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Ionospheric scintillation can cause severe degradation in the GNSS services, particularly at the polar and equatorial regions, by deteriorating the positioning performance in terms of accuracy. The continuity of the GNSS service is also affected as severe scintillation may lead to the loss of lock of the receiver. In this work, the occurrence probability of amplitude scintillation index S4 has been statistically related to the ionospheric irregularity index of ROTI. Simultaneously measured S4 and TEC data from the GAGAN TEC network stations, installed across India, were used for the purpose. TEC is used for the derivation of ROTI. The distribution of the S4 observed over different finite ranges in ROTI was separately matched with standard parametric models. This was done for both quiet and disturbed geomagnetic conditions. Log-normal distribution has been found to be the best match for most of the cases. The parameters for the matched log-normal distributions were found to vary systematically with the mean ROTI values in the range. Therefore, these distribution parameters were modelled in the terms of the corresponding ROTI values. The derived model was successfully validated with independent data and the conformity was established using statistical methods. This work may find usefulness in estimating the probable scintillation strength S4 once the ionospheric irregularity is known in terms of ROTI. Moreover, given an observed value of ROTI, the probability of losing receiver lock can be determined, provided the corresponding threshold S4 that the receiver can withstand is known. [ABSTRACT FROM AUTHOR]

Abstract The vertical ionospheric TEC values obtained from GAGAN grid based ionospheric delay correction values over the sea in the Indian equatorial region have been compared with the corresponding values derived from the International Reference Ionosphere model, IRI-2016. The objective of this work is to study the deviation of the vertical TEC derived from the IRI model from ground truths over the sea for different conditions. This will serve the basic intention of assessing the candidature of the IRI model as an alternative ionospheric correction model in navigation receivers in terms of accuracy. We have chosen different solar activity periods, seasons, geomagnetic conditions, locations etc. for our comparison and analysis. The TEC values by the IRI-2016 were compared with the actual measured values for the given conditions and errors were obtained. The measured vertical TEC values at the ionospheric grid points were derived from the GAGAN broadcast ionospheric delay data and used as reference. The IRI model with standard internal functions was used in estimating the TEC at the same ionospheric grid points. The errors in the model derived values are statistically analysed. Broadly, the results show that, for the Indian sector over the sea, the IRI model performs better on quiet days in off equatorial regions, particularly in the northern region. The overall performance degrades for other conditions with the model generally underestimating the true TEC values and most severely in the equatorial region. The performance is worst in this region for the disturbed days of the equinoctial period. The comparison study is also done with the TEC data measured directly by dual frequency GPS receivers. The results were found to be in general agreement with those obtained by comparing the model with GAGAN broadcast data as reference. This study will be useful in considering the IRI-2016 model for real time estimates of TEC as an alternative to the current parametric model in a satellite navigation receiver in absence of other options. [ABSTRACT FROM AUTHOR]

Abstract: The ionospheric delay experienced by the satellite navigation signals depends upon the Total Electron Content (TEC) and needs to be corrected. While the single frequency receivers always use parametric models to correct this delay, dual frequency receivers, when suffers a loss of lock of one of its signal, also has to resort to these models. Here, an alternative method, based on Doppler, surrogated by range rate variation, has been attempted to estimate the ionospheric delay using a Kalman filter. GPS data have been used for all visible satellites over four days selected around the equinox and solstice with nominal geomagnetic conditions and estimations done in continuous and calibrated modes. Results of continuous estimation, obtained for a mid latitude station, showed moderate accuracy while it was significantly better for the calibrated mode with no seasonal dependence. Estimations done for station within the extent of equatorial anomaly, has not only resulted in relative deterioration in performance, but also shown seasonal dependence. Compared with estimates of Klobuchar model, the Calibrated estimation showed superior performance, conspicuously in the mid latitude station. However, for the continuous mode, performance was at par with the model at higher latitudes but inferior to it in regions within the extent of the equatorial anomaly. [Copyright &y& Elsevier]

Abstract: It is important to use models developed specifically for the equatorial ionospheric estimation for real-time applications, particularly in Satellite Navigation. This work demonstrates a methodology for improved predictions of VTEC in real time using the model developed for the equatorial ionosphere by the authors. This work has been done using TEC data of the low solar activity period of 2005 obtained using dual frequency GPS receivers installed under the GAGAN project of ISRO. For the purpose, the model is first used in conjunction with Kriging technique. Improvement in accuracy is observed when compared with the estimations from the model alone using the measurements as true reference. Further improvement is obtained by Bayesian combination of these estimates with independent Neural Network based predictions. Statistical performance of improvement is provided. An improvement of ∼1m in confidence level of estimation of VTEC is obtained. [Copyright &y& Elsevier]

Abstract: Increasing demand of bandwidth in communication satellites has forced satellite links to be designed in Ku bands and above. But at these frequencies, rain and other tropospheric elements result in large attenuation. To mitigate the tropospheric attenuation of microwave satellite signals above 10GHz using any standard Fade Mitigation Technique (FMT), it is essential to have a priori knowledge about the level of attenuation. Hence, short-term rain attenuation prediction models play a key role in maintaining the link in which necessary compensation can be applied depending on the early information of attenuation. This paper presents a method of attenuation prediction using Adaptive Artificial Neural Network. Here In situ Learning Algorithm (ILA) has been used to enable the system to track the non-stationary nature of the attenuation. To validate this, Ku Band data, collected at three different sites in India have been used for the purpose of prediction. The performance of the algorithm is determined through the estimation of prediction accuracy by comparing the predicted values with the measured data. Results obtained using the mentioned technique shows considerably good accuracy even up to 20s of prediction interval with acceptable ratio between the under and over predictions. The prediction performance is evaluated for different prediction intervals. Furthermore the present model is also compared with the persistence model and the relative performance is quantified. [Copyright &y& Elsevier]

Abstract: The temporal variation of the equatorial electrojet is estimated utilising a suitably designed Kalman filter and using the established empirical relations between the anomaly component of equatorial TEC and the modified electrojet. TEC data obtained from dual frequency GPS receivers are used for the purpose. Estimation requires the a-priori knowledge of the peak electrojet value of the day and hence can be made in post temporal scenario only. Initial results obtained during a low solar activity time in an equinoctial month shows acceptable accuracy of the proposed algorithm. Limited analysis is done by segregating the results into temporal sessions of pre-attainment and post-attainment of the electrojet peak. [Copyright &y& Elsevier]

Abstract: The Ionospheric Total Electron Content is responsible for the group delay of the signals from the Navigation satellites. This delay causes ranging error, which in turn degrades the accuracy of position estimated by the receivers. For critical applications, single frequency receivers resort to Satellite Based Augmentation Systems in order to have improved accuracy and integrity. The performance of these systems in terms of accuracy can be improved if predictions of the delays are available simultaneously with real measurements. This paper attempts to predict the Total Electron Content using adaptive recurrent Neural Network at three different locations of India. These locations are selected at the magnetic equator, at the equatorial anomaly crest and outside the anomaly range, respectively. In-situ Learning Algorithm has been used for tracking the non-stationary nature of the variation. Prediction is done for different prediction intervals. It is observed that, for each case, the mean and root mean square values of prediction errors remain small enough for all practical applications. Analysis of Variance is also done on the results. [Copyright &y& Elsevier]

• Study of Ionospheric scintillations indices S4 and ROTI in Indian region. • Relation between ROTI & S4 is useful for standard GNSS receivers to estimate S4. • Correlation of ROTI & S4 is better than 0.6, yet an estimate of S4 is unreliable. • Using obtained relation, a range of S4 values can be estimated from ROTI value. Ionospheric irregularities cause fluctuations (scintillations) in received satellite signal strength, degrading communication and navigation signals. The scintillation strength depends on the scale sizes of the irregularities and the frequency of the radio signal. The parameter S4 index describes the scintillation strength derived from the amplitude of high rate received signal sampled at 50 Hz, usually from special GNSS receivers. Most GNSS receivers have a lower data sampling rate, so an alternate parameter, Rate of Change of TEC Index (ROTI), is derived from the total electron content (TEC) sampled at 30 s. Here, the correlation between ROTI and S4 is performed for the Indian ionospheric region, with different elevation mask angles and ROTI calculated over different averaging intervals. An estimate of the S4 value from ROTI is unreliable even though the correlation between them is better than 0.6. Different elevation mask angles and ROTI with varying averaging intervals do not significantly affect the correlation between S4 and ROTI. Further, the correlation and occurrence percentages of S4 corresponding to ROTI are analyzed. An 80% confidence range is obtained to correlate the observed ROTI with the corresponding S4 index. These probability estimates of S4 values derived from ROTI values also vary with latitude. This relation between ROTI and S4 will be beneficial in using standard GNSS receivers for the studies of ionospheric scintillations. [ABSTRACT FROM AUTHOR]

• A detail study of rain fade at Ka band over tropical region has been done. • Performance comparison of experimental link at 20/30 GHz with ITU-R model done. • "s" parameter relating fade slope to filter characteristics is calculated. Dynamic characteristics of Ka band rain fade (or rain attenuation) over tropical region are relatively less studied, though they are essential for implementation of suitable fade mitigation techniques. In this paper, dynamic characteristics of rain fade at 20 GHz and 30 GHz from GSAT-14 satellite are reported with experimental measurements at Ahmedabad, India. ITU-R model for fade slope probability distribution has been analyzed in this paper and Ka band rain fade has been investigated based on this fade-slope analysis. The ITU-R model is found to closely match 20 GHz measurements, though differences increase at 30 GHz. Fade slopes are found to be within ±0.5 dB/s at both 20 and 30 GHz. Both positive and negative slopes of the fade slope distributions are quasi-similar. ITU-R model's s parameter, relating fade slope to filter characteristics, is found to be very close to the measured value for this region. Distributions of fade slopes are also found to be dependent on the standard deviation values similar to ITU-R recommendation at 20 GHz, but significant deviation is observed at 30 GHz. This observed frequency dependency might be due to increase in cloud attenuation at high frequencies or may be due to limited data set. Results indicate needs of further experimental data at high frequencies from other tropical regions to gain further insight into the fade slope characteristics. This will ultimately help in designing a better prediction model in future. [ABSTRACT FROM AUTHOR]