Dual-frequency 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.

Development and performance evaluation of Correntropy Kalman Filter for improved accuracy of GPS position estimation

It is well known that a Global Positioning System (GPS) receiver needs to ‘see’ at least four satellites to provide a three-dimensional fix solution. However, if any GPS receiver is operated in urban canyons, the visibility further reduces. To improve the position estimation accuracy, a novel kinematic positioning algorithm designated as Correntropy Kalman Filter (CKF) is proposed in this study. Instead of minimum mean square error (MMSE), the correntropy criterion (CC) is used as the optimality criterion of CKF. Like the traditional Kalman Filter (KF), the prior estimate of the state and covariance matrix are computed in CKF, and a novel fixed-point algorithm is then used to update the posterior estimates. The data of a dual-frequency GPS receiver located at the Indian Institute of Science (IISc), Bangalore (13.021°N/77.5°E) is collected from Scripps Orbit and Permanent Array Centre (SOPAC) to implement the proposed algorithm. The results of the proposed CKF algorithm are promising and exhibit significant improvement in position estimation compared to the conventional methods.

GPS Receiver Position Augmentation Using Correntropy Kalman Filter in Low Latitude Terrain

If any Global Positioning System (GPS) receiver is operated in low latitude regions or urban canyons, the visibility further reduces. These system constraints lead to many challenges in providing precise GPS position accuracy over the Indian subcontinent. As a result, the standalone GPS accuracy does not meet the aircraft landing requirements, such as Category I (CAT-I) Precision Approaches. However, the required accuracy can be achieved by augmenting the GPS. Among all these issues, the predominant factors that significantly influence the receiver position accuracy are selecting a user/receiver position estimation algorithm. In this article, a novel method is proposed based on correntropy and designated as Correntropy Kalman Filter (CKF) for precise GPS applications and GPS Aided Geosynchronous equatorial orbit Augmented Navigation (GAGAN) based aircraft landings over the low latitude Indian subcontinent. The real-world GPS data collected from a dual-frequency GPS receiver located in the southern region of the Indian subcontinent (IISc), Bangalore with Lat/Long: 13.021°N/ 77.5°E) is used for the performance evaluation of the proposed algorithm. Results prove that the proposed CKF algorithm exhibits significant improvement (up to 34%) in position estimation compared to the traditional Kalman Filter.

A novel kinematic positioning algorithm for GPS applications in urban canyons

Evolve a new kinematic positioning algorithm to estimate the position accurately in low latitude region and assess the algorithm's performance in terms of accuracy and precision is the objective of this research paper. The Extended Kalman Filter (EKF) has been widely recognized as one of the most powerful state estimation techniques in estimating system of state variables and deteriorating measurement noise. The Extended Kalman Filter is required because the uncertainty in the Minimum Mean Square Error (MMSE) estimation can be minimized. In this article, the researcher carried out a new kinematic positioning algorithm named as Cross-Correntropy Kalman Filter (CCKF) to enhance the position accuracy and performance of the Global Positioning System (GPS) receiver. Primarily, the augmentation depends on the Cross-Correntropy criterion which is a measure of local similarity index, and a novel Fixed-Point algorithm for updating subsequent estimates. In this work, the researcher presents a thorough derivation of the method is suggested, and how to estimate the Global Positioning System receiver position in low latitudes of the Indian geographical area accurately. Furthermore, in this research paper, a comparison between estimated receiver positions, measured error and performance metrics (2-Dimensional & 3-Dimensional) together with graphical illustrations for both the algorithms (EKF & CCKF) are presented. Performance of the two algorithms (EKF and CCKF) are evaluated by considering the data of a dual frequency GPS receiver located at IGS station: IISc Bangalore (lat/lon: 13.01° N/77.56° E). The re-enactment outcomes of the proposed algorithm show that the estimated receiver position is much closer to the receiver true position. In this article, batch processing data of IGS station, IISc Bangalore, obtained from IGS network of Scrips Orbit Permanent Array Center (SOPAC) is given as input and output yields in East, North and Up directions of the receiver position.

Evaluation of the SSN and F10.7 indices as a cause for performance variation of the IRI 2016 and IRI-Plas 2017 models

This paper mainly focuses on the evaluation of efficiency of the Sunspot Number (SSN) and the 10.7 cm Solar Radio Flux (F10.7) indices as a cause for the variation of the performance of the latest versions of the International Reference Ionosphere model (IRI 2016 with NeQuick, IRI01-corr and IRI2001 options for the topside electron density) and the IRI Extended to the Plasmasphere (IRI-Plas 2017) for the modeling of the Total Electron Content (TEC). The Global Positioning System (GPS)-derived TEC data obtained from the dual frequency GPS receivers located at Observation Rock, OBSR (geog 46.90°N, 238.18°W, Geom. 52.46°N) and Husband, HUSB (44.12°N, 238.15°W, Geom. 49.73°N) in the West Pacific region during the recent solar maximum (2012-2014) years have been considered for the validation of the performance of the models. The results show that both the GPS-derived TEC (GPS VTEC) and modelled (IRI 2016 and IRI-Plas 2017 VTEC) seasonal diurnal values tend to peak at 00:00 UT (16:00 LT) and 20:00 (12:00 LT) with the highest being observed mostly at 20:00 (12:00 LT); while, their minima are mostly observed at about 13:00 UT (05:00 LT). In addition, in utilizing the SSN, the best performance is generally observed in the June solstice months, especially by the IRI-Plas 2017 model. However, for the equinoctial and December solstice months, the best performance is generally observed by the IRI 2016 model with NeQuick and IRI01 options. It has also been shown that the root-mean-square deviations between the GPS-derived and modelled VTEC diurnal variation in using the F10.7 index are generally less than those of the SSN option in all months, revealing that both the IRI 2016 and IRI-Plas 2017 models generally show better performance using F10.7 index than the SSN. Hence, the F10.7 option is recommended for better TEC modeling employing the IRI 2016 and IRI-Plas 2017 models during the recent solar maximum years over the West Pacific region. In addition, both models cannot effectively estimate the geomagnetic storm time TEC variation.

Surveying Drifting Icebergs and Ice Islands: Deterioration Detection and Mass Estimation with Aerial Photogrammetry and Laser Scanning

Icebergs and ice islands (large, tabular icebergs) are challenging targets to survey due to their size, mobility, remote locations, and potentially difficult environmental conditions. Here, we assess the precision and utility of aerial photography surveying with structure-from-motion multi-view stereo photogrammetry processing (SfM) and vessel-based terrestrial laser scanning (TLS) for iceberg deterioration detection and mass estimation. For both techniques, we determine the minimum amount of change required to reliably resolve iceberg deterioration, the deterioration detection threshold (DDT), using triplicate surveys of two iceberg survey targets. We also calculate their relative uncertainties for iceberg mass estimation. The quality of deployed Global Positioning System (GPS) units that were used for drift correction and scale assignment was a major determinant of point cloud precision. When dual-frequency GPS receivers were deployed, DDT values of 2.5 and 0.40 m were calculated for the TLS and SfM point clouds, respectively. In contrast, values of 6.6 and 3.4 m were calculated when tracking beacons with lower-quality GPS were used. The SfM dataset was also more precise when used for iceberg mass estimation, and we recommend further development of this technique for iceberg-related end-uses.

TEC Regional Modeling and prediction using ANN method and single frequency receiver over IRAN

In order to study in the field of the dynamics and continuous variations in the ionosphere, the ionospheric measurement tools such as ionosondes, incoherent scatter radars, satellites, and Global Positioning System (GPS) networks should be used. Total Electron Content (TEC) is a key parameter in the investigation and identification of ionosphere layer. From observations of dual frequency GPS receivers, the ionospheric TEC can be extracted. Global Ionospheric Maps (GIM) are auxiliary Maps for a study on ionosphere layer in around the world. It is necessary to produce the regional TEC map for precise studying of the ionospheric TEC. Bernese software is used to extract TEC by dual frequency GPS receivers. Regional modeling of ionospheric TEC by Artificial Neural Network (ANN) is a significant domain for prediction TEC at both single and double frequency GPS receivers. Five locations in Iran during the period of 2006-2010 were identified and used in the development of an input space and ANN architecture for the TEC modeling. The input space included the day number (seasonal variation), hour (diurnal variation), sunspot number (a measure of the solar activity) and magnetic index (a measure of magnetic activity). Based on the results, the ANN have capability and flexibility to model and to predict TEC. TEC predicted by ANN A (NN TEC) and TEC obtained from the IRI2007 version of the International Reference Ionosphere (IRI TEC) are compared during equinoxes and solstices. Results show that ANN predicts GPS TEC more accurately than the IRI over Iran. The IRI-2007 model is not a suitable method to produce TEC over IRAN.

Real-time multi-GNSS single-frequency precise point positioning

Precise Point Positioning (PPP) is a popular Global Positioning System (GPS) processing strategy, thanks to its high precision without requiring additional GPS infrastructure. Single-Frequency PPP (SF-PPP) takes this one step further by no longer relying on expensive dual-frequency GPS receivers, while maintaining a relatively high positioning accuracy. The use of GPS-only SF-PPP for lane identification and mapping on a motorway has previously been demonstrated successfully. However, the performance was shown to depend strongly on the number of available satellites, limiting the application of SF-PPP to relatively open areas. We investigate whether the applicability can be extended by moving from using only GPS to using multiple Global Navigation Satellite Systems (GNSS). Next to GPS, the Russian GLONASS system is at present the only fully functional GNSS and was selected for this reason. We introduce our approach to multi-GNSS SF-PPP and demonstrate its performance by means of several experiments. Results show that multi-GNSS SF-PPP indeed outperforms GPS-only SF-PPP in particular in case of reduced sky visibility.

A novel approach to design measurement models of single-frequency GPS receivers for cost-effective structural monitoring networks

The global positioning system (GPS) has become an effective tool for measuring the dynamics of engineering structures from their absolute displacements. Although dual-frequency GPS receivers are widely adopted for structural monitoring applications to guarantee coordinate accuracy of the order of a few centimetres, the associated cost is too high to place larger number of these sensors. It has been recently recognised that single-frequency GPS receivers have great potential to be the smart sensors for measuring structural displacement due to their low-cost, small form-factor and low power consumption. Hence, the structural monitoring using single-frequency GPS receivers provides a much more cost-effective and compact tool for these applications; however, it remains a challenge to resolve the ambiguities rapidly and reliably. To address such a technical issue, this research focuses on design of GPS measurement models that can be effectively implemented in the structural monitoring networks. The main feature of these models is that they simultaneously model all available observables within the network, including not only GPS satellite ranging signals but also some dynamic constraints of structural monitoring networks, such as nominal coordinates and baselines of the target antennas. A series of tests has been carried out, using simulated satellite geometries and measurements, to assess impact of the proposed models on kinematic positioning performance. The results indicate that overall performance of the models that employ the single-frequency GPS observables and the dynamic constraints can be significantly improved by employing the redundant measurements.

Sagnac Effect and SET Error Based Pseudorange Modeling for GPS Applications

Radio Frequency (RF) signals broadcast by Global Positioning System (GPS) satellites have become an indispensable part of military and civil applications, right from missile guidance and civil aviation to farming and surveying. But the critical services provided by the GPS demand millimeter level positional accuracy. Over the years, research has been conducted and it is realized that the positional inaccuracy is mainly due to error in the pseudorange. The pseudorange is derived from the signal emission time (SET) of the satellite (known location) and signal reception time at the receiver (unknown location). Hence the better receiver position accuracy can be achieved by precise estimation of SET and accurate satellite position. If these errors are not corrected, they lead to inaccurate satellite position, which will propagate directly in to the range measurements. In this paper, an algorithm is proposed to estimate the SET and correct the satellite position for sagnac effect. Here the error in SET is modeled by taking into account the satellite clock aging parameters and the elliptical trajectory of the satellite, similarly the sagnac effect is corrected considering the earth's rotation during the signal propagation time. The implementation of the proposed algorithm for the ephemeris data collected from the Dual Frequency (DF) GPS receiver located in Indian subcontinent proved better positional accuracy. The SET and sagnac impact are assessed for all the tracked satellites, but the impact analysis pertaining to only SV PRN07 (Space Vehicle Pseudorandom Noise) are presented in this paper. The proposed algorithm can be implemented to achieve higher accuracy for navigation applications like civil aviation, category I/II Precision Approach (PA) aircraft's landing and Real Time Kinematic (RTK) positioning.

TEC and Instrumental Bias Estimation of GAGAN Station Using Kalman Filter and SCORE Algorithm

The standalone Global Positioning System (GPS) does not meet the higher accuracy requirements needed for approach and landing phase of an aircraft. To meet the Category-I Precision Approach (CAT-I PA) requirements of civil aviation, satellite based augmentation system (SBAS) has been planned by various countries including USA, Europe, Japan and India. The Indian SBAS is named as GPS Aided Geo Augmented Navigation (GAGAN). The GAGAN network consists of several dual frequency GPS receivers located at various airports around the Indian subcontinent. The ionospheric delay, which is a function of the total electron content (TEC), is one of the main sources of error affecting GPS/SBAS accuracy. A dual frequency GPS receiver can be used to estimate the TEC. However, line-of-sight TEC derived from dual frequency GPS data is corrupted by the instrumental biases of the GPS receiver and satellites. The estimation of receiver instrumental bias is particularly important for obtaining accurate estimates of ionospheric delay. In this paper, two prominent techniques based on Kalman filter and Self-Calibration Of pseudo Range Error (SCORE) algorithm are used for estimation of instrumental biases. The estimated instrumental bias and TEC results for the GPS Aided Geo Augmented Navigation (GAGAN) station at Hyderabad (78.47°E, 17.45°N), India are presented.

How can GOCE and TerraSAR-X contribute to the topside ionosphere and plasmasphere research?

In this study we analyze concurrent observations derived from two low-Earth orbit satellite missions—the GOCE (Gravity field and steady state Ocean Circulation Explorer) satellite with unprecedented low orbit of 250 km and the TerraSAR-X satellite with orbit altitude of 515 km. The science payloads of both satellites have no instruments for ionospheric research, but they include a dual-frequency GPS (Global Positioning System) receiver. GPS measurements from the precise orbit determination GPS antenna on board these satellites can be used to determine TEC (total electron content) at the topside ionosphere-plasmasphere system for different conditions. Results derived from these missions are presented for the first time. The comparison was done for June and December solstice conditions at low and moderate solar activity levels for quiet geomagnetic activity. We obtain the quantitative estimates of electron content and its percentage contribution to the ground-based GPS TEC for topside ionosphere/plasmasphere at altitudes 250–500 km and above 500 km. Similarities and differences in such electron content distribution on a global scale are discussed. Distinctive features of the Weddell Sea Anomaly and equatorial ionization anomaly are evident in the topside TEC derived from GOCE GPS data. We report the Weddell Sea Anomaly presence in TEC at early morning local time.

TEC variations during geomagnetic storm/substorm with Pc5/PI2 pulsation signature

The electron density integral along the paths between a GPS satellite and receiver is known as Total Electron Content (TEC), and this parameter is used in studying the ionosphere behaviors. TEC can be obtained by means of many methods. A space-based radio navigation system, such as Global Positioning System (GPS), offers good opportunities for studying the ionosphere. The TEC is calculated from the group path delay and phase advance in GPS satellite signals along the slant paths connecting GPS receivers and satellites at 22,000km. Locally, a dual frequency GPS receiver was installed in Helwan, Egypt (29.86°N, 31.32°E) in November 2009. Here, GPS data were analyzed to establish a daily observation of Vertical TEC in a region located near to the northern crest of the ionospheric equatorial anomaly. During a moderate geomagnetic storm, observed on 02–05 May 2010, a number of ionospheric/magnetic phenomena were observed. Also, observations for Pc5/Pi2 pulsations were recorded during the geomagnetic storm phases. These geomagnetic observations are taken from MAGDAS-magnetometer station, located at Aswan (23.59°N, 32.51°E). More than 10 TECU increase in the ionospheric TEC values were recorded during the daytime of 02 May, followed by a large reduction during 03 May, reference to the pre-storm conditions. This confirms the enhancement in the geomagnetic H-component peak during the storm’s initial phase and its reduction during the main phase.

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 hybrid LS-HE and LS-SVM model to predict time series of precipitable water vapor derived from GPS measurements

Precipitable water vapor (PWV) is one of the key parameters for the analysis of global climate systems, formation of clouds, and short-term forecasts of precipitation. Since water vapor is highly variable both spatially and temporally, it can be problematic, requiring modeling and efficient techniques in order to measure its quantity. Global Positioning System (GPS) stations provide total water column at high quality under all weather conditions with high temporal resolution. The technique used in this work is Precise Point Positioning (PPP) for single dual-frequency GPS receiver aided by precise orbit and clock product. Zenith wet delay (ZWD) is estimated through a stochastic process called random walk at 5-min intervals. The seasonal and diurnal variations of PWV for four GPS stations located in Europe are investigated. Latitude and station height are the most influential factors for the amount of water vapor when we compare their amplitudes and offsets to NASA Water Vapor Dataset-M (NVAP-M) and station heights. The area-averaged accumulated rain time series retrieved from Tropical Rainfall Measuring Mission (TRMM) product also shows that high PWV levels do not necessarily lead to rainfall. The amplitudes of diurnal and semi-diurnal variations are found to be of much lower amount than those of seasonal variations. In order to validate the capability of GPS-sensed PWV measurements, two episodes in wintertime and summertime are simulated using Weather Research Forecast (WRF) model. Both simulated PWV and observational PWV are consistent with each other. The main objective of this paper is to model and forecast the PWV time series which would be useful information on climatology and meteorology. Due to the distinct harmonic characteristics of PWV time series, the least squares harmonic estimation (LS-HE) is applied to time series of PWV derived from 4-year GPS station observations. Subsequently, least squares support vector machine (LS-SVM) optimized by cross-validation strategy is used to model non-harmonic components of the signal. The underlying motivation for using LS-SVM is the proficiency of this methodology to precisely model time series data when the underlying model is usually non-stationary, not defined a priori, and non-linear which are major characteristics of PWV data. The modeled time series shows that the hybrid approach (LS-HE and LS-SVM) can efficiently filter white noise in the observations and then perform forecasting. The bias (∼0.37 mm) and standard deviation (∼3 mm) of observed and predicted values presents the sound capability of the proposed model.

Simplistic approach for water vapour sensing using a standalone global positioning system receiver

Precipitable water vapour (PWV) is an important input for numerical weather prediction model, meteorology and high-precision navigational applications. Conventional methods for the determination of PWV using radiosonde are not sufficient owing to poor temporal resolution, whereas radiometer-derived PWV is reliable only in fair weather conditions. Global positioning system (GPS) is a very useful and cost-effective tool to determine PWV continuously in all weather conditions. The processing of GPS data to extract the PWV information is, however, very complicated due to very small effect of the PWV (~0.5% of total delay) on GPS frequencies than other sources of delay and errors and requires a network of GPS in differential configuration for such purpose. The authors show how the problem can be handled in a standalone dual-frequency GPS receiver in a relatively less complicated manner with reasonable accuracy. The performances of different dry tropospheric delay models are also investigated. The methodology is tested with GPS measurements at Kolkata (22.57°N, 88.37°E) and Bangalore (13.01°N, 77.5°E). The results indicate that the proposed methodology can be implemented for PWV estimation using single GPS receiver with satisfactory performance.

IRI-vTEC versus GPS-vTEC for Nigerian SCINDA GPS stations

Following the recent proliferation of dual-frequency GPS (Global Positioning System) receiver systems across the African continent, there is a growing number of papers that compare vertical Total Electron Content (vTEC) values derived from the International Reference Ionosphere (IRI) model with those obtained from the GPS receiver measurements. In this work we report an investigation of IRI-vTEC versus GPS-vTEC comparisons for three Nigerian SCINDAGPS stations (Nsukka, Ilorin, and Lagos) for which data are available in the year 2012, and present a further review of the differences/similarities observed between them. Since a major interest in this work is to use the GPS measurements to improve the predictions of the IRI model for the region, we present a detailed regression analysis of differences between the two sources in a manner that will benefit this application.

Validation of IRI-2012 TEC model over Ethiopia during solar minimum (2009) and solar maximum (2013) phases

This paper investigates the capacity of the latest version of the International Reference Ionosphere (IRI-2012) model in predicting the vertical Total Electron Content (vTEC) over Ethiopian regions during solar minimum (2009) and solar maximum (2013) phases. This has been carried out by comparing the IRI-2012 modeled and experimental vTEC inferred from eight ground based dual frequency GPS (Global Positioning System) receivers installed recently at different regions of the country. In this work, the diurnal, monthly and seasonal variation in the measured vTEC have been analyzed and compared with the IRI-2012 modeled vTEC. During the solar minimum phase, the lowest and highest diurnal peak of the experimental vTEC are observed in July and October, respectively. In general, the diurnal variability of vTEC has shown minimum values around 0300UT (0600LT) and maximum values between around 1000 and 1300UT (1300 and 1600LT) during both solar activity phases. Moreover, the maximum and minimum monthly and seasonal mean hourly vTEC values are observed in October and July and in the March equinox and June solstice, respectively. It is also shown that the IRI-2012-model better predicts the diurnal vTEC in the time interval of about 0000–0300UT (0300–0600LT) during the solar minimum phase. However, the model generally overestimates the diurnal vTEC except in the time interval of about 0900–1500UT (1200–1800LT) during the solar maximum phase. The overall result of this work shows that the diurnal vTEC prediction performance of the model is generally better during the solar minimum phase than during solar maximum phase. Regarding the monthly and seasonal prediction capacity of the model, there is a good agreement between the modeled and measured monthly and seasonal mean hourly vTEC values in January and December solstice, respectively. Another result of the work depicts that unlike the GPS–TEC the IRI-2012 TEC does not respond to the effect resulted from geomagnetic storms.

Estimation Of Global Positioning System Measurement Errors For GAGAN Applications

The Global Positioning System (GPS) has been in use for providing positioning, navigation and timing (PNT) services in many parts of the world. There are several errors that affect the positional accuracy of GPS. Prominent among them are ephemeris errors, satellite and receiver clock errors, multipath errors, signal propagation errors such as ionospheric delay, tropospheric delay, and instrumental biases of the satellite and receiver. In this paper, prominent estimation techniques to characterize various GPS measurement errors are reviewed. The GPS data in the Receiver INdependent EXchange (RINEX) format obtained from a Dual frequency GPS receiver is used in this analysis. Among all the errors, ionospheric delay is found to be the most dominant. However, these delay measurements are affected by the satellite and receiver instrumental biases. The instrumental biases exist as the signals at the two GPS frequencies experience different delays inside the satellite and receiver hardware. For estimation of the instrumental biases Extended Kalman filter(EKF) technique is adopted. The user equivalent range error (UERE) obtained due to all the error sources is of the order of few metres. After accounting for various errors, the estimation accuracy is significantly improved.The positional accuracy of the GPS Aided GEO Augmented Navigation (GAGAN) system is basically dependent on ranging errors and the satellite constellation geometry(1). The lineof-sight ionospheric measurements derived from the Global Positioning System (GPS) observables are corrupted by the instrumental biases present in both the GPS satellites and the receivers. The instrumental bias and Total Electron Content (TEC) results (Hyderabad GAGAN station (78.47°E, 17.45°N)) obtained using theExtended Kalman filter technique are presented in this paper. In this work the main focus is towards development of an improved error free approach for aircraft navigation. The existing navigation solutions are based on iterative algorithms and require an initial estimate of the receiver position for computing the navigation solution which is not possible in all conditions. Accurate, reliable and cost effective navigation solution algorithms that suit the Indian geographic area and area of application are proposed in this work. A non-iterative point solution approach algorithm computes the navigation solution based on the available satellite positions and pseudorange measurements(3). The improvement in GPS system has given a feasibility of automated operation of satellite based augmenting system in aircraft navigation can augment GAGAN and improves the positional accuracy of the user. In this paper a communication approach , high, accurate processing using EKF scheme is proposed for GPS aided geo augmented navigation (GAGAN) application. The effectiveness of the proposed approach for higher performance is evaluated under variable conditions.

Satellite Clock Error and Orbital Solution Error Estimation for Precise Navigation Applications

Global Positioning System (GPS) is a satellite-based navigation system that provides a three-dimensional user position (x,y,z), velocity and time anywhere on or above the earth surface. The satellite-based position accuracy is affected by several factors such as satellite clock error, propagation path delays and receiver noise due to which the GPS does not meet the requirements of critical navigation applications such as missile navigation and category I/II/III aircraft landings. This paper emphasizes on modelling the satellite clock error and orbital solution (satellite position) error considering the signal emission time. The transmission time sent by each satellite in broadcast ephemerides is not accurate. This has to be corrected in order to obtain correct satellite position and in turn a precise receiver position. Signal transmission time or broadcast time from satellite antenna phase center is computed at the receiver using several parameters such as signal reception time, propagation time, pseudorange observed and satellite clock error correction parameters. This corrected time of transmission and broadcast orbital parameters are used for estimation of the orbital solution. The estimated orbital solution was validated with the precise ephemerides which are estimated by Jet Propulsion Laboratory (JPL), USA. The errors are estimated for a typical day data collected on 11th March 2011 from dual frequency GPS receiver located at Department of Electronics and Communication Engineering, Andhra University College of Engineering, Visakhapatnam (17.73°N/83.319°E).