Tropospheric Signal Enhancements Measurement & Its

Indian Journal of Radio & Space Physics Vol. 8, October & December 1979, pp. 303-305

Tropospheric Signal Enhancements Measurement & Its Dependence on Elevation Angle & Season

AB GHOSH & S K SARKAR

Radio Science Division, National Physical Laboratory, New Delhi 110012

Received 9 April 1979; revised received 31 August 1979

The records of 40 and 41 MHz beacon transmissions from INTASAT during the period Nov. 1974 to Sep. 1976 are examined for tropospheric effect. It has been found that the tropospheric effects mostly occur at elevation angles less than ISO and the most favourable elevation angle is around 5°. The comparison of satel• lite beacon signals with 7 GHz propagation measurement reveals that whenever satellite beacon at low eleva• tion angles detects tropospheric effects, microwave signal suffers deep fades (sometimes as high as 10 dB). The satellite beacon as well as microwave link measurements were also compared with refractivity gradients obtained from radiosonde measurements.

1. Introduction Elevation angle All possible observations Anomalous enhancement and/or fluctuations of analyzed and signal enhance• the amplitude of radio beacons received from satel• ments are found to predomi• lites at low elevation angles have been reported by nate at elevation angle less , several authors.1-12 More recently in this laboratory than,...., 15° detailed analyses have provided conclusive evidence 2.2 Terrain and Systems Characteristics of Microwave Link showing that satellite radio beacon technique can be (7 GHz) used to detect the tropospheric changes when the The line-of-sight microwave link is situated bet• satellite elevation angle is below 15°.]3-16The tro• ween Sonepat and Delhi over a path length of 41 km. pospheric effects detected through satellite radio The terrain consists of rolling plains and no unusual beacon technique have also been verified by micro• features exist over the entire path. The transmitting wave signal amplitude fading observations at 7 GHz end is at Sonepat (28°56'34"; 77°01'36") while the over Delhi/Sonepat path.17 Satellite beacon records receiving end is at Delhi (28°38'; 77°10'36"). The from INTASAT transmissions obtained at Delhi height of the receiving antenna is 30m while the during the period Nov. 1974 to Sep. 1976 and the height of the transmitting antenna is 90m. The corresponding microwave field strength measure• propagation experiments are conducted on 24 hr ments are presented and discussed in this paper. basis since Dec. 1974 over this link. The signal amplitude measurements are recorded in the tower 2. Path Details and Systems Characteristics of the National Physical Laboratory, New Delhi, The details of the satellite beacon system along by means of a voltmeter-type strip-chart recorder. with the terrain and microwave link characteristics The operational characteristics of the transmitting are given below. and receiving systems are as follows. 2.1 Satellite Radio Beacon System Radio frequency 7617 MHz Transmitting power 1 W The satellite beacon system, used at the National Type of modulation FM/FDM Physical Laboratory has the following characteris• tics. Receiver sensitivity -90 dBm Transmitting antenna gain 45 dB Sensitivity of the -80 dBm (AGe over 12 dB Receiving antenna gain 28 dB receiver range) Type of transmitting antenna Dish with horn feed Operating frequency 40 and 41 MHz Type of receiving antenna Dish with horn feed Antenna used Simple dipole in the horizon• tal plane with gain 3 dB. 3. Results Satellite used Orbiting sun-synchronous The changes in refractive index with position satellite,INTASAT cause rays propagating through the atmosphere to 303

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change their direction. Tropospheric effects on z

8 IJ1 10 satellite signals are greater at low elevation angles '1:1- 9 WZ 8 than at higher ones because of the increased length 00 ~ 7 WW 6 of the signal path in the air, especially in the lower eLL 5 j a 4 layers of atmosphere. The extent of elevation angles W 0 3 !t z 2 for receiving satellite signals gets further down to V1

horizon during the time when there is temperature :;;E inversion and/or high humidity lapse rate. Measure• Cl: I- -80 ments of signal's in the vhf and microwave frequency LLZ { W w -60 fi ranges using satellites or radar with aircraft or ~ ~-40 capture balloons and targets are made to study the t~~~-"" ~ -20 . . ~ JAN. FEB. MAR.APR MAY JUNE JULY AUG. SEP. OCT NOli. DEC. elevation angle dependence of signal strength.1-12 MONTHS OF YEAR

Using drift measurements from ATS-6, field strength Fig. 2 - Comparison of satellite beacon observations at low measurements of tropospheric origin at IS and elevation angles with radiosonde measurements 30 GHz showed attenuation of signals which decreas• 10 ed with increase in elevation angle. Most of the 9 • • 8 ••••-••••••••• experiments were conducted above 10 GHz and • •• ..-•.....• . hence attenuations due to rain, cloud, water vapour ---. make it difficult to study signal enhancements due to higher tropospheric refractivity gradients. Using • orbiting satellites, Hartmannl,2 reported signal strength measurements of tropospheric origin in the range 2-28° but did not discuss about the elevation angle dependence. The present work stresses on the elevation angle dependence of tropospheric enhancement at 1° ele• vation intervals. All the available satellite beacon " . • recordings during the period Nov. 1974 to Sep. 1976 ·. were examined for tropospheric effects and 58 cases - 50 -100 -150 -200 of tropospheric origin are identified. The tropo• INITIAL REFRACTIVITY GRADIENT, t.N/km spheric effects observed are signal enhancements Fig 3 - Comparison of initial refractivity gradient with (2-15 dB) and/or scintillations of the radio beacon satellite beacon measurements and microwave link fade signal amplitude. From the ephemeries of the satel• depth measurements lite INTASA T, the elevation angle of the satellite corresponding to the time of tropospheric effect is measurements made available from India Meteoro• computed. The distribution of the elevation angles logical Departmen t, Delhi. Refractivity profile is shown in the histogram (Fig. 1). informations are obtained from the India Meteoro• The atmospheric parameters, viz. temperature, logical Department where two flights are made daily, pressure and humidity, are taken from radiosonde one at 0000 hrs GMT while the other at 1200 hrs 10 GMT. Using the above parameters, the surface U tropospheric refractivity and the corresponding 0: W initial refractivity gradients are computed. Taking If• U.z the data for two years, the number of cases of tro• VlW ~L pospheric enhancements for different months and OW the corresponding average surface refractivity gra• I-Zc::u5 LL« dients from radiosonde data are plotted in Fig. 2. OIZ The amplitude measurements were made from VlW W Dec. 1974 to Dec. 1976 at 7 GHz on 24 hr basis. Vl « U The data have been analyzed in terms of hourly o median values of field strength and hourly average o 5 10 15 ELEVATION ANGLE, deg fading depthsP Considering the individual tropo• spheric cases recorded by satellite beacon, correspon• .Fig. 1 - Elevation angle dependence of tropospheric fading (Total no. of cases identified = 58; Effects north of station ding refractivity gradients and microwave link fade = 44) depth measurements are compared (Fig. 3). 30+

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OHOSH & SARKAR : STUDY OF TROPOSPHERIC SIONAL ENHANCEMENTS

4. Discussion tion of Fig. 3. From these relations, it can be From Fig. 1, it is found that the tropospheric inferred that the possibility of receiving signal from effects begin to arise as the elevation angle starts satellites at lower elevation angle is high when the decreasing from 15°. It is further found that the microwave link fade depth is high. number of events start increasing as one goes towards lower elevation angle tilI there is an appa• Acknowledgement rent peak at 5°. The downward trend of the histo• The authors are thankful to Dr Y V Somayajulu gram below 5° is because of the ground clutter and for having suggested the problem and for his muItipath scattering which makes it difficult to guidance during the progress of the work. Thanks distinguish between the signal and noise pick-ups. are also due to Dr A P Mitra and Dr B M Reddy Secondly, as the satellite goes down to lower ele• for their useful discussions in the present work. vation angle, the path traversed by the signal is References more. Hence although there is signal enhancement, 1. Hartmann G, Rep. No. 31 (I), Mittai Lungen und dem the atmospheric path loss is more and so the proba• Max Planck lnstitut fur Aeronomic, 1967. bility of receiving enhancements goes down. Detail• 2. Hartmann G, J. atmos. terr. Phys., 31 (1969),663. ed analysis on the apparent peak is under progress 3. Blomquist A, FOA-3 rep. 3516-60, Research Institute of and a quantitative interpretation will be reported in National Defence, Stockholm 80, Sweden, Nov. 1968. a subsequent paper. 4. Fengler 0 & Fengler C, Proceedings of the symposium on future applications of satellite beacon experiments, Lindau, It is seen from Fig. 2, that using radiosonde FRO, 1970, 10-1. observations, the occurrence of higher tropospheric 5. Crane R K, Proc. IEEE,59 (1971), 173. refractivity gradients is more frequent during pre• 6. Edenhofer P, Olesner D, Harnischmacher E & Stein V, monsoon period in May and June. The satelJite Space Res., 12 (1972), 1195. beacon observations while following a similar trend 7. Strickland J I, J. Recherches Atmospheriques, 5 (1974), show a significant occurrence after the monsoon. The 451. . 8. Inkster DR & Rogers R R, J. Recherches Atmospheri• present study thus shows a discrepancy from initial ques, 5 (1974), 428. refractivity gradient measurements. It is also found 9. Vickers W W & Lopez ME, Radio Sci., 5 (1975),491. that whenever there was high initial refractivity 10. Thomson P T, Colloquium on UK results/rom ATS~6 gradient, the satellite beacon signals showed enhance• mm-wave propagation experiments, 1977, 13/1. ments when a particular orbit was available over 11. Pratt T & Browning D J, Colloquium on UK results from ATS-6 mm-wavepropagation experiments, 1977, 14/1. Delhi during that time. This being the limitation of 12. Vogel W J, Straiton A W & Fannin B M. Radio Sci., a signal orbiting satellite, more observations are 12 (1977), 757. planned using geostationery satelJites or NNSS 13. Somayajulu Y V, T~hi Ram Tyagi & Ohosh A B, satellites to study the seasonal dependence of tropo• J .atmos. terr. Phys., 37 (1975), 1603. spheric disturbance on satellite beacon signals. 14. Ghosh A B, lonospheric and tropospheric refract"on errors in satellite systems and radars, Ph D thesis, Delhi Uni• From Fig. 3 it can be seen that as the refractivity versity, Delhi, 1976•. gradient decreases from -40 to -200 N/km, the 15. Mitra A P, Somayajulu Y V, Singal S p, Mazumdar S C, satellite elevation angle decreases from 15 to 2°. Tyagi T R, Reddy B M, Aggarwal S K, Gera B S; This indicates that as the refractivity gradient Ghosh A B & Sarkar S K, Boundary-layer Meteorol., 11 (1977), 103. f-'. decreases, the possibility of receiving signal from I 16. Ohosh A B & Somayajulu Y Y, sent for publication in I;,. satellites at lower elevation angle increases. Further, J. atmos. terr. Phys., 1979. ~Fl"i· as the refractivity gradient decreases from -40 to 17. Sarkar S K, Radio climatological effects on 'tropospheric -200 N/km, the microwave link fade depth also radio wave propagation over the Indian subcontinent, Ph D increases from 5 to 8 dB, as shown in the top por- thesis; Delhi UniverSity, Delhi, 1978.

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