rates using ionosphere data assimilation

AbstractFlux tube integrated Rayleigh‐Taylor instability growth rates computed by using the results of ionosphere data assimilation are used for the first time to investigate global plasma bubble occurrence. The study is carried out by assimilating total electron content measurements using ground‐based Global Positioning System (GPS) receivers into thermosphere ionosphere electrodynamic general circulation model, and the growth rates are calculated by using standalone model run without assimilation (control run) as well as using prior (or forecast) state output of the assimilation run. The growth rates are compared with the rate of change of total electron content index (ROTI), estimated from global network of GPS receivers, as well as all‐sky airglow observations carried out over Taiwan on the nights of 16 and 17 March 2015. In contrast to the growth rates using the control run, results using data assimilation show remarkable agreement with the ROTI. Further, the all‐sky images reveal intense plasma bubbles over Taiwan on the night of 16 March, when the corresponding assimilated growth rate is also pronounced. Similarly, the absence of plasma bubbles in the all‐sky images on the night of 17 March (St. Patrick's Day storm) is supported by smaller growth rates predicted by the assimilation model. Significant improvements in the calculated growth rates could be achieved because of the accurate updating of zonal electric field in the data assimilation forecast. The results suggest that realistic estimate or prediction of plasma bubble occurrence could be feasible by taking advantage of the data assimilation approach adopted in this work.

THE DETERMINATION OF ATMOSPHERIC WATER CONTENT FROM METEOROLOGICAL AND GPS DATA

The Global Positioning System (GPS) based on satellites and the networks of dual frequency receivers are actively used for geodetic and geophysical applications, as well as for studying the ionosphere and troposphere. The atmospheric water content is in the focus of research as a key parameter for determining of the accuracy of weather forecasting and hydrological monitoring. The precision of atmospheric water content calculations depends on the accuracy of determination of the delays of signals propagating from GPS satellites to ground-based GPS receivers when geodynamic measurements are conducted. This paper describes a technique that allows us to estimate the integrated water vapor (IWV) in the atmosphere from measurements of GPS satellite signal delays. We consider remote sensing of the lower atmosphere by GPS measurements to detect the water vapor content in the conventional vertical column to the top level of the troposphere (up to 12 km above the Earth's surface). In studies of the propagation of signals from GPS satellites to ground receivers, the atmospheric water vapor is taken into account as a ‘wet’ component (ZWD) of the zenith tropospheric delay (ZTD). ZTD is the sum of ZHD (hydrostatic or ‘dry’ delay) and ZWD (‘wet’ delay). ZWD values can be converted with a very high confidence in integrated water vapor (IWV) values for each installed GPS receiver.

TEC prediction performance of the IRI-2012 model over Ethiopia during the rising phase of solar cycle 24 (2009–2011)

This paper discusses the performance of the latest version of the International Reference Ionosphere (IRI-2012) model for estimating the vertical total electron content (VTEC) variation over Ethiopian regions during the rising phase of solar cycle 24 (2009–2011). Ground-based Global Positioning System (GPS) VTEC data, inferred from dual-frequency GPS receivers installed at Bahir Dar (geographic latitude 11.6°N and longitude 37.35°E, geomagnetic latitude 2.64°N and longitude 108.94°E), Nazret (geographic latitude 8.57°N and longitude 39.29°E, geomagnetic latitude −0.25°N and longitude 111.01°E), and Robe (geographic latitude 7.11°N and longitude 40.03°E, geomagnetic latitude −1.69°N and longitude 111.78°E), are compared to diurnal, monthly, and seasonal VTEC variations obtained with the IRI-2012 model. It is shown that the variability of the diurnal VTEC is minimal at predawn hours (near 0300 UT, 0600 LT) and maximal between roughly 1000 and 1300 UT (1300–1600 LT) for both the experimental data and the model. Minimum seasonal VTEC values are observed for the June solstice during the period of 2009–2011. Moreover, it is shown that the model better estimates diurnal VTEC values just after the midnight hours (0000–0300 UT, 0300–0600 LT). The modeled monthly and seasonal VTEC values are larger than the corresponding measured values during the period of 2009–2010 when all options for the topside electron density are used. An important finding of this study is that the overestimation of VTEC values derived from the model decreases as the Sun transitions from very low to high solar activity. Moreover, it is generally better to use the model with the NeQuick option for the topside electron density when estimating diurnal, monthly, and seasonal VTEC variations.

Comparison of GPS-TEC with IRI-2012 TEC over African equatorial and low latitude regions during the period of 2012–2013

This paper presents the performance of the latest version of the International Reference Ionosphere (IRI-2012) model in estimating the Vertical Total Electron Content (VTEC) variation over equatorial and low latitude East Africa regions during the period of 2012–2013. This has been conducted by comparing the ground-based Global Positioning System (GPS) VTEC inferred from nine dual frequency GPS receivers installed at different regions in the continent. In this work, the diurnal, monthly and seasonal variation in the measured VTEC have been analyzed and compared with the VTEC obtained from IRI-2012 model. It is depicted that the lowest diurnal peaks of the modeled and measured VTEC values are observed in the June solstice months, whereas the highest values are observed in the equinoctial months. The variability of the diurnal GPS-VTEC is also found to be minimal nearly at 03:00 UT and maximal mostly between 09:00 and 14:00 UT. Moreover, the maximum and minimum monthly mean hourly measured VTEC values inferred from both the Arba Minch and Entebbe stations are observed in October and July, respectively. Similarly, the highest and the lowest seasonal mean hourly measured VTEC values are observed in the September equinox and in the June solstice, respectively. The model predictions generally follow the diurnal variations of the measured VTEC, with minimum value at predawn hours and maximum at noontime hours (10:00–13:00 UT). It has been shown that the model better estimates the diurnal VTEC values mostly just after midnight hours (00:00–03:00 UT). Good agreements between the modeled and measured monthly and seasonal mean hourly VTEC values obtained from Arba Minch station are also observed in the equinoctial months. But, for the Entebbe station, the modeled monthly and seasonal VTEC values are larger than the corresponding measured VTEC values by about 75% and 60%, respectively. In addition, there are large discrepancies observed between the diurnal measured and modeled VTEC values (especially in the equinoctial months) in the time interval between 09:00 and 14:00 UT.

TEC prediction performance of IRI‐2012 model during a very low and a high solar activity phase over equatorial regions, Uganda

AbstractThis paper examines the capacity of the latest version of the International Reference Ionosphere (IRI‐2012) model in predicting the vertical total electron content (VTEC) variation over Uganda during a very low solar activity phase (2009) and a high solar activity (2012) phase. This has been carried out by comparing the ground‐based Global Positioning System (GPS) VTEC inferred from dual‐frequency GPS receivers installed at Entebbe (geographic latitude 0.038°N and longitude 32.44°E; geomagnetic latitude −9.53°N and longitude 104.06°E) and Mbarara (geographic latitude −0.60°N and longitude 30.74°E; geomagnetic latitude −10.02°N and longitude 102.36°E). In this work, the diurnal, monthly, and seasonal variations in the measured VTEC have been analyzed and compared with the VTEC derived from IRI‐2012 model. It has been shown that the lowest diurnal peak GPS‐VTEC values are observed in the June solstice months during both the low and the high solar activity phases. Similarly, the highest diurnal peak IRI‐2012 VTEC values are observed in equinoctial months during both phases. The variability of the VTEC in both the experimental and model is minimal nearly at 03:00 UT (06:00 LT) and maximal mostly between 10:00 and 13:00 UT (13:00–16:00 LT) during both phases. The diurnal highest peak modeled VTEC value observed during the high solar activity phase is almost twice larger than the diurnal highest peak modeled VTEC value depicted during the low solar activity phase. Moreover, the highest monthly mean hourly measured VTEC value observed in October during the high solar activity phase is larger by more than twice the corresponding highest monthly mean hourly measured VTEC value observed in November during the low solar activity phase. Similarly, the lowest monthly measured VTEC value observed in July during the high solar activity phase is larger by more than twice the corresponding lowest measured monthly VTEC value observed in the same month during the low solar activity phase. It has also been shown that the highest measured seasonal mean hourly VTEC values are observed in the December solstice and the March equinox, respectively, during the low and the high solar activity phases, while the corresponding lowest measured VTEC values are observed in the June solstice during both phases. The model prediction generally follows the monthly and seasonal variations of measured VTEC, with the highest and the lowest monthly values being observed in equinoctial and solstice months, respectively, during both phases. The overall results show that the modeled diurnal, monthly, and seasonal VTEC values are generally larger than those corresponding measured VTEC values observed during both the low and the high solar activity phases, with the largest deviations being observed during the low solar activity phase. Moreover, the model does not respond to the effects resulting from the storm. Hence, unlike the measured VTEC values, the modeled VTEC values are unaffected by the storm.

Intensity of low-latitude nighttime F-region ionospheric density irregularities observed by ROCSAT and ground-based GPS receivers in solar maximum

This study examines the global correlation between the instantaneous total amplitude of ion density fluctuations observed by ROCSAT and the phase fluctuation of the total electron content (TEC) recorded by worldwide ground-based Global Positioning System (GPS) receivers during the high solar activity period of March 1999–December 2002 for Kp<3. The Hilbert–Huang transform (HHT) is applied to compute the instantaneous total amplitude of ROCSAT ion densities. The event-based and climatological comparisons of the total amplitude and occurrence probability of irregularities observed by ROCSAT show that the total amplitude can reveal both the occurrence probability of irregularities and the associated intensity. The noise level of the total amplitude is about 103.5 (near 3000) ions/cm3. The high correlation (correlation coefficient=0.81) between the GPS TEC phase fluctuation index FP and instantaneous total amplitude of ROCSAT electron densities suggests that the total amplitude can be used to globally monitor the intensity of irregularities at equatorial and within the latitude belt of ±15°. The relationship between the ionospheric background ionization and the irregularity intensity is further investigated.

Patterns of GPS-TEC variation over low-latitude regions (African sector) during the deep solar minimum (2008 to 2009) and solar maximum (2012 to 2013) phases

Due to the unique geometry of the geomagnetic fields near the magnetic equator and low-latitude regions, the satellite communication system in the African sector is strongly influenced by the effects resulting from the accumulation of electrons in their ionosphere. Hence, this paper investigates the patterns of the vertical total electron content (VTEC) variation detected by the Global Positioning System (GPS) over low-latitude regions during a very low (2008 to 2009) and a high solar activity (2012 to 2013) phases. The study has been carried out by considering eight ground-based dual-frequency GPS receivers installed recently at different regions in Ethiopia. In this work, the diurnal, monthly, and seasonal variations in the GPS-VTEC have been analyzed. It has been found that the diurnal variability of VTEC has shown minimum values at around 0300 UT (0600 local time (LT)) and maximum values nearly between 1000 and 1300 UT (1300 and 1600 LT) during both the low and the high activity phases. Moreover, the maximum and minimum of monthly mean hourly VTEC values are observed in October and July, respectively, during both the low (2009) and the high solar activity (2012) phases. It has also been depicted that seasonal mean hourly VTEC values have shown maxima and minima in the March equinox and the June solstice, respectively, during both the low and the high solar activity phases.

A study of El Niño-Southern oscillation impacts to the South China Sea region using ground-based GPS receiver

We observe an ENSO activity by using ground-based GPS receiver as an effort to study the effects of global warming and climate change in the tropical region. The precipitable water vapor (PWV) derived from Global Positioning System (GPS) meteorology in line with the sea surface temperature anomaly (SSTa) is used to indicate their response on ENSO activities. The PWV data used in this study was taken from the station at Universiti Malaysia Sabah, Kota Kinabalu (UMSK) over 2011, together with NTUS station (in the Singapore), PIMO (in Philippines) and BAKO (in Indonesia) are also compared. The relationship between PWV and SSTa at all stations on weekly basis exhibited modest with correlation coefficients between −0.30 and −0.78 significantly at the 99% confidence level. The negative correlation indicates that during a La Niña phase, the PWV is increased when the sea surface temperatures getting cold causes warm air mass in the central Pacific moved to west Pacific. The increased of PWV causes the GPS signals will be getting slower.