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GPS satellites of opportunity for ionospheric monitoring

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... [,,. They represent the possible nonsynchronization of the four measurements recorded by the i-th receiver (Coco, 1991; Sato, 1995). If the four measurements are collected at exactly the same instant, these bias terms will be zero. ...
... The terms b,,,, b,,,, and b,,, represent the interchannel biases between @: , and @: , , Pfi,, and Pf,, respectively. The underlying reason for the presence of these interchannel delays results from the fact that the L, and L, signals must travel through different hardware paths inside the receiver as well as in the satellite transmitter (Coco, 1991 ). The signals transmitted from GPS satellites at the two L-band frequencies have been carefully synchronized and calibrated by Rockwell International , the manufacturer of the GPS satellites, before the satellites are launched (Coco, 1991). ...
... The underlying reason for the presence of these interchannel delays results from the fact that the L, and L, signals must travel through different hardware paths inside the receiver as well as in the satellite transmitter (Coco, 1991 ). The signals transmitted from GPS satellites at the two L-band frequencies have been carefully synchronized and calibrated by Rockwell International , the manufacturer of the GPS satellites, before the satellites are launched (Coco, 1991). But this synchronization and calibration work might not be as thorough for the GPS receivers due to the cost factor. ...
Article
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Precise determination of GPS phase ambiguity integers on the fly is a key requirement for accurate kinematic positioning. A fully automatic approach is developed to resolve phase integers and perform airborne GPS positioning for aerial triangulation using dual-frequency GPS data. Although always helpful, static initialization is not required; the problem of signal interruptions (losses of lock) while an airplane is maneuvering distant from a base station has also been addressed. Test results show that positions using precise phase data are reliably determined with this approach.
... On the other hand, taking this advantage, we can use dual-frequency GPS measurements to remote sensing the ionosphere (i.e. to estimate the TEC of the ionosphere). One of the challenging problems in making absolute TEC measurements using dual-frequency observations of the GPS satellites is to estimate the satellite and receiver L1/L2 differential delays (Coco 1991; Bishop et al., 1992; Klobuchar et al., 1993). In this paper the author proposes an alternative algorithm, based on the single-site modeling technique, to estimate the sum of the satellite and receiver L1/L2 differential delay for each tracked GPS satellite, using 24-hour data sets. ...
... The codes transmitted by GPS satellites at the two L-band frequencies (L1 and L2) are carefully synchronized so that they are broadcast simultaneously. Absolute simultaneity is not possible, however, so the time difference between the transmitted times at the two frequencies is called the satellite L1/L2 differential delay or satellite differential delay (Coco, 1991) Both the satellite and receiver differential delays introduce error in the measurement of TEC. According to Wilson et al. (1994), ignoring the satellite differential delays and the receiver differential delay when computing line-of-sight TEC measurements from GPS observables may result in an error of 3 ± ns and 10 ± ns respectively. ...
... : A dual-frequency GPS receiver measures pseudo-ranges and carrier phases at L1/L2 and its observables are used to compute TEC. The "phase leveling" technique (e.g. Coco et al., 1991; Lin, 1998) is used to compute precise phase-derived slant TEC for each tracked satellite at each observation epoch. These slant TEC measurements are the sum of the real slant TEC, the GPS satellite differential delay S b and the receiver differential delay R b . ...
Article
The ionospheric delay in the propagation of Global Positioning System (GPS) signals is one of the main sources of error in GPS precise positioning and navigation. A dual-frequency GPS receiver can eliminate (to the first order) the ionospheric delay through a linear combination of the L1 and L2 observations. On the other hand, taking this advantage, a dual-frequency GPS receiver can be used to remote sensing the ionosphere. One of the challenging problems in making absolute ionospheric delay measurements using the dual-frequency observations to the GPS satellites is to estimate satellite and receiver L1/L2 differential delays. In this paper an algorithm is proposed which can estimate the sum of the satellite and receiver L1/L2 differential delays of each tracked GPS satellite using a single-site modeling technique. The estimation method, test results, and comparison with the results of other organizations are described here. The test results indicate that the estimation precision of the proposed algorithm is about 0.43 nanosecond (ns) in differential delay. The standard deviation of the estimated satellite differential delay differences between the values determined by the proposed algorithm and other organizations is generally less than 1 ns (from 0.33 ns to 1.07 ns).
... However, this difference does not take into account any error in the receiver's clock relative to the satellite's clock and therefore the range is only approximate and is therefore called a pseudorange. The ionosphere has a refractive index at radio frequencies, which is different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground (Coco, 1991; Wanninger, 1993; Klobuchar, 1996). One of the significant effects is that the GPS signal traversing the ionosphere undergoes an additional delay proportional to the total number of electrons in the cross-section volume measured in TEC units. ...
... However, this difference does not take into account any error in the receiver's clock relative to the satellite's clock and therefore the range is only approximate and is therefore called a pseudorange. The ionosphere has a refractive index at radio frequencies, which is different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground (Coco, 1991; Wanninger, 1993; ). One of the significant effects is that the GPS signal traversing the ionosphere undergoes an additional delay proportional to the total number of electrons in the cross-section volume measured in TEC units. ...
Article
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With the recent increase in the satellite-based navigation applications, the ionospheric total electron content (TEC) and the L-band scintillation measurements have gained significant importance. In this paper we present the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, for the first time, using the Indian GPS network of 18 receivers located from the equator to the northern crest of the equatorial ionization anomaly (EIA) region and beyond, covering a geomagnetic latitude range of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005. The diurnal variation in TEC at the EIA region shows its steep increase and reaches its maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad and occurs around 16:00 LT. A short-lived day minimum occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region. Beyond the crest region the day maximum values decrease with the increase in latitude, while the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00 LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the equator and about 5 to 90 TEC units at the EIA crest region, which correspond to range delay variations of about 1 to 8 m at the equator to about 1 to 15 m at the crest region, at the GPS L1 frequency of 1.575 GHz. The day-to-day variability is also significant at all the stations, particularly during the daytime hours, with maximum variations at the EIA crest regions. Further, similar variations are also noticed in the corresponding equatorial electrojet (EEJ) strength, which is known to be one of the major contributors for the observed day-to-day variability in TEC. The seasonal variation in TEC maximizes during the equinox months followed by winter and is minimum during the summer months, a feature similar to that observed in the integrated equatorial electrojet (IEEJ) strength for the corresponding seasons. In the Indian sector, the EIA crest is found to occur in the latitude zone of 15° to 25° N geographic latitudes (5° to 15° N geomagnetic latitudes). The EIA also maximizes during equinoxes followed by winter and is not significant in the summer months in the LSSA period, 2004?2005. These studies also reveal that both the location of the EIA crest and its peak value in TEC are linearly related to the IEEJ strength and increase with the increase in IEEJ.
... TOPICS OF IONOSPHERIC RESEARCH RELATED TO THE IGS NETWORK The availability of corrected GPS-data obtained at a number of globally distributed IGS stations provide new chances for ionospheric research (e.g. Coco, 1991). The effective use of IGS stations in regional and/or global monitoring has been demonstrated already by several authors (e.g. ...
... The final conclusion will be made after the additional data analysis using precise orbital parameters provided by IGS. It is very important now to continue the precise GPS measurements in the Sakhalin-Kamchatka area, as the region Avacha Bay near Petropawlovsk is one of the dangerous regions according to the long term predictions for 1991-1995(Fedotov at al. in Volcanology and Seismology, 1993. The Avacha Bay may well be happen to be a site of release of seismic energy, sufficient for one earthquake of 8.5 M accumulated in the Kamchatka region (Fedotov and al. 1994, Bull. ...
... GPS geodesists estimate ionospheric and tropospheric delays only to eliminate them. But ionospheric physicists are beginning to use GPS as a tool to study the ionosphere [Coco, 1991]. We believe that in a similar manner meteorologists may be able to exploit GPS as a means of studying the refractivity of the atmosphere and tropospheric water vapor distribution in particular. ...
Article
Full-text available
We present a new approach to remote sensing of water vapor based on the Global Positioning System (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground-based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time-varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, and government and military agencies, in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitate water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change.
... The ionosphere has a refractive index at radio frequencies, which is different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground receiver (Coco, 1991;Wanninger, 1993;Klobuchar, 1996). One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in column of 1 m 2 cross-sectional area along the ray path from the satellite to receiver. ...
Article
The ionospheric total electron content (TEC), derived by analyzing dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Varanasi (geomagnetic latitude 14°, 55′N, geomagnetic longitude 154°E) is studied. Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the solar minimum period from May 2007 to April 2008. It is found that the daily maximum TEC near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semiannual variation is seen with two maxima occurring in both spring and autumn. Statistical studies indicate that the variation of EIA crest in TEC is poorly correlated with Dst-index (r = −0.03) but correlated well with Kp-index (r = 0.82). The EIA crest in TEC is found to be more developed around 12:30 LT.
... The dynamic model describes the time-dependent relationship between successive values of the same state. The ionospheric effect is usually considered to be a time-dependent signal that continuously changes in time as the electron density varies with time and location in the ionosphere (Coco, 1991;Klobuchar, 1991;Ming, 1999). To characterise the temporal behaviour of the ionosphere, we have calculated and analysed the autocorrelation of phase DD ionospheric residuals and thereby obtained the dynamic model. ...
Article
The ionospheric effect is considered to be one of the most important error sources limiting the quality of GPS kinematic positioning. Over longer distances, differential ionospheric residuals become larger and may affect the ambiguity resolution process. We present here a Kalman-filter-based GPS ionosphere model for long-baseline kinematic applications. This observational model includes the differential ionosphere as an additional unknown factor with position coordinates and ambiguities, while the temporal correlations of the state vector are specified in the dynamic model. The temporal behaviour of ionospheric residuals is determined by the analysis of their autocorrelation function. This newly developed method was applied on a set of data collected by a roving receiver located offshore of Oran (Algeria). The results show that for baselines of about 80 km, the root mean square is at the level of a few centimetres. For tests of baselines of about 51 km, the comparison between short- and long-baseline solutions revealed that mean differences of a few millimetres and 2 cm are obtained for the horizontal coordinates and vertical component, respectively, and the standard deviation (sigma) of differences on the scale of a few centimetres.
... Davies, 1991). Recently, space-based radio navigation systems such as the US Global Positioning System (GPS) offer new opportunities for studying the ionosphere on a global scale (e.g. Coco, 1991; Wilson et al., 1995; Zarraoa and Sardon 1996). This is possible because GPS satellites transmit coherent dual-frequency signals in the L-band, low enough to measure a significant ionospheric contribution. ...
Article
Full-text available
When travelling through the ionosphere the signals of space-based radio navigation systems such as the Global Positioning System (GPS) are subject to modifications in amplitude, phase and polarization. In particular, phase changes due to refraction lead to propagation errors of up to 50 m for single-frequency GPS users. If both the L1 and the L2 frequencies transmitted by the GPS satellites are measured, first-order range error contributions of the ionosphere can be determined and removed by difference methods. The ionospheric contribution is proportional to the total electron content (TEC) along the ray path between satellite and receiver. Using about ten European GPS receiving stations of the International GPS Service for Geodynamics (IGS), the TEC over Europe is estimated within the geographic ranges -20°leqleq40°E and 32.5°leqleq70°N in longitude and latitude, respec- tively. The derived TEC maps over Europe contribute to the study of horizontal coupling and transport proces- ses during significant ionospheric events. Due to their comprehensive information about the high-latitude ionosphere, EISCAT observations may help to study the influence of ionospheric phenomena upon propagation errors in GPS navigation systems. Since there are still some accuracy limiting problems to be solved in TEC determination using GPS, data comparison of TEC with vertical electron density profiles derived from EISCAT observations is valuable to enhance the accuracy of propagation-error estimations. This is evident both for absolute TEC calibration as well as for the conversion of ray-path-related observations to vertical TEC. The combination of EISCAT data and GPS-derived TEC data enables a better understanding of large-scale ionospheric processes.
... where S 1 = 0 and S 2 = R, N e is the integrated electronic density in a cubic meter, and S is the path distance of a column with height R (Jursa, 1985;Chamberlain and Hunten, 1987;Coco, 1991;Hunt et al., 2000). The TEC was calculated considering only satellite measurements taken at an elevation angle above 30°. ...
Article
Full-text available
The ionospheric dynamics in the South America (SA) sector during geomagnetic disturbed period from 21 to 24 June 2015 is investigated through ground ionosonde stations and Global Navigation Satellite System (GNSS) receivers, supported by Very Low Frequency (VLF) and magnetometer data. These disturbances were caused by 3 interplanetary shocks (IS) derived from 3 consecutives coronal mass ejections (CME) from the same solar active region; the first two CME were caused by filament eruptions, and the third was a much larger full halo CME, associated with a M2.6 solar flare. The first 2 shocks were compressive and did not cause an immediate response to the ionosphere in the analyzed region, while the third shock increased considerably the electron density from low to high-latitudes, triggering the second strongest geomagnetic storm of the 24th solar cycle. It was possible to observe the expansion of the crest of equatorial ionospheric anomaly (EIA) at midlatitudes and high-latitudes mainly due to prompt penetration electric field (PPEF) during the main phase and the recovery phase of the geomagnetic storm during the day.
... , respectively. They result from the fact that the L1 and L2 signals travel through different hardware paths inside the receiver as well as the satellite transmitter (Coco, 1991). Therefore, the interchannel biases are dependent on both the satellite and the receiver. ...
... TOPICS OF IONOSPHERIC RESEARCH RELATED TO THE IGS NETWORK The availability of corrected GPS-data obtained at a number of globally distributed IGS stations provide new chances for ionospheric research (e.g. Coco, 1991). The effective use of IGS stations in regional and/or global monitoring has been demonstrated already by several authors (e.g. ...
... The interchannel biases for i-th receiver are caused by time non-synchronization of the four measurements. This non-synchronization results from the fact that the L1 and L2 signal must travel through different hardware paths inside the receiver and transmitter (Coco, 1991). ...
Article
Compared to the conventional ground measurement of gravity, airborne gravimetry is relatively efficient and cost-effective. Especially, the combination of GPS and INS is known to show very good performances in the range of medium frequencies in the gravity signal and it is relatively cheap and efficient. Conventionally, gravity estimation using GPS/INS was analyzed through the estimation of INS system errors using GPS position and velocity updates. In this case, the complex navigation equations must be integrated to obtain the INS position, and the gravity field must be stochastically modeled as a part of the state vector. The vertical component of the gravity vector is not estimable in this case because of the instability of the vertical channel in the solution of the inertial navigation equations. In this study, a new algorithm using acceleration updates instead of position/velocity updates has been developed. Because we are seeking the gravitational field, that is, accelerations, the new approach is conceptually simpler and more straightforward. In addition, it is computationally less expensive since the navigation equations do not have to be integrated. It is more objective, since the gravity disturbance field does not have to be explicitly modeled as state parameters. An application to real test flight data as well as an intensive simulation study have been performed to test the validity of the new algorithm. The results from the real flight data show very good accuracy in determining the down component, with accuracy better than +/-5 mGal. Also, a comparable result was obtained for the horizontal components with accuracy of +/-6 to +/-8 mGal. The resolution of the final result is about 10 km due to the attenuation with altitude. The inclusion of a parametric gravity model into the new algorithm is also investigated for theoretical reasons. The gravity estimates from this filter showed strong dependencies on the model and required extensive computation with no improvement over the approach without parametric gravity model.
... The third segment is the 'user segment' includes GPS receiver and makes use of the transmitted signals by satellites . The ionosphere has a variable refractive index at radio frequencies and can affect GPS signals in a number of ways as they pass from satellite to ground receiver (Coco 1991; Wanninger 1993; Klobuchar 1996). One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in a column of 1 m 2 cross-sectional area along the ray path from the satellite to the receiver. ...
... The third segment is the 'user segment' includes GPS receiver and makes use of the transmitted signals by satellites . The ionosphere has a variable refractive index at radio frequencies and can affect GPS signals in a number of ways as they pass from satellite to ground receiver (Coco 1991; Wanninger 1993; Klobuchar 1996). One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in a column of 1 m 2 cross-sectional area along the ray path from the satellite to the receiver. ...
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The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16′ N, longitude 82°, 59′ E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense—but still moderate class—storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on 20 November 2007, 9 March 2008 and 11 October 2008 were selected and storm induced features in the vertical TEC (VTEC) have been studied considering the mean VTEC value of quiet days as reference level. The possible reasons for storm time effects on VTEC have been discussed in terms of local time dependence, storm wind effect as well as dawn-dusk component of interplanetary electric field (IEF) Ey intensity dependence.
... The remaining terms b^ , b£ 2 , and b£ 3 are the relative interchannel biases between <J>£j and <^\ 2 ♦ Pij, and P£ 2 , respectively. They result from the fact that the LI and L2 signals travel through different hardware paths inside the receiver as well as the satellite transmitter (Coco, 1991). Therefore, the interchannel biases are dependent on both the satellite and the receiver. ...
Article
Full-text available
This report presents the results of investigations to determine very accurate position coordinates using the Global Positioning System in the absolute (point) positioning mode. The most common method to obtain very accurate positions with GPS is to apply double-differencing procedures whereby GPS satellite signals are differenced at a station and these differences are again differenced with analogous differences at other stations. The differencing between satellites eliminates the large receiver clock errors, while the between-station differences eliminate the large satellite clock errors (as well as some other errors, such as orbit error). However, only coordinate differences can be determined in this way and the accuracy depends on the baseline length between cooperating stations. The strategy with accurate point positioning is to estimate GPS satellite clock errors independently, thus obviating the between-station differencing. The clock error estimates are then used in an application of a single-difference (between-satellite) positioning algorithm at any site to determine the coordinates without reference to any other site. Using IGS orbits and stations, the GPS clock errors were estimated at 30-second intervals and these estimates were compared to values determined by JPL. The agreement was at the level of about 0.1 nsec (3 cm). The absolute positioning technique was tested at a stationary site (IGS station) whose coordinates are known. The differencess between the estimated absolute position coordinates and the known values had a standard deviation less than 4 cm in all three dimensions, with mean differences ranging from 3.4 cm to 6.3 cm.
... The 'third segment' is the user segment, which includes GPS receiver, who is making use of the transmitted signals by satellites. The ionosphere has a refractive index at radio frequencies, which is different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground receiver [17][18]. One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in column of 1 m 2 cross-sectional area along the ray path from the satellite to receiver. ...
Article
Full-text available
In this paper we studied the effect of geomagnetic storm on Global Positioning System (GPS) derived total electron content (TEC) at low latitude Varanasi (Geomagnetic lat 14°, 55' N, geomagnetic long 154° E) during the period of May 2007 to April 2008. During this period 2 storms were found, which were occurred on 20 November 2007 and 9 March 2008. In this study vertical total electron content (VTEC) of single Pseudorandom Noise (PRN) and average of VTEC of same PRN before 10 days of storm, which is called background TEC, were used to see the effect of these storms on the variation of TEC. From this study this is found that during the storm of March 2008 the TEC increases in main phase of storm while in the case of November 2007 storm, TEC decreases during the main phase of storm but increases in the recovery phase (next day) of storm.
... The third segment is the 'user segment', which includes GPS receiver, which makes use of the transmitted signals by satellites. The ionosphere has a variable refractive index at radio frequencies, which are different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground receiver (Coco, 1991; Wanninger, 1993 ). One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in column of 1 m 2 cross-sectional area along the ray path from the satellite to receiver. ...
Article
Results pertaining to the response of the low latitude ionosphere to a major geomagnetic storm that occurred on 24 August 2005 are presented. The dual frequency GPS data have been analyzed to retrieve vertical total electron content at two Indian low latitude stations (IGS stations) Hyderabad (Geographic latitude 17°20′N, Geographic longitude 78°30′E, Geomagnetic latitude 8.65°N) and Bangalore (Geographic latitude 12°58′N, Geographic longitude 77°33′E, Geomagnetic latitude 4.58°N). These results show variation of GPS derived total electron content (TEC) due to geomagnetic storm effect, local low latitude electrodynamics response to penetration of high latitude convection electric field and effect of modified fountain effect on GPS–TEC in low latitude zone.
... The effect of the ionosphere on GPS measurements is generally regarded as a time-dependent signal that continuously changes in time as the electron density varies with time and location in the ionosphere (Klobuchar, 1991;Coco, 1991). In addition to the dependence on time, the differential ionospheric effect on double-differenced observations is also a function of the length of the baseline, i.e., the distance between the two receivers ( Bender and Larden, 1985;Dong and Bock, 1989;Goad and Yang, 1997). ...
Article
Full-text available
This paper presents a real-time kinematic positioning approach which uses the Global Positioning System (GPS) for precise ocean surface monitoring. Correct resolution of the associated ambiguity integers of GPS carrier phase measurements is obtained on the fly. A GPS buoy campaign was used to compare the sea surface heights determined using a tide gauge and a nearby GPS buoy. Test results show that centimeter level accuracy in sea surface height determination can be successfully achieved using the proposed technique. The centimeter level agreement between the two methods also suggests the possibility of using this inexpensive and more flexible GPS buoy equipment to enhance (or even to replace) the current use of tide gauge stations.
... The third segment is the 'user segment' includes GPS receiver and makes use of the transmitted signals by satellites . The ionosphere has a variable refractive index at radio frequencies and can affect GPS signals in a number of ways as they pass from satellite to ground receiver (Coco 1991; Wanninger 1993; Klobuchar 1996). One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in a column of 1 m 2 cross-sectional area along the ray path from the satellite to the receiver. ...
Article
Full-text available
The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16′N, longitude 82°, 59′ E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense—but still moderate class—storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on 20November 2007, 9March 2008 and 11October 2008 were selected and storm induced features in the vertical TEC (VTEC) have been studied considering the mean VTEC value of quiet days as reference level. The possible reasons for storm time effects on VTEC have been discussed in terms of local time dependence, storm wind effect as well as dawn-dusk component of interplanetary electric field (IEF) Ey intensity dependence. KeywordsGlobal positioning system–Ionospheric total electron contents–Geomagnetic storm
... Recently, space-based radio navigation systems such as the US Global Positioning System (GPS) offer new opportunities for studying the ionosphere on a global scale (e.g. Coco, 1991;Wilson et al., 1995;Zarraoa and Sardon 1996). This is possible because GPS satellites transmit coherent dual-frequency signals in the L-band, low enough to measure a significant ionospheric contribution. ...
Article
When travelling through the ionosphere the signals of space-based radio navigation systems such as the Global Positioning System (GPS) are subject to modifications in amplitude, phase and polarization. In particular, phase changes due to refraction lead to propagation errors of up to 50 m for single-frequency GPS users. If both the L1 and the L2 frequencies transmitted by the GPS satellites are measured, first-order range error contributions of the ionosphere can be determined and removed by difference methods. The ionospheric contribution is proportional to the total electron content (TEC) along the ray path between satellite and receiver. Using about ten European GPS receiving stations of the International GPS Service for Geodynamics (IGS), the TEC over Europe is estimated within the geographic ranges -20°leqleq40°E and 32.5°leqleq70°N in longitude and latitude, respec- tively. The derived TEC maps over Europe contribute to the study of horizontal coupling and transport proces- ses during significant ionospheric events. Due to their comprehensive information about the high-latitude ionosphere, EISCAT observations may help to study the influence of ionospheric phenomena upon propagation errors in GPS navigation systems. Since there are still some accuracy limiting problems to be solved in TEC determination using GPS, data comparison of TEC with vertical electron density profiles derived from EISCAT observations is valuable to enhance the accuracy of propagation-error estimations. This is evident both for absolute TEC calibration as well as for the conversion of ray-path-related observations to vertical TEC. The combination of EISCAT data and GPS-derived TEC data enables a better understanding of large-scale ionospheric processes.
... The ionosphere has a refractive index at radio frequencies which is different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground (Klobuchar, 1996;Wanninger, 1993;Coco, 1991). One such effect is the addition of ionospheric delay to GPS signals, thereby introducing an external bias source to pseudorange and carrier phase observations which is difficult to correct in single frequency receivers. ...
Article
Full-text available
A system has been built around a dual-frequency NovAtel MiLLennium (TM) Global Positioning System (GPS) receiver in order to measure the Total Electron Content (TEC) of the ionosphere and to detect radio wave scintillation. The software has been designed and developed to provide interactive control of the receiver and the logging of its data to an external removable disk. In addition, it features a graphical user interface which includes displays both in real-time and for the previous 24 hours of satellite locations and measured TEC values. Adequate calibration of TEC values is widely recognised as a difficult problem and in this implementation, calibration of the real-time results has been achieved satisfactorily to first order. The real-time displays make the equipment particularly useful for campaigns, but the more useful application is for routine unattended logging. Two systems have been deployed, one each in Malaysia and Indonesia under the Regional Engagement program. The intent is to monitor both ionospheric behaviour and GPS performance in equatorial regions as we approach the next peak in solar cycle activity in about the year 2000. A system based on a NovAtel Millennium L1/L2 dual frequency Global Positioning System (GPS) satellite receiver has been developed to monitor ionospheric Total Electron Content (TEC) and to detect ionospheric scintillation. Software has been written to control the logging of data from the receiver to an Iomega Jaz 1 Gbyte removable disk unit. Real time displays on the Notebook screen permit the inspection of both slant and vertical TEC values taken over the preceding 24 hours, together with the locations of satellites currently in view. The system is suitable both for long term unattended routine data gathering and for more intensive short term campaigns. Two receiver system units have been deployed in Indonesia and Malaysia for the purpose of monitoring the effects of the equatorial ionosphere on GPS navigation performance as the current solar cycle approaches its 11 year maximum activity. DASD
... Scintillation measurements are a key diagnostic tool for detecting the spatial and temporal distribution of electron density irregularities in the E-and F-regions (Aarons, 1982) and could possibly provide information on the physical processes that lead to the formation and dynamics of such irregularities (Basu and Basu, 1989). The launch of the GPS satellites provided with the opportunity of a large-scale scientific research on the L-band scintillations (Coco, 1991) prior to which VHF scintillation studies were mainly carried out. Enhanced scintillation activity has been observed in the equatorial ionization anomaly (EIA) regions (10-20 ) on either side of the magnetic equator (e.g., Aarons, 1982;Thomas et al., 2001;Paznukhov et al., 2012). ...
Conference Paper
Scintillation measurements are the key diagnostic tool for detecting irregularities in electron density of the ionosphere. Morphological study of L-band amplitude scintillations were carried out at Suva using GSV 4004B GPS Ionospheric Scintillation and TEC monitor (GISTM) during 2010-11. The receiver recorded scintillation index S4, and the correction to S4 (S4 Cor) due to multipath effects. The final S4 (S4 FIN) values were computed by subtracting the S4 Cor from the recorded S4 in a Random Sum Spectrum (RSS) sense. This S4 FIN has been used to categorize weak (0.2 ≤ S4< 0.3), moderate (0.3 ≤ S4 < 0.45) and strong (0.45 ≤ S4) scintillation events and then the monthly and seasonal percentage occurrences of different category of scintillation events were studied. From 480 scintillation events, 84.4% were weak, 14.6% were moderate and 1% were strong. Scintillations were most pronounced during daytime with January showing the highest occurrences. Seasonal analysis revealed that scintillations occurred more often during the hot and wet season as compared to the cold and dry season. Annually, it was vividly clear that the daytime scintillation occurrence outweighed the nighttime occurrence with peak occurrence at around 05:00-09:00 LT. Geomagnetic disturbances effects on scintillations showed more prominent on D-days. Seasonally, during hot and wet season, the scintillations seems to be increased on D-days, while during the cold and dry season, scintillations does not vary significantly. The scintillations were also analyzed under the moderate (-100 nT ≤ Dst < -50 nT), intense (-200 nT ≤ Dst < -100 nT) and very intense (Dst < -200 nT) storms. The storms were also grouped according to the occurrence times of their main phase into category A (daytime), B (pre-midnight) and C (post-midnight). A total of 17 storms occurred during these two years, out of which 14 were moderate and 3 were intense. Generally, in the entire three categories (A, B, C), the post-storm scintillations were slightly enhanced, however, the scintillation occurrences are still unpredictable since it displays varied behavior during storms.
... Scintillation measurements are a key diagnostic tool for detecting the spatial and temporal distribution of electron density irregularities in the E-and F-regions (Aarons, 1982) and could possibly provide information on the physical processes that lead to the formation and dynamics of such irregularities (Basu and Basu, 1989). The launch of the GPS satellites provided with the opportunity of a large-scale scientific research on the L-band scintillations (Coco, 1991) prior to which VHF scintillation studies were mainly carried out. Enhanced scintillation activity has been observed in the equatorial ionization anomaly (EIA) regions (10-20 ) on either side of the magnetic equator (e.g., Aarons, 1982;Thomas et al., 2001;Paznukhov et al., 2012). ...
... Earlier, it has been confirmed that TEC values differ based on the geographical location and show noticeable variability during different solar activity periods (Radzi et al. 2013;Huang et al. 2013). The distribution and characteristics of TEC variations due to solar and geomagnetic activities at equatorial, lower, middle and high latitudes are investigated by several researchers (Lanyi and Roth 1988;Coco 1991;Goodwin et al. 1995;Ho et al. 1996;Mannucci et al. 1998;Brunini et al. 2003;Wu et al. 2004;Bhuyan and Borah 2007;Mukherjee et al. 2010). Also, significant alterations of TEC in response to different geomagnetic storms over equatorial and low-latitude regions have been demonstrated in many reports (Rao et al. 2009;Chakraborty et al. 2015). ...
Article
Full-text available
The present study investigates the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish Permanent GNSS Network (TPGN) and International GNSS Services (IGS) observations during the period from January 2015 to December 2015. The corresponding TEC predicted by the International Reference Ionosphere (IRI 2012) and Standard Plasmasphere Ionosphere Model (SPIM), and interpolated from Global Ionosphere Maps (GIMs) are evaluated to realize their reliability over the region. We studied the diurnal and monthly behavior of TEC and the relative TEC deviations along with the upper and lower quartiles to represent its spatio-temporal variability. The diurnal variation of GNSS-derived TEC indicates its maximum peak value around 10.00 UT which decreases gradually to attain minimum value after midnight. The monthly maximum value of TEC is observed in March followed by May and August, and the lowest value is seen during September. Studies show that the monthly relative deviation of TEC variability lies in the range of -1 to 4 units for all stations with the maximum difference between positive and negative variability remaining around 5. The studies also cover seasonal variation, grand-mean of ionospheric TEC and TEC intensity from the TPGN. The seasonal ionospheric VTEC pattern over all stations depicts slight increment in VTEC distribution during March equinox compared to September equinox. The December solstice perceived relatively higher VTEC than June solstice. The overall of VTEC values enhanced at all stations towards end of the year 2015 compare to mid of year due to the high solar activity. The maximum grand-mean of VTEC is registered in March equinox while the lowest value is seen in September irrespective of all stations. The measured grand-mean intensity variations of VTEC values are in ascending phase during March, May, August and November months, but in descending phase during February, April, June and September months. The latitudinal study shows daytime TEC slowly decreasing with latitudes with a latitudinal gradient range of 0.1-0.2 TECU/degree. Additionally, the TEC analysis during the strong geomagnetic storm period (07-11 September 2015; SYM-H -120 nT) infers relatively better predictability of the SPIM towards a complete understanding of the lower mid-latitude ionospheric dynamics and its effects on radio propagations, particularly over the Turkish region.
... However, absolute simultaneity is not possible, so that a time difference between the signal transmissions exists. This is called satellite inter-frequency bias (IFB) and each satellite has a unique IFB (Coco 1991). The receiver experiences a similar phenomenon because L 1 and L 2 signals travel through different hardware paths inside the receiver. ...
Article
The differential code bias (DCB) is the differential hardware (e.g., the satellite or receiver) delay that occurs between two different observations obtained at the same or two different frequencies. There are two approaches used to estimate DCBs for receivers and satellites: the relative and absolute methods. The relative method utilizes a GPS network, while the absolute method determines DCBs from a single station (zero difference). Three receiver types based on the pseudo-range observables were used here to collect the GPS data: Codeless Tracking, Cross Correlation, and Non-Cross Correlation styles. According to its types, GPS receivers have responded to restrictions on the GPS signal structure in different ways. The main goal of the current research is providing a method to determine the DCBs of GPS satellites and dual frequency receivers. The developed mathematical model was based on spherical harmonic function and geometry-free combination of pseudo-range observables (C/A or/and P-code) according to receiver type. A new elevation-dependent weighting function with respect to GPS satellites in our algorithm was applied. The applied weighting function was used to consider the quality variation of satellite DCBs, which is caused by pseudo-range measurement errors. The code of the proposed mathematical model was written using MATLAB and is called “zero difference differential code bias estimation (ZDDCBE)”. This code was tested and evaluated using data from IGS GNSS stations and different types of GPS stations out of IGS network installed in Egypt and Saudi Arabia. The estimated values from the ZDDCBE code show a good agreement with the IGS analysis centers with a mean error of estimation for the receiver DCB equal 5.94%. Therefore, the ZDDCBE code can be used to estimate the DCB for any type of receiver regardless if the receiver is from IGS network or not.
... It is also demonstrated that TEC values differ based on the geographical location and show appreciable variability during different solar activity periods (Huang et al., 2013;Adewalea et al., 2013;Radzi et al., 2013). The distribution and characteristics of TEC variations due to solar and geomagnetic activities at equatorial, lower, middle and high latitudes are investigated by several researchers (Lanyi and Roth, 1988;Coco, 1991;Goodwin et al., 1995;Ho et al., 1996;Mannucci et al., 1998;Brunini et al., 2003;Wu et al., 2004;Mukherjee et al., 2010;Bhuyan and Rashmi, 2007). The seasonal variations of TEC also depend on the solar zenith angle, thermospheric composition and the ratio of O/N 2 , as discussed in the literature (Rishbeth and Setty, 1961). ...
Article
Global Positioning System (GPS) is a remote sensing tool of space weather and ionospheric variations. However, the interplanetary space-dependent drifts in the ionospheric irregularities cause predominant ranging errors in the GPS signals. The dynamic variability of the low-latitude ionosphere is an imperative threat to the satellite-based radio communication and navigation ranging systems. The study of temporal and spatial variations in the ionosphere has triggered new investigations in modelling, nowcasting and forecasting the ionospheric variations. Hence, in this paper, the dynamism in the day-to-day, month-to-month and seasonal variability of the ionospheric Total Electron Content (TEC) has been explored during the solar maximum period, January-December 2013, of the 24th solar cycle. The spatial and temporal variations of the ionosphere are analysed using the TEC values derived from three Indian low-latitude GPS stations, namely, Bengaluru, Guntur and Hyderabad, separated by 13-18 degrees in latitude and 77-81 degrees in longitude. The observed regional GPS-TEC variations are compared with the predicted TEC values of the International Reference Ionosphere (IRI-2012 and 2007) models. Ionospheric parameters such as Vertical TEC (VTEC), relative TEC deviation index and monthly variations in the grand-mean of ionosphere TEC and TEC intensity, along with the upper and lower quartiles, are adopted to investigate the ionosphere TEC variability during quiet and disturbed days. The maximum ionospheric TEC variability is found during March and September equinoxes, followed by December solstice while the minimum variablitity is observed during June solstice. IRI models are in reasonable agreement with GPS TEC but are overestimating during dawn hours (01:00-06:00 LT) as compared to the dusk hours. Higher percentage deviations are observed during equinoctial months than summer over EIA stations, Guntur and Hyderabad. GPS TEC variations are overestimated during dawn hours for all the seasons over Bengaluru. It has also been observed that positive storm effect (enhancement of TEC) is observed during the main phase of the March storm, 2013 (March 16-18, 2013) while both positive and negative storm effects (depletion of TEC) are registered during the main phase of the June storm, 2013 (June 28-30, 2013) at Bengaluru and Guntur, respectively. IRI-2012 model has slightly large discrepancies with the GPS-VTEC compared with the IRI-2007 model during the June storm, 2013 over Guntur station. This analysis highlights the importance of upgrading the IRI models due to their discrepancies during quiet and disturbed states of the ionosphere and developing an early warning forecast system to alert about ionosphere variability.
... The third segment is the 'user segment', which includes GPS receiver, which makes use of the transmitted signals by satellites. The ionosphere has a variable refractive index at radio frequencies, which are different from unity and can affect GPS signals in a number of ways as they pass from satellite to ground receiver (Coco, 1991;Wanninger, 1993). One of the significant effects is that the GPS signals traversing the ionosphere undergoes an additional delay proportional to the total electron content (TEC), which is defined as total number of free electrons in column of 1 m 2 cross-sectional area along the ray path from the satellite to receiver. ...
Article
Full-text available
The ionosphere responses to space weather events (solar flare and geomagnetic storm) observed by using ground-based receivers of the global positioning system (GPS) have been investigated in this paper. A solar flare can be defined as the sudden and explosive release of energy (˜1019 -- 1025 J) from a localized active region of the Sun, mainly in the form of electromagnetic radiation across the entire spectrum. The heated plasma producing radiation resides in the closed loops created during the eruption process. The present paper describes the effect of solar flare on ionospheric total electron content which deduced by using data recorded by Global Positioning system (GPS) recorded in RINEX FORMAT. Total 12 flare have been selected to study the effect of solar flare on ionosphere. The enhancement on ionospheric TEC has been found during the period of solar flare. The signature of traveling ionospheric disturbances (TIDs), launched by Atmospheric Gravity Waves (AGWs) is predicted during the period of intense geomagnetic storms. Key Words: GPS, Ionosphere, TEC, Solar Flare, Geomagnetic Storms.
... The GPS signal traversing the ionosphere undergoes an additional delay proportional to the total number of electrons in the cross-sectional volume measured in TEC units. The dual frequency GPS receivers use two frequencies , L1 (1.575 GHz) and L2 (1.227 GHz), to compensate for the ionospheric delay, a measure of TEC, at least to a first order approximation, taking advantage of the dispersive nature of the ionosphere, where the refractive index is a function of frequency (Coco, 1991; Wanninger, 1993; Klobuchar, 1996). The GPS data provides an efficient way to estimate TEC with a greater spatial and temporal coverage (Davies and Hartmann, 1997; Hocke and Pavelyev, 2001). ...
Article
Full-text available
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 (
... The ionospheric time delay was found to vary between 16 ns to 100 ns while tropospheric time delay was found to vary between 1 ns to 350 ns. GPS can be used for ionospheric monitoring also [19] Some of the ionospheric parameters estimated at 1.575 GHz are given in Table 1. ...
Article
Full-text available
Wave propagation effects on L and C band frequencies with respect to satellite aided communication, navigation and surveillance are discussed. Theoretical background on ionospheric and tropospheric parameters is presented. Using the available theoretical models, for typical conditions the propagation errors are estimated. Importance of bit error rate is briefly mentioned.
... To measure precise vertical TEC by single GPS receiver, a new method is proposed in this paper. To reduce the pseudorange error and measure changes of the ionospheric delay correctly, the post-processed measurement is used to calibrate the relative cycle ambiguity with dual-frequency GPS receiver and estimate the "phase-leveling" result [2], because the group delay is irregularity but absolute and the carrier phase advance is relative range error in the ionosphere. By using the SLM assumption [3], the slant TEC (STEC) can be converted into VTEC of certificated height, and then, IFB could be measured with two satellites of the highest and the second highest elevation by the weighted least square, and also, one-minute average vertical TEC would be estimated. ...
Conference Paper
Full-text available
By GPS dual-frequency receiver, the Total Electron Content (TEC) along the path from satellite to receiver can be measured directly. But the dual-frequency measurement can be severely colored by the thermal noise in the GPS receiver and the bias in the GPS satellite. The new two steps method is proposed to estimate the precise quantities of TEC. In the first step, the "phase leveling" results of the ionospheric delay would be measured. By filtering the carrier and code measurement together, the "post-processing measurement" is used to average the code measurements from carrier phase track to calibrate the relative carrier ambiguity, but the bias of the GPS receiver and satellites is remained in this step. The remained bias is called inter-frequency bias (IFB). In the second step the SLM (single lay model) assumption and the least squares are used to determine IFB. Through one-minute average of the results measured in the first step, one-minute average of vertical TEC (VTEC) can be measured and IFB can be calibrated. Setting two GPS receivers through one antenna tests this proposed method, one minute average of VTEC with two receivers are measured respectively ; the difference between VTEC measured by receiver1 and receiver2 are calculated and the value of the difference are very little. It shows that the method can calibrate IFB, and also can measure the one-minute average of vertical TEC by single GPS observer.
... With the advancement of GPS technology and the spread of the GPS network around the world to remote locations such as Antarctica, the application of GPS to tasks including monitoring atmosphere dynamics, positioning and tracking, meteorology, and geophysical surveying has become possible. Recently, GPS has become a powerful tool for quantifying the ionospheric total electron content (TEC) [e.g., Coco 1991, Wanninger 1993, Klobuchar 1996] and the at-mospheric precipitable water vapor content (PWV) [e.g., Bevis et al. 1994, Businger et al. 1996, Rocken et al. 1997] in a cost-effective manner with global coverage and superior temporal and spatial resolution. This application of GPS can improve our understanding of the mechanisms driving ionospheric irregularities and the evolution of water vapor, two important factors in the relationship between solar activity and our atmosphere. ...
Article
Full-text available
This short-term work characterized the upper and lower levels of the atmosphere through Global Positioning System (GPS) measurements. The observations were conducted during the 2009 equinoxes from two pairs of conjugate polar observing stations: Husafell, Iceland (HUSA) and Resolute in Nunavut, Canada (RESO) and their conjugate pairs at Scott Base (SBA) and Syowa (SYOG) in Antarctica, respectively. The total electron content (TEC), an indicator of the upper atmosphere, and the precipitable water vapor (PWV), which served as the lower atmospheric response, were retrieved and analyzed. The results reveal a good relationship between TEC and PWV at each station during the onset day of the equinoxes, whereas an asymmetrical response was observed in the beginning of and after the equinoxes. In addition, the conjugate pairs were only consistent during the autumnal equinox. Thus, the high correlation was observed following the seasonal pattern for the onset day, while strong and moderate correlations were found only for the vernal equinox in Antarctica and the Arctic, respectively. This relationship reflects the fact that the intensity of solar activity during the solar minimum incident on the lower atmosphere through the conjugate points is associated with the variation of the Sun’s seasonal cycle, whereas the TEC and PWV showed an opposite relationship.
Article
Measurements at GPS ground stations of the International GPS Service (IGS) havebeen used to derive the total electron content (TEC) of the ionosphere over Europe and overthree North American stations for the 6–11 January 1997 storm event. The derived TEC dataindicate large deviations from the average behaviour especially at high latitudes on thenight-side/early morning longitude sector.The high-latitude perturbation causes a well-pronounced positive phase on the day-sidesector over Europe.Both meridional winds as well as transient electric fields are assumed to contribute to thesignature of the ionospheric perturbation propagating from high to low latitudes. Theobservations indicate a subsequent enhanced plasma loss which is probably due to theequatorward expansion of storm induced composition changes.
Article
Total electron content (TEC) measured simultaneously using Global Positioning System (GPS) satellites at 18 locations in North-South and East-West directions across the Indian subcontinent during 2003–2004 was used to study the diurnal, seasonal, and annual TEC variations. The TEC exhibits features like the equatorial noon time bite-out, annual and semiannual variations, the equatorial ionization anomaly, and day-to-day variability. Daytime variability is less at and near the equator than at the crest of the anomaly, whereas nighttime variability is high compared to the daytime variability in all seasons and at all latitudes. The time of occurrence of the diurnal maximum in TEC also varies with season. Measured TEC were compared with those predicted by the International Reference Ionosphere (IRI). It was observed that while IRI TEC is greater than those measured at about all local times, the TEC predicted by the IRI with a set of regional foF2 coefficients are closer to the measured values.
Article
Presented is a new method for retrieving the topside electron density distribution from space-based observations of the total electron content. By assuming an adequate topside density distribution, the profile reconstruction technique utilizes ionosonde and oxygen–hydrogen ion transition level measurements for uniquely determining the unknown ion scale heights and the corresponding ion and electron density profiles. The method is tested on actual measurements from the CHAMP satellite. Important applications are envisaged, such as developing and evaluating empirical and theoretical ionosphere–plasmasphere models.
Chapter
The Global Positioning System is a one-way ranging system. The GPS satellites emit signals — complex modulated radio waves — which propagate through space to receivers on or near the earth’s surface.1 From the signals it intercepts, a receiver measures the ranges between its antenna and the satellites. In this chapter, we will examine the nature of the GPS signals. After a brief review of the fundamentals of electromagnetic radiation, we will describe the structure of the GPS signals. Since the signals, in propagating to a receiver, must travel through the ionosphere and the neutral atmosphere, we will examine the effect these media have on the signals. Finally, we will look at the propagation phenomena of multipath and scattering and the effects they have on the measurements made by a GPS receiver.
Chapter
The objective of this chapter is to highlight the contribution of Global Navigation Satellite System (GNSS)–derived data to the monitoring and studying of the upper region of the Earth's atmosphere ionized mainly by solar radiation. GNSS signals travel across the ionosphere to reach receivers on the Earth or on low orbiting satellites (LEOs). The effect of the ionosphere plasma on such signals is to introduce a propagation delay that is proportional to the inverse of the squared frequency of the radio signal and to the total number of free electrons encountered by the signal along the ray path from the satellite to the receiver. This number is called “total electron content” or TEC and is very variable with time and location. When dual-frequency receivers are used to monitor GNSS signals for navigation and positioning purposes, first-order TEC effects can be removed and an estimate of TEC itself can be obtained. TEC represents a good indicator of ionosphere conditions and the availability of a very large number of GNSS receivers that provide TEC information makes it an essential contributor to ionosphere monitoring and science. An additional effect on GNSS signals is the presence of ionosphere plasma irregularities located particularly on high and low latitudes that induces rapid changes in amplitude and phase of the received signals called ionosphere scintillations. These rapid fluctuations of the GNSS signal degrade the system performance but at the same time give important information about ionospheric irregularities of different scales. This chapter starts with an outline about the ionosphere and the way TEC is estimated from GNSS signals. The way electron density profiles are obtained using radio occultation of signals between GNSS and LEO satellites is also mentioned. Finally, the main contribution of GNSS derived information to the ionosphere monitoring and science follows.
Article
 An algorithm for very accurate absolute positioning through Global Positioning System (GPS) satellite clock estimation has been developed. Using International GPS Service (IGS) precise orbits and measurements, GPS clock errors were estimated at 30-s intervals. Compared to values determined by the Jet Propulsion Laboratory, the agreement was at the level of about 0.1 ns (3 cm). The clock error estimates were then applied to an absolute positioning algorithm in both static and kinematic modes. For the static case, an IGS station was selected and the coordinates were estimated every 30 s. The estimated absolute position coordinates and the known values had a mean difference of up to 18 cm with standard deviation less than 2 cm. For the kinematic case, data obtained every second from a GPS buoy were tested and the result from the absolute positioning was compared to a differential GPS (DGPS) solution. The mean differences between the coordinates estimated by the two methods are less than 40 cm and the standard deviations are less than 25 cm. It was verified that this poorer standard deviation on 1-s position results is due to the clock error interpolation from 30-s estimates with Selective Availability (SA). After SA was turned off, higher-rate clock error estimates (such as 1 s) could be obtained by a simple interpolation with negligible corruption. Therefore, the proposed absolute positioning technique can be used to within a few centimeters' precision at any rate by estimating 30-s satellite clock errors and interpolating them.
Article
Radio occultation studies of the terrestrial atmosphere are possible through the use of signals transmitted by satellites of the Global Positioning System (GPS) and received by one or more other satellites in low earth orbit (LEO). The perturbed phase of the occulted signal gives direct information on the refractivity profile in the region of occultation, from which vertical profiles of density, pressure and temperature can be retrieved. The technique requires the use of the dual GPS frequencies in order to isolate and remove most of the ionospheric effect. Analysis of the effect of the ionosphere and methods of removing it will be presented. For the recovery of atmospheric profiles, two major issues are addressed. The first is how accurately can refractivity be retrieved for a region in which there is a large horizontal refractivity gradient; the second considers the separation of temperature and moisture in the lower troposphere. Based on model simulations, the capability of GPS to provide atmospheric profiles is assessed.
Article
Observing the Global Positioning System with a satellite in low earth orbit in an occulting geometry provides a powerful means of imaging the ionosphere. Tomographic imaging of the ionosphere from space and ground is examined using singular value decomposition analysis. The resolution and covariance matrices are examined, and simulations are performed that indicate that space data are significantly more effective than ground data in resolving both horizontal and vertical structures, such as the E layer, can be probed with occultation data.©1994 John Wiley & Sons Inc
Article
A preliminary analysis was made of ionospheric slab thickness, τ, and total electron content, TEC, for southern Australia using GPS satellite measurements. It was found that at mid-latitudes τ has similar overall diurnal, seasonal and latitudinal variations in the southern hemisphere as in the northern hemisphere. However, there are appreciable differences between τ in the two hemispheres which would justify appropriate modifications to ionospheric models based on northern hemisphere data before being applied confidently to the southern hemisphere. The usefulness of GPS satellites together with ionosondes over a spread of latitudes was demonstrated in determining long-term variations of TEC and τ over a large area. It was concluded that as few as four GPS receivers could provide TEC for the whole of Australia in real-time, though approximately six receivers in convenient locations would be required in practice.
Article
The ability to monitor space weather in near-real time is required as our society becomes increasingly dependent on technological systems such as the Global Positioning System (GPS). Certain critical applications such as railway control, highway traffic management, emergency response, commercial aviation, and marine navigation require high-precision positioning. As a consequence, these applications require real-time knowledge of space weather effects. In recent years, GPS itself has become recognized as one of the premier remote sensing tools to monitor space weather events. For this reason, Space Weather has opened a special section called ``Space Weather Effects on GPS.'' Papers in this section describe the use of GPS as a monitor of space weather events and discuss how GPS is used to observe ionospheric irregularities and total electron content gradients. Other papers address the implications that these space weather features may have on GPS and on Global Navigation Satellite System (GNSS) operations in general. Space weather impacts on GPS include the introduction of range errors and the loss of signal reception, both of which can have severe effects on marine and aviation navigation, surveying, and other critical real-time applications.
Article
In this paper we presented the model calculation results of the total electron (TEC) over Europe sector obtained with using the Global Self-Consistent Model of the Thermosphere, Ionosphere and protonosphere (GSM TIP) during August 11, 1999 Solar Eclipse. The model was developed in WD of IZMIRAN and describes the three-dimensional time-dependent distribution of the upper atmosphere parameters between 80km to 15 Earth's radii including electron concentration. The numerical model results of the TEC calculation have been compared with the experimental data of TEC obtained with GPS measurements on the European station located near far from the path of totality. Model results have shown substantial decreasing of TEC (25–30%) with time delay about 30 min. Comparison with experimental TEC data show a reasonable agreement for a number of station such as Hers, Graz, Lamkowko, Sofia, Ankara etc. though there are some differences in the details.
Article
The in situ measurements of electron contents from GRACE K-band (dual-frequency) ranging system and CHAMP planar Langmuir probe were used to validate the international reference ionosphere (IRI) models. The comparison using measurements from year 2003 to 2007 shows a general agreement between data and the model outputs. The improvement in the newer IRI model (IRI-2007) is evident with the measurements from the GRACE satellites orbiting at the higher altitude. We present the comparison between the models and data comprehensively for various cases in solar activity, local time, season, and latitude. The IRI models do not well predict the electron density in the years 2006 and later, when the solar activity is extremely low. The IRI models generally overestimate the electron density during local winter while they underestimate during local summer. In the equatorial region, the large difference at local sunrise lasts for all years and all seasons. The IRI models do not perform well in predicting the anomaly in the polar region such as the Weddell Sea Anomaly. These discrepancies are likely due to smoothed (12-month averaged) solar activity indices used in the IRI models and due to insufficient spherical harmonic representation not able to capture small spatial scales. In near future, further improvement on the IRI models is expected by assimilating those in situ satellite data by implementing higher resolution (spatial and temporal) parameterizations.
Chapter
An algorithm for absolute positioning through satellite clock estimation has been developed. Using IGS precise orbits and measurements, the GPS clock errors were estimated at 30-second intervals and these estimates were compared to values determined by JPL. The agreement was at the level of about 0.1 nsec (3 cm). The clock error estimates were then used in an application of a single-differenced (between satellite) positioning algorithm in static and kinematic mode. For the static case, an IGS station was selected and the coordinates were estimated. The estimated absolute position coordinates and the known values had a mean difference of up to 18 cm with standard deviation less than 2 cm. For the kinematic case, data (every second) obtained from a GPS buoy were tested and the result from the absolute positioning was compared to a DGPS solution. The mean difference between two algorithms is less than 45 cm and the standard deviation is less than 30 cm. It was proved that a higher rate of satellite clock determination is necessary to do absolute kinematic positioning at better than 10 cm precision.
Chapter
Mankind’s earliest navigational experiences are lost in the shadows of the past. But history does record a number of instances in which ancient mariners observed the locations of the sun, the moon, and the stars to help direct their vessels across vast, uncharted seas. Bronze Age Minoan seamen, for instance, followed torturous trade routes to Egypt and Crete, and, even before the birth of Christ, the Phoenicians brought many shiploads of tin from Cornwall. Twelve hundred years later, the Vikings were probably making infrequent journeys across the Atlantic to settlements in Greenland and North America.
Chapter
Mankind’s earliest navigational experiences are lost in the shadows of the past. But history does record a number of instances in which ancient mariners observed the locations of the sun, the moon, and the stars to help direct their vessels across vast, uncharted seas. Bronze Age Minoan seamen, for instance, followed torturous trade routes to Egypt and Crete, and, even before the birth of Christ, the Phoenicians brought many shiploads of tin from Cornwall. Twelve hundred years later, the Vikings were probably making infrequent journeys across the Atlantic to settlements in Greenland and North America.
Article
Forecasting the ionospheric space weather is crucial for improving the accuracy of the global navigation satellite systems (GNSS). Nonetheless, comprehending the nonhomogeneous ionospheric variability under space earth environmental conditions is a major challenge, and so is developing an accurate ionospheric forecasting model. The complex spatial and temporal variations in the ionospheric region are the results of the solar and interplanetary activities, in addition to the magnetosphere, mesosphere, thermosphere, stratosphere, troposphere, and lithosphere processes. Thus, this calls for an urgent need to develop a suitable ionospheric forecasting algorithm to capture the ionospheric perturbations. Total electron content (TEC) is the key parameter derived from GNSS receivers to represent the status of the ionosphere. This paper introduces a novel ionospheric forecasting algorithm based on the fusion of principal component analysis and artificial neural networks (PCA–NN) methods to forecast the ionospheric TEC values. Solar index (F10.7), geomagnetic index (Ap index), and 20-year TEC data (1997–2016) over a Japan Grid Point (34.95 °N and 134.05 °E) were used to apply artificial intelligence methodologies. The experimental results underscore the reliability of the proposed algorithm in forecasting the ionospheric time delay effects.
Article
The main source of error in the estimation of TEC (total electron content) from dual Global Positioning System (GPS) data is the effect of the differential satellite and receiver instrumental delay biases. These biases are normally estimated simultaneously with the TEC. However, the additional estimation of the instrumental biases may constitute an insurmountable burden in some practical applications like real-time estimation of TEC, or the estimation may be difficult or correlated to the ionospheric parameters, particularly in situations where the TEC behavior may be harder to model (equatorial or auroral zone, ionospheric storms, etc.). A priori values of the instrumental biases, estimated under good conditions or with global networks, could solve those problems if we could determine how stable those instrumental biases are in time and how often we need to check or reestimate their values. In this paper we will present our estimation of the GPS satellite and receiver instrumental biases from 19 months of data and the study of their variation during that time. We will also show some situations of changes in the instrumental biases and the possible influence of antispoofing (AS). The main conclusion of this work is that the variation of the estimated differential GPS satellite biases during the 19 months is smaller than 1 ns (1ns=2.86×1016e/m2) in most of the cases, with a mean RMS of 0.15 ns. For the GPS receivers used, that variation is greater than for the satellites, with the larger variations corresponding to physical changes in the receivers. The difference of the estimated differential instrumental biases between two consecutive days is in practically all cases smaller than 0.5 ns for the GPS satellites and smaller than 1 ns for the GPS receivers. Regarding the influence of AS, we have detected some significant changes in the instrumental biases of some satellites and some stations whether AS is activated or not. Our main conclusion is that due to the stability of the GPS instrumental biases, only an estimation or calibration of them (under optimal conditions) from time to time is required.
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