Performance evaluation of IRI-2007 at equatorial latitudes and its Matlab version for GNSS applications

International Reference Ionosphere (IRI) model is the widely used empirical model for ionospheric predictions, especially TEC which is an important parameter for radio navigation and communication. The Fortran based IRI-2007 does not support real-time interactive visualization and debugging. Therefore, the source code is converted into Matlab and is validated for the purposes of this study. This facilitates easy representation of results and for near real-time implementation of IRI in the applications including spacecraft launching, now casting, pseudolite based navigation systems etc. In addition, the vertical delay results over the equatorial region derived from IRI and GPS data of three IGS stations namely Libreville (Garbon, Africa), Brasilia (Brazil, South America) and Hyderabad (India, Asia) are compared. As the IRI model does not account for plasmasphere TEC, the vertical delays are underestimated compared to vertical delays of GPS signals. Therefore, the model should be modified accordingly for precise TEC estimation.     

Quality management for space systems in ISRO

In a little over four decades, the Indian Space Program has carved a niche for itself with the unique application driven program oriented towards National development. The end-to-end capability approach of the space projects in the country call for innovative practices and procedures in assuring the quality and reliability of space systems. The System Reliability (SR) efforts initiated at the start of the projects continue during the entire life cycle of the project encompassing design, development, realisation, assembly, testing and integration and during launch. Even after the launch, SR groups participate in the on-orbit evaluation of transponders in communication satellites and camera systems in remote sensing satellites. SR groups play a major role in identification, evaluation and inculcating quality practices in work centres involved in the fabrication of mechanical, electronics and propulsion systems required for Indian Space Research Organization's (ISRO's) launch vehicle and spacecraft projects. Also the reliability analysis activities like prediction, assessment and demonstration as well as de-rating analysis, Failure Mode Effects and Criticality Analysis (FMECA) and worst-case analysis are carried out by SR groups during various stages of project realisation. These activities provide the basis for project management to take appropriate techno-managerial decisions to ensure that the required reliability goals are met. Extensive test facilities catering to the needs of the space program has been set up. A system for consolidating the experience and expertise gained for issue of standards called product assurance specifications to be used in all ISRO centres has also been established.     

Signal Reconstruction from the Output Phases and Magnitudes of Two-Dimensional Filters

Algorithms are presented for reconstruction of a two-dimensional complex signal by knowing the phases and magnitudes of different filtered versions of the signal for a special class of filters. The algorithms have simple geometric interpretation and are easily extended to N-dimensional systems.     

GPS navigation with increased immunity to modeling errors

A robust extended Kalman filter (EKF) is presented for GPS navigation with outlying errors in the GPS measurements due to failed satellites or unmodeled errors. In this approach, the innovation step of the conventional EKF is modified using the robust statistics concept to obtain a more accurate state estimate in the presence of outlying errors. Performance of the proposed robust EKF in comparison with the conventional EKF, and adaptive EKF is demonstrated through simulation results.     

Analysis of parallel resonant converter operating above resonance

A high-frequency DC-DC converter using a parallel resonant inverter operating above resonance is analyzed using a state-space approach and a constant current model. Closed-form solutions are obtained under steady-state conditions. The analysis shows that with some changes most equations are of the same form as those corresponding to operations below resonance. The analysis is also used to obtain the design curves and to provide a simple design procedure, which is illustrated by a design example. Experimental results obtained with a prototype MOSFET unit (using metal-oxide-semiconductor field-effect transistors as switches and operating above 90 kHz) are presented to support the theory     

Photoionization balance and ion composition model in the equatorial D-region over Thumba

Positive and negative ion profiles, together with the λ-profile (ratio of negative ions to electrons), have been derived for Thumba (8°54′N) for noon conditions and for the height range 50–90 km, using the latest available input parameters.     

Relative importance of solar X-rays below 10 Å and pulsed HF radiowave absorption at 2.132 MHz frequency over Delhi under solar flare conditions

The computed photoionization rates in the discrete wavelength bands in the XUV spectra under quiet sun, moderate flare, strong flare and outstanding flare conditions have been presented graphically as a function of altitude in the height range 50 – 90 km to show the relative importance of solar X-rays below 10 Å. Ionospheric absorption data, measured by A₁ technique at the Ionosphere Research Centre at the University of Delhi, Delhi (28.6°N) at a single radio wave frequency of 2.132 MHz have been analyzed under solar flare conditions, with a special emphasis on a solar flare event on 3 February, 1983 which started at 1115 IST, reached its peak at 1140 IST and persisted till 1400 IST, during which time the radio wave absorption exceeded 60 or 65 dB for nearly three and half hours. During solar flare, a direct correlation between large enhancements of solar X-ray flux and ionospheric absorption is observed.     

Analysis of a parallel resonant converter with secondary-sideresonance

A high-frequency (HF) link parallel resonant DC/DC converter operating in the lagging power factor mode with the resonating capacitor on the secondary side of the HF transformer is analyzed using a state-space approach. Closed-form solutions (except for the duration of diode conduction) are obtained for steady-state conditions, and design curves are obtained. A method of obtaining optimum operating point under certain constraints is developed and is used as the basis of a simple design procedure. A theoretical study comparing the performance of three MOSFET-based 1-kW converters with different transformer turn ratios under load changes from rated-load to 10% load is carried out. Experimental results obtained with these converters with different transformer turn ratios are also presented     

New approach for suppression of FM jamming in GPS receivers

A new approach is proposed for the suppression of FM jammers in C/A code GPS receivers. This approach is based on the cascading of the augmented-state approximate conditional mean (ASACM) filter and the discrete wavelet transform (DWT) filter. An ASACM filter for single interference suppression in spread-spectrum systems has been reported in the recent literature. However, the formulation of the ASACM filter and its performance analysis is lacking in the case of suppression of multiple FM interferences. The ASACM filter is formulated here for the suppression of multiple FM jammers. Further, a DWT filter using biorthogonal wavelet is suggested for the suppression of FM jammers. Finally, these two filters are cascaded to get an optimum performance for higher jammer to signal ratio, which is of importance in the GPS, for the case of suppression of multiple FM jammers. The performance of the proposed filters is analyzed through simulation examples for the suppression of single and multiple FM jammers in GPS receivers