Animesh Maitra

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

Abstract VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earths horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst

Rain Attenuation Modeling in the 10-100 GHz Frequency Using Drop Size Distributions for Different Climatic Zones in Tropical India

Rain drop size distributions (DSD) are measured with disdrometers at flve difierent climatic locations in the Indian tropical region. The distribution of drop size is assumed to be lognormal to model the rain attenuation in the frequency range of 10{100GHz. The rain attenuation is estimated assuming single scattering of spherical rain drops. Difierent attenuation characteristics are observed for difierent regions due to the dependency of DSD on climatic conditions. A comparison shows that signiflcant difierences between ITU-R model and DSD derived values occur at high frequency and at high rain rates for difierent regions. At frequencies below 30GHz, the ITU-R model matches well with the DSD generated values up to 30mm/h rain rate but difier above that. The results will be helpful in understanding the pattern of rain attenuation variation and designing the systems at EHF bands in the tropical region.

Improving Rain Attenuation Estimation: Modelling of Effective Path Length Using Ku-Band Measurements at a Tropical Location

Rain attenuation is an important aspect of signal propagation above 10GHz frequency. The attenuation time series generation from point rain rate measurement is crucial due to unavailability of actual signal measurements. In this paper, a simple and realistic approach has been demonstrated for better estimation of rain attenuation using Ku-band signal propagation data and ground rain rate measurements at Kolkata, India. The ITU-R model of rain attenuation has been modifled by incorporating an efiective slant path model. The efiective slant path has been estimated and modelled in terms of a power-law relationship of rain rate data of 2007{2008. The methodology has been validated with the measured data of 2006. Comparison with ITU-R and SAM clearly demonstrates the improved predictability of the proposed model at the present tropical location.

Rain-Induced Scintillations and Attenuation of Ku-Band Satellite Signals at a Tropical Location

The phenomenon of scintillations in relation to rain attenuation of Ku-band satellite signals has been studied at a tropical location. The standard deviation (σ) of scintillations increases with attenuation up to a value in the range of 6-7 dB, beyond which σ decreases with attenuation. A technique is proposed to obtain some effective values of structure constant (C_n²) of refractive-index variation from the experimental observations of σ and rain rate (R). The value of C_n² also increases with attenuation up to values in the 6-7-dB range and decreases beyond that value. The eddies in turbulent raining medium grow with rain rate, and consequently with attenuation, causing an increase in the outer scale (L_O) of turbulence and thus increasing σ until L_O reaches the size of the first Fresnel zone. In a further development, the contribution of LO toward C_n² decreases, resulting in the decrease of fast fluctuations with rain attenuation.

Rain Depolarization Measurements on Low Margin Ku-Band Satellite Signal at a Tropical Location

The phenomenon of depolarization of a Ku-band satellite signal caused by rain has been studied with a simple experimental system in which the attenuation of the copolar component and the enhancement of the cross-polar component signal have been measured at a tropical location. The degradation of cross-polarization discrimination (XPD) due to rain over the Earth space path has been obtained from these measurements and compared to that estimated from the ITU-R model, indicating the characteristic feature of a tropical location.

Multi-technique observations of convective rain events at a tropical location

The effects of the convective rain on various atmospheric parameters have been investigated at Kolkata (22.57°N, 88.37°E), India, during pre-monsoon and monsoon period of 2013. Various parameters like cloud base height and depth, liquid water content, rain rate and rain drop size distribution (DSD) are observed concurrently. The atmospheric electric field measured with an electric field monitor and the attenuation and depolarisation of satellite signals measured by a Ku-band receiving system are also studied during rain events. The instability indices obtained from radiometric measurements and the rain height profiles from micro rain radar are used to classify rain into two types, namely, convective and stratiform. The signatures of rain events on multi-technique observations are studied to indicate the various aspects of convective processes at a tropical location.

Multi-technique observations on precipitation and other related phenomena during cyclone Aila at a tropical location

A severe cyclone, named Aila, passed over Kolkata on 25 May 2009. The strong convective activities manifested through rainfall during the cyclone were studied with multi-technique observations involving micro rain radar, disdrometer, rain gauges, and a Ku-band satellite signal receiving system. A number of features of precipitation, namely the presence of large rain drops, the large vertical extent of the precipitating layer, an unusual enhancement of cross-polar component, and strong scintillations of the co-polar component of the satellite signal due to strong turbulences associated with the cyclone, were observed. This study leads to a more comprehensive understanding of the precipitation associated with a cyclonic storm.

Time series predictor of Ku band rain attenuation over an Earth–space path at a tropical location

SUMMARY A channel model is proposed to predict the time series of Ku band rain attenuation during a rain event at a tropical location. The model is based on considering the Gaussian distribution of the conditional occurrence of rain attenuation with a particular value of the attenuation occurring before. The mean (μ) and standard deviation (σ) for the distribution are modeled with the experimental data. The measured attenuation at a particular time instant is used to obtain μ and σ and to predict the attenuation after certain interval. The channel model has tested well giving the predicted attenuation that agrees with the measured value with a mean error within 15% higher than 1 dB. Validity of the model is also tested with the first-order and second-order statistics of attenuation occurrence, on a long-term basis. The method can also be applied even if attenuation measurements are missing for certain period of time with increased error. Copyright

Ionospheric electron content observations during the total solar eclipse of February 16, 1980

Abstract Observations on the Faraday rotation of a transionospheric VHF signal obtained from a network of four stations near the path of totality during the total solar eclipse of 16 February 1980 are reported. A small decrease of 3–4% in the total ionization has been obtained around the time of totality. Absence of any periodic structure following the eclipse indicates that the TIDs are not of significant amplitude in the present case to be detected by the Faraday rotation technique.

Some melting layer characteristics at two tropical locations in Indian region

Hydrometeors pose a serious threat to satellite communication operating above 10 GHz. The designing line of sight link usually is based on the ITU-R models, which are often inadequate for tropical region. ITU-R model uses a yearly averaged constant rain height for the attenuation calculation, which may not be valid for tropics. This paper reports the study of rain height based on Micro Rain Radar and Radiosonde observations at two tropical locations in Indian region. Results suggest a possible modification in the rain attenuation model taking into account the melting layer height variation with the season and rain rate.