Structure of the equatorial lower ionosphere from the Thumba Langmuir probe experiments

A Langmuir probe designed and developed at the Physical Research Laboratory, Ahmedabad has been used on a variety of rockets since 1966 from the Thumba Equatorial Rocket Launching Station, TERLS (8°31'N, 76°52'E, dip.lat. 0°47'S) to study the structure of the equatorial lower ionosphere. Good quality data is available from a set of twenty five rocket flights conducted during the period 1966 to 1978. This data has been obtained using a single standardised instrument at a single location and using a uniform procedure for data reading and analysis, and adopting a calibration procedure to convert the measured probe currents into electron densities which involves a height dependent calibration factor. The data has been used to establish the gross features of the equatorial lower ionosphere under daytime, night time, morning twilight and evening twilight periods.     

Parameterization of surface temperature retrieval for remote sensing experiments

A simple parameterization has been developed for determining the actual surface temperature from the effective brightness temperature measured radiometrically in the 11 μm window region. This algorithm allows the computation of atmospheric correction without performing detailed radiative transfer calculations. Correction due to atmospheric water vapor is represented in terms of the integrated water vapor burden. Correction due to variation of surface emittance is represented in terms of its deviation from unity. Parameteric representation has also been developed for simultaneous variation of both parameters. The parameterization is based on model calculations performed with a line-by-line radiative transfer program. Sensitivity of the retrieved surface temperature to uncertainties of water vapor burden and surface emittance have also been examined.     

Role of neutral winds in generating irregularities in equatorial F-region

Electron density and neutral wind velocity measurements were carried out by rocketborne probes from rocket ranges in India. The experiments were carried out at the time of the onset of spread-F at sunset hours. The results show that a neutral wind velocity in north-south direction greater than 100 m/sec is required to trigger spread-F. It is suggested that spread-F is generated by the interaction of neutral gas with ionospheric plasma.     

An algorithm for estimation and separation of ephemeris and clock errors in SBAS

The estimation and separation of ephemeris and clock errors is an integral part of a SBAS (Space Based Augmentation System). Generally, the global solution is based on the full state approach for satellite errors (ephemeris and clock) and station errors, using a large least square estimator; or the other way is to sequentially estimate the ephemeris and clock through a Kalman filter, using a complex model of the satellite dynamics. In this paper, the estimation and separation of ephemeris and clock errors is addressed through a unique approach of combining both the methods. The algorithm employs measurements, which are pre-processed for various errors and known biases. A single difference technique is used to separately estimate the ephemeris and clock components. The ephemeris Kalman filter uses a priori information of ephemeris errors along with measurements through a minimum variance estimator to provide ephemeris error estimate. A similar approach is adopted in the clock error estimation process, to provide clock and clock rate estimates. The algorithm results are presented using simulated data for known errors in ephemeris/clock and subsequent retrieval. This algorithm estimates these errors as corrections to the broadcast Global Positioning System (GPS) navigation data, required by a SBAS user for accuracy improvement.     

An Adaptive Threshold System for Nonstationary Noise Backgrounds

The problem of detecting target signals in an ocean environment using active sonar is complicated by the nonstationary background which usually consists of both ambient ocean noise and reverberation. on. In this paper a signal processing system capable of detecting a signal in nonstationary noise is introduced. This system makes use of the mean and variance time functions of the nonstationary noise background in order to design estimation filters which will cope with the nonstationarity. Appropriate statistics of the noise and signal (tone burst) plus noise have been obtained and are used to determine the probabilities of false alarm and detection and the receiver operating characteristics.     

Importance of electric field measurement over low latitudes at stratospheric heights by balloons

The vertical field in the stratosphere around 35 km is predominantly of atmospheric origin whereas the horizontal electric field at these altitude is mainly of ionospheric origin. The electrical coupling between ionosphere and atmosphere is not known for low latitudes. Balloon borne electric field measurements are planned from Hyderabad, India (geographic latitude 17.5° N) to understand this coupling. Measurement of stratospheric electric fields are also important from the point of view of the sun-weather relationship. It si suggested that the balloon borne electric field measurements are important to understand the electrodynamics of the middle atmosphere.     

Absorption and vibrational spectra of methylamine and its ions using quantum chemical methods

We present vibrational spectra of methylamine and its ions using the MP2 method and absorption spectra using time dependent density functional method. On the basis of these calculations electronic transitions of methylamine were successfully determined. We also have derived electron affinity and ionization potential for methylamine. The ionization potential 8.98 eV from this work is in excellent agreement with the experimental value of 9.04 eV. We found that like neutral methylamine, its cation also displays strong σ → σ∗ electronic transitions in the vacuum UV region whereas its anion shows strong electronic transitions in both the vacuum and far UV regions. The detection of methylamine in astronomical objects may benefit greatly from the accurate new data on its far-UV and near-IR lines.     

Closed-Form Solutions of Target-Tracking Filters with Discrete Measurements

Using two separate algorithms, called Kalman's recursive and Vaughan's nonrecursive, exact closed-form solutions of the steady-state two-state exponentially correlated velocity (ECV) and three-state exponentially correlated acceleration (ECA) target-tracking filters are derived for discrete position measurements. For this particular application, the superiority of Vaughan's nonrecursive over Kalman's recursive algorithm is observed.     

Performance of a Modified Applebaum Adaptive Array

The performance of a modified Applebaum adaptive array is studied. The new array is obtained by removing the desired signal component from the output signal fed back to the correlator of an Applebaum type adaptive array. Various signal scenarios, including single desired signals or multiple simultaneous desired signals are examined. The new array is less sensitive to beam pointing errors and does not cause power inversion of desired signals. In the case of multiple simultaneous desired signals, the new array does not degrade the SNRs of strong desired signals.     

Variations in Electron Content Ratio and Semi-thickness Ratio during LSA and MSA periods and some Cyclone Genesis Periods using COSMIC satellite observations

In this study for the first time, COSMIC satellite data have been used to deduce values of ionospheric Electron Content Ratio (ECR) and Semi-thickness Ratio (Rtb) for Low Solar Activity (LSA) (2008) and Moderate Solar Activity (MSA) (2012) periods over the Indian low-latitude (15–30°N) region with 80–95°E longitude. These two ratios provide sensitive information about bottom and topside ionosphere for different geophysical conditions. Extraction of suspected patterns and discrepancies unfold that the deviations between ECR and Rtb values during LSA period are comparatively higher than that of MSA period when the diurnal variability in these two parameters is flatter along with the diurnal-dips during pre-noon hours. The correlative relationship of ECR exhibits low association with NmF2 and anti-correlation with HmF2, whereas its correlation with Rtb is extremely high. During Cyclone Genesis Period (CGP) strong dips in ECR and Rtb values with respect to pre and post CGP occurred which helps to take decisive conclusion about the ionospheric variations to be dominant through getting relatively higher Ne concentration in the bottom side part of the ionosphere.