Indian Journal of Radio & Space Physics

(CODEN : IJRSAK    ISSN : 0367-8393)

 

VOLUME 31

NUMBER 5

OCTOBER 2002

 

CONTENTS

 

Short period oscillations in mesosphere

237

H Chandra, S Kovalam & R A Vincent

 

 

 

Seasonal variation of horizontal winds and momentum fluxes from 53 MHz VHF radar observations over Gadanki, a tropical station in India

 

250

B Vasantha, D Narayana Rao, S Vijaya Bhaskar Rao, M Venkat Ratnam,

K Mohan & S Kamala

 

 

 

Radio visibility at microwave frequencies

264

P K Karmakar, M Maiti & A Mitra

 

 

 

Chemical composition of size-separated aerosols at two rural locations in the

  Himalayan region

 

270

P D Safai, P S P Rao, G A Momin, K Ali, S Tiwari , M S Naik & J C Kuniyal

 

 

 

An adaptive polynomial path loss model at UHF frequencies for mobile railway communications

 

278

K Ravindra, A D Sarma & M V S N Prasad

 

 

 

Emission characteristics of dry and wet loamy sand soil layered pack at microwave frequencies

 

285

O P N Calla & R K Singh

 

 

 

 

 

Indian Journal of Radio & Space Physics

Vol. 31, October 2002, pp. 237-249

 

 

Short period oscillations in mesosphere

 

H Chandra

Physical Research Laboratory, Ahmedabad 380 009

and

S Kovalam & R A Vincent

Department of Physics, University of Adelaide, S Australia 5005

Received 8 November 2001; revised 2 May 2002; accepted 17 June 2002

Long sequences of the hourly average zonal and meridional wind components obtained by MF spaced antenna radars at Davis (69oS, 78oE), Buckland Park (34oS, 138oE) and Christmas Island (2oN, 157oW) are spectrally analyzed to study the short period oscillations in mesospheric winds. Data lengths covering a minimum of four years at each of the stations, five altitudes used for averaging (84-92 km) and the windows of 10, 20 and 40 days used in estimating power spectra give high reliability and spectral resolution. Apart from the tidal components at 24, 12, 8 and 6 h there are oscillations with periods ranging between 4.8 h and 12 h. Moving power spectra are obtained to study the seasonal dependence of the short period (smaller than 12 h) oscillations. The wave activity is strongest during June solstice and weakest during equinoxes at each of the locations. The presence of similar features at high, mid and low latitudes indicates the global nature of such oscillations. The seasonal dependence of the wave activity is in agreement with the recently reported observations of waves with periods between 7.5 and 10.5 h seen in the meteor radar data at south pole. Oscillations with periods of 4.9-5.2 h and 7.2-7.5 h are also seen.

 

Indian Journal of Radio & Space Physics

Vol. 31, October 2002, pp. 250-263

 

 

Seasonal variation of horizontal winds and momentum fluxes from 53 MHz VHF radar observations over Gadanki, a tropical station in India

B Vasantha, D Narayana Rao, S Vijaya Bhaskar Rao, M Venkat Ratnam, K Mohan & S Kamala

Department of Physics, S V University, Tirupati 517 502

Received 14 August 2001; revised 27 May 2002; accepted 25 June 2002

Diurnal, monthly and seasonal variation of horizontal winds and momentum fluxes in troposphere and lower stratosphere are presented in this study. Data collected using Indian MST radar for a period of three years (September 1995- August 1998) are used for the present study. The observed mean zonal wind is westward (easterly jet) with a maximum of
~ 42 ms
-1 around 16 km during the monsoon season. The maximum mean meridional wind is ~5 ms-1 and is northward during winter and 3 ms-1 in the altitude range of 10-14 km during summer. The maximum zonal variances are ~ 40 m2s-2 at an altitude of 16 km during monsoon season. The meridional wind variances are of 9 m2s-2 at ~15 km during winter. The maximum vertical shears of zonal winds occur at altitudes of 12 and 18 km during the monsoon and post-monsoon seasons, respectively. The vertical shears of meridional winds are maximum at an altitudes of 11 and 15 km during winter season. In summer, the maximum vertical flux of eastward momentum is about 1.0 m2s-2 around 11 km, and westward momentum flux of 0.3 m2s-2 is observed around 14.5 km during post-monsoon. The diurnal variations show different values on individual days.

 

 

Indian Journal of Radio & Space Physics

Vol. 31, October 2002, pp. 264-269

 

 

Radio visibility at microwave frequencies

 

P K Karmakar, M Maiti & A Mitra

Institute of Radiophysics and Electronics, University of Calcutta, 92, A P C Road, Calcutta 700 009

Received 6 July 2001; revised 30 January 2002; accepted 6 March 2002

Radio visibility through water vapour has been studied over Calcutta at microwave/ millimetre wave frequencies. Due to large abundance of water vapour over Calcutta, the integrated water vapour content restricts the radio visibility approximately up to 5 km at different microwave/millimetre wave frequencies. The present study has been restricted to only the total absorption (dB) part of the propagation parameters of radio waves. The vertical layers of atmosphere have been divided into 10 m slabs with the assumption that the meteorological parameters remain unchanged quantitatively within any of these slabs. The height at which the variation of total absorption (dB) is less than or equal to 1% of that of the immediate preceding height (which is one slab thickness less), is, here, defined as 1% height limit. It has been found that beyond this height the radio receivers at microwave/millimetre wave frequency suffer no appreciable change in the total absorption.

 

Indian Journal of Radio & Space Physics

Vol. 30, October 2002, pp. 270-277

 

 

Chemical composition of size-separated aerosols at two rural locations in the Himalayan region

P D Safai, P S P Rao, G A Momin, K Ali, S Tiwari & M S Naik

Indian Institute of Tropical Meteorology, Pune 411 008

and

J C Kuniyal

G B Pant Institute of Himalayan Environment and Development, Kullu 175 126

Received 19 February 2002; revised 20 May 2002; accepted 28 August 2002

Size-separated atmospheric aerosols were collected at Kothi (a hill top) and Mohal (a valley), situated at the foothills of western Himalayas during the summer season of 1999. Bimodal distribution of aerosols was observed at both the locations. However, at Kothi, fine size particles dominated (62%), whereas at Mohal, coarse size particles contributed more (75%). The SO4 and NO3 particles together contributed ~ 30% of the total measured chemical composition of aerosols. Burning of biomass and emissions from tourist vehicles could be the main local sources for these components. However, aerosols showed alkaline nature (S-S+ ratio < 1) due to the neutralizing effect of some cations such as Ca and NH4.

Indian Journal of Radio & Space Physics

Vol. 31, October 2002, pp. 278-284

 

 

An adaptive polynomial path loss model at UHF frequencies for mobile railway communications

K Ravindra & A D Sarma

R & T Unit for Navigational Electronics, Osmania University, Hyderabad 500 007

and

M V S N Prasad

Radio & Atmospheric Science Division, National Physical Laboratory, New Delhi 110 012

Received 22 November 2001; revised 16 April 2002; accepted 10 July 2002

Study of channel behaviour for efficient spectral usage has been the key aspect in the design of third generation (3G) mobile communications systems. This paper deals with modelling of UHF signals acquired in a moving train corresponding to a distance of 180 km in northern India starting from New Delhi to Sharanpur, covering various environmental zones. A new sixth order adaptive polynomial model is proposed to further model the deviation of earlier adaptive three coefficient model. The standard deviations of the predictions made by the proposed model from the experimental data are within 2.1 dB throughout the region of experimentation in contrast to a maximum of 24 dB reported earlier.

 

Indian Journal of Radio & Space Physics

Vol. 31, October 2002, pp. 285-292

 

Emission characteristics of dry and wet loamy sand soil layered pack
at microwave frequencies

O P N Calla

International Centre for Radio Science, "OM-NIWAS", A-23 Shastri Nagar, Jodhpur 342 003

and

R K Singh

Sanjay Gandhi Colony, Hanuman Nagar, Kankar Bagh, Patna 800 020

Received 4 January 2001; revised 10 December 2001; accepted 23 April 2002

The emissivity estimated at different moisture content and at different frequencies is reported for a typical loamy sand soil. It is observed that the emissivity of soil pack depends on frequency as well as on moisture content and the depth of layer at which moisture is present. The estimated emissivity also shows dependence on look angle and polarization. The estimated values of emissivity can be used for designing passive remote sensing instruments.