Indian Joumal of Radio '" Space PbyslcsVol. I, Marcb 1971, pp. 1-8

Atmospheric Radio Noise Measurements in India'So V. C. AIYA*, A. R. K. SASTRY 8: A. P. SHIVAPRASAD.

Dep~nt of Elcct~l Communication Engineering, Indian Insti+ttte oto Science, Bangalore 12

Manuscript received 21 December 1971

A review is glven of the instruments developed for the study of the amplitude and timecharacteristics and the structure of radio noise bursts resulting from lightnin~ flashes. Theuse of such instruments, suitably modified for a study of thunderstorms, thunderclouds, convec-tion ceHs in thunderclouds and lightning is discussed. Most of the parameters investigatedhave a log-normal distribution. This tog-normat law is an approx~te repre.sentation, butthe accuracy realized is of the order of 90%. An integrated account Incorporattng the latestresults on atmospheric noise data for the land mass of India and on the characteristics oftropical thunderstorms and lightnin~, is furnished. . ~ :~. ~ . I

1. Introduction

ELECTRICAL discharges accompanying. light-ning flashes in thunderstorms radiate asignificant amount of ~nergy over a WIde

range of the electromagnetic spectru~. Theseradiations are propagated through space In accor-dance with the known laws. The nature andmagnitude of these radiations received ~t .a poi!ltare determined by the source characteristics, VIZ.the location of thunderstorms, their growth anddecay, and the flashing characteristic~ o! the light-ning discharge. When. these radiations .fromlightning flashes as received at a. point are plc~edup by a receiver tuned to a certain .frequency ~Itha specified bandwidth, the output IJ1 the receiveris called atmospheric radio noise. To the extentto which the aerial and the associated circuits ofthe receiver have an effect on the waveform ofthe received radiation, the characteristics of atmo-spheric radio noise will differ from those of theoriginal radiation. . ..

The output in the receiver ansmg from onecomplete flash in a lightning ~ischarge is a 'n<,>iseburst' having a structure of Its .o~n. InvestJ~a-tions on the structure and characteristics of such norsebursts, viz. both amplitude and time characteristics,and the collection and assessment of data basedon such investigations are of real significa.nce f<,>rassessing the interfering effect of atmos~hen~ radionoise to the different classes of services In thevarious frequency bands. If such studies aresupplemented by a proper study of thundercloudsand thunderstorms, the results can be much moreuseful. With this end in view, a series of investi-gations have been carried out in I~dia ove~ a per~odof two decades. The object of this paper IS to givean integrated account of the results of such ~nvesti-gations, highlighting the result~ of experimentaland theoretical work done dunng the last twoyears.

1. Work on Atmospheric Noise in IndiaThe land mass of India extends from the tropical

to the temperate regions and is a well-known global

=Present address: Director, National Council of Edu-cational Research & Training, New Delhi 16.

centre of intense thunderstorm activity. Therefore,the local and near sources of the noise playa moreimportant role in determining the characteristicsof atmospheric noise in India than in countrieslocated at very high latitudes. Consequently, thesevere interference arising from atmospheric radionoise to communication systems has attractedconsiderable attention over a period of three de-cades. The All India Radio, the Department ofCivil Aviation and the Department of WirelessPlanning & Coordination have evinced sustainedinterest in studies of atmospheric radio noise. Thefirst measurements were in the mf and hf bandsand were largely confined to high amplitude noiseimpulsesv+. The Radio Research Board of theUnited Kingdom carried out worldwide measure-ments of atmospheric radio noise and, for thispurpose, included Calcutta, Delhi and Colomboin the list of observation stations. In their method,a signal electronically keyed at the rate o.f 10 wordsper minute was fed along WIth the ~eceIved atmo-spheric noise. The strength of the SIgnal was gra-dually raised till the observer recorded them with95% of the time intelligibility. Measurements weretaken every hour at a predetermined number ofspot frequencies with a suitably chosen bandwidth.The results have been incorporated in a specialreports. A modified form of this method suitablefor broadcasting was developed in India and exten-sively used by the All India Radio for measurementsat a number of places", The National Bureauof Standards, USA, designed the ARN-2 noiserecorder for measuring atmospheric radio noisein an objective way". This recorder has been incontinuous use at Delhi for collecting the requireddata on a worldwide basis. Utilizing the datacollected with the ARN-2 noise recorder at differ-ent stations in the world and together with someother data, the CCIR8 have provided noise dataOn a worldwide basis in their report No. 322.Primarily, these estimates are related to the worlddistribution of thunderstorm days.

For presenting the data, the year has beendivided into four seasons, viz. March-May, June-August, September-November and December-Feb-ruary. Each day has been divided into 6 four-hourtime periods, viz. 00-04 hrs LMT, 04-08 hrs LMT,

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INDIAN J. RADIO SPACE PHYS., VOL. 1. MARCH 1972

and so on. The aggregate of the correspondingone four-hour time period of the day throughouta season constitutes a time block. Actual mea­surements carried out by the different adminis­trations in India show' that the noise values: <j.S

furnished by the CeIR are low in several casesand do not generally corr~3P.o.ndto theac~ualitiesof the situation. The CCIR qata do not adequatelytake into account' the effect of local sources whichare extremely important for India. The ARN -2.recorder has a limited dynamic range.

The CCIR, however, proposes to prepare a com­prehensive revision of report No. 322 with dueconsideration fa the "alternative m~thods of pre .•.sentation of the data9 •.

3. Sources of the Noise

Every lightning. flash can give rise to a noiseburst. As is now. known, lightning flashes areof different types. There are lightning flashes inwhich the electrical discharge strikes the ground(ground flashes), those in which the discharge occurswithin the cloud only (cloud flashes) and thoseoccurring between the cloud and the air. But,in all types of flashes, there are electrical dischargesoccurring inside the cloud. It is now known thatthese discharges inside the cloud are responsiblefor the radiations giving rise to a noise burst atany frequency in the 1£, mf, hf and vhf bands.Ordinarily, in tropical latitudes, the cloud baseis at a height of about 3 km above the mean sea­level. Radiations from discharges within the cloudcan be received up to a distance of about 300 kmby the direct ray. Hence, all sources lying withina distance of 300 km from the point of observationare called local sources. The days of a season onwhich such local sources are present are called daysof local activity. Experimental investigations haveshown that the number of days of local activityis approximately three times the number of thunder­storm days as recorded by the weather offices.Obviously, local sources are responsible for thehighest amplitude noise bursts.

Sources lying between 300 and 1000 km arecalled near sources. Radiations from such sources inthe mf and hf bands should ordinarily be expectedto be received via the ionosphere. Consequently,the reception of radiations from such near sourcesdepends on propagation conditions via the iono­sphere and such reception may not be possible duringcertain hours of day at certain frequencies. Sourceslying beyond 1000 km are called distant sources.Whenever propagation conditions at any givenfrequency are favourable, the signal from suchsources can appear as noise in the receiver. It isexpected that there can be a very large numberof sources lying beyond 1000 km. Hence, a verylarge number of noise bursts of varying amplitudesshould be received from different sources. Itfollows, therefore, that the noise arising from suchsources would correspond practically to continuousnoise. The noise arising from local sources is alwaysin the form of distinct and well-separated noisebursts. The noise arising from near sources ismostly in the form of distinct noise bursts. But,when there is a number of near sources active,the number of noise bursts received per minutemay be very large. This may give rise to noisewhich is almost of ·the continuous form.

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4. Forms and CharacteristiCs- of ,Radio' Noise ,:

4.1 Forms of Noise

Noise, as understood today, is a complex pheno­. menan. Ther~ is what is caUed fluctuation noise. and this does arise) from several known factorseitherinth,e.eqllipment .or in the channel. Mathe­maticaJ models for the treatment of this noise arealready available. The second form of hoise, known

.,as .....' hnp\lhive.,\ noise', appears intermittently atrandom intervals and for random durations. Themedian value of the duration is defined in relationto the service for .which the word' impul{;e ' is used.Thus, for voice communication, the ear can detectan impulse when it lasts about a millisecond.Therefore, noise of millisecond duration would beimpulsive. Similarly, the ear gets the .im.pressionof continuous noise when the. noise lasts ,~or morethan 200 milliseconds. But, the problem is.slightlydifferent in data communication. Suppose the bitthat is transmitted is of 1 [Lsec duration. Then,anything that lasts about a microsecond wouldbe an impulse for this purpose. It will thus beseen that the definition of an impulse is relative.to the type of transmission of the signal that isenvisaged. Since generalizations of impulsive noiseare misleading without the appreciation of thiscrucial distinction, it is being stressed here. Evenimpulsive noise is a random process. The ampli­tude, the duration and the time interval betweenimpulses vary in a random way. The problemwith which an engineer is most concerned is to findout how a particular impulsive noise conformswithin limits of error to one or other known formsof statistical distribution.

In some cases, noise lasts for an extended periodof the time corresponding to many times the dura­tion of an impulse as understood for a particularservice. But, after the appearance of the noisefor such an extended period, there is a pause whenthere is no noise. Thereafter, the same processrepeats itself. In such a case, the problem posedis one of noise bursts. The duration and the timeinterval between such noise bursts are randomvariables and the types of distribution they followneed examination. The amplitude is a parameterdifficult to define for a noise burst as a whole; butit can be done in various ways. First, one canthink of the average or rms value of the envelopevoltage in a noise burst. This approach is verysatisfactory when the noise in the burst correspondsto fluctuation noise. One can also think of theintegrated effect of the noise in the burst whichwould affect a particular type of service. In sucha case, there would be what is called a ' quasi-peakamplitude'. This quasi-peak amplitude and themanner in which it has to be measured (and studied)would vary from service to service. Thus, in voicecommunication, the charging and discharging timeconstants of the equivalent electrical circuit of theear become important. In some forms of data'communication, the bandwidth employed limitsthe frequencies in the pulse and hence would indi­rectly give the time over which the primary pulsesin the noise will have to be integrated.

Within a noise burst itself, there can be a numberof quasi-peak amplitudes as described above,specially from the standpoint of data. communi­cation. Consequently, it is to be expected that they

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,I AIYA etal.: ATMOSPHERIC RADIO NOISE MEASUREMENTS IN INDIA

f~llbw,acertain distribution 'as far as their ampU•.,tude and time characteristics arecbncerned. Infor.,mation' on this point is vital for high reliabilitydata. ;communication. This, therefore, poses theneed for double statistics. Thus, there is the needfor the amplitude and the time statistics of noiseDursts::in .their entirety. This may be called themacroscopic problem. Then,. there is the needfor the amplitude and time characteristics of thepeak amplitudes or the im.pulses within a noiseburst .. Such peak amplitudes -within a noise burstgive rise to a variety of complications. First, theduration of a noise burst may be many times theduration' .of the bit that is transmitted. In sucha case,- there would be clusters of errors arisingout of one noise burst. In optimal design, it isnecessary to reduce the number of errors in suchclusters of errors.

4<.2 .Classiftcation and Measurement ofAtmospheric Radio Noise

.Extensive investigations carried out over a periodof a decade revealed that atmospheric radio noisegenerally appears in the form of a continuous back­ground '.over which are superposed distinct noisebursts. Further studies of the noise revealed thatit is possible to classify atmospheric radio noiseinto three types10,1l:, .Type A: Low amplitude continuous noise. Thisarises principally from distant sources.

Type B: This is a burst form of the noise. Inthis case, the bursts as heard are not very sharp.The number of bursts received per minute is ordi­narily 20/min, correct to a factor of two eitherway. ,This burst form of the noise generally arisesftomnear sources.

Type C: This is also a burst form of noise. Thebursts are ordinarily of shorter duration and theamplitudes are high. Noise in this form has beeninvariably traced to local sources. The numberof bursts received per minute can vary over a widerange from a few to more than 40.

Over the entire mf and hf band, the magnitudeof type A noise is of about the same order or evenless than the other sources of interference takentogether. It is difficult to separate out this formof the noise and carry out precise measurements.Experience has shown that it is more practical tolump this noise with other interference and statethe level of the background.

Ordinarily, for the major portion of the time,atmospheric noise received in tropical regions con­sists mostly of noise bursts. Therefore, it becomesnecessary to study the distribution of the durationand the time interval between such noise bursts.It-is also necessary to study an amplitude parameter,which can be associated with a noise burst, as alsothe structure of a noise burst.

Over a short period of time, say 5 min, engineersexpect at least 90% of the time satisfactory service.Therefore, the highest amplitude noise that is presentfor 10% of the time is the one of real significance.Consequently, highest amplitude noise bursts occu­pying' 10% of the time have to be studied and theirdistributions generalized. Such amnlitude studiesare called 'short-term characteristics'. The medianvalue of the amplitude for such a short term canbestudie'd at the chosen hour over all the days ofthe seaSon.. This will furnish the required long-

term characteristics. The long-term median 'valueof the amplitude can be plotted against the- hourof the day for the 24 -hours of the day. If thisgraph shows an hour-to-hour change in the samedirection for several hours, then during those hOUTS,there is a - systematic variation. This can arisefrom a variety of causes, viz. growth and decayof thunderstorms, changes in the condition of pro­pagation, etc.

The common international practice is to lumpfour hours of the day together and divide the dayinto 6 four-hour time blocks as mentioned earlier.Noise data on a seasonal basis are furnished for eachtime block. The same practice can be followedfor the furnishing of noise data on the basis ofstudies of noise bursts.

Studies on the time characteristics of noise burstspertain to a study of the duration of a noise burstand the time interval between such noise bursts.Such studies are obviously carried out over a shortperiod of time, e.g. 5 min. The study of the sttuc­ture of a noise burst is primarily concerned withthe peak amplitude pulses of a certain durationwithin the noise burst. What is significant is ,thedistribution of the amplitudes of such pulses, theirduration and the time interval between them.

When studies on the lines indicated above arecarried- out, one is studying basically the noiseburst which arises from a lightning flash and is;therefore, a natural unit for dealing with atmo­spheric radio noise.

All such studies posed a variety of problemsfor instrumentation. These were tackled duringdifferent stages of the investigations. An integratedaccount of the instruments required and developedis furnished in the next section.

5. Instrumentation

5.1 Quasi-peak Meter

This method12 was specifically developed formeasuring the interfering effect of atmosphericnoise to listening where the ultimate judge is thehuman ear. Extensive measurements have beenmade at Poona and Bangalore, based on this method.

The noise meter essentially consists of an aerialsystem, a communications receiver and a noiseoutput meter which is fed from the output of thedetector in the receiver. The output meter con!:istsof a semi-logarithmic amplifier and a detector unitwith a charging time constant of 10 msec and adischarging time constant of 500 msec. The timeconstants have been selected after extensive trialand error experiments in such a way that the outputunit has characteristics very similar to those ofaverage human ear. A microammeter is used inthe discharging path for recording purposes. Thefrequency response of the output unit is flat withinthe frequency range 100-5000 Hz.. The signal tobe used for calibrating the noise meter is a sinewave modulated by a 400 Hz note to a level of100%.

This method of measurement gives the quasi­peak value of the individual audible noise burstsin the form of kicks in the microammeter of theoutput unit and the average value of the envelopevoltage in the form of a; steady reading of the meterfor the continuous. background noise. Continuousaural monitoring becomes necessary to avoid the

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INDIAN J. RADIO SPACE PHYS., VOL. 1. MARCH 1972

recording of man-made noise and station interference.The sensitivity of the receiver used in the noisemeter can be changed in steps of 6 dB, with about20 dB amplitude range in each position. This cansuitably cover the amplitudes arising from a singlefrontal storm which have been observed to lie ina range of 20 dB.

By subjective experiments, a criterion whichsuitably takes into account the annoyance feltby the ear due to the bursts has been developed.According to this criterion, the annoyance is feltonly when there are at least 10 bur:stsjmin. Byextensive investigations on the time characteristicsof noise bursts, it has been found that the medianvalue of noise burst duration is about 500 msec.Hence, ten bursts per minute occupy roughly 10%of time. Thus, Aiya's annoyance criterion refersto 90% of time intelligibility. The arithmeticalaverage of the quasi-peak values of ten highestnoise bur~ts received per minute over a short periodof 5 min represents the annoyance felt by the earand is termed as 'the noise burst level'. Themean value of noise burst field strength is mostfrequently found to be about 3 dB below the' noiseburst level' defined above.

5.2 Peak Meter

The peak amplitude corresponds to the highestpulse amplitude occurring in a noise burst and isnecessary for the design of data communicationsystems. Optimal design of high reliability datacommunication systems requires also the numberof peaks in a noise burst and their time functionswhich are discussed in a subsequent section.

This peak amplitude is measured by feeding theoutput of the last intermediate frequency stageof the receiver to an output unit having chargeand discharge time constants of 1 and 160 msecrespectively13, This unit is calibrated by usingcontinuous wave signals and taking their peakamplitude.

5.3 Average and rms Meters

The average and rms values of the amplitudeof a noise burst are required for deducing the conti­nuous noise equivalent for being used in the designof data communication system. These values aremeasured by feeding the output of the last inter­mediate-frequency stage of the receiver to outputunits with charge and discharge time constantsof 170 and 500 msec respectively for the averagevalue, and of 80 and 500 msec respectively for therms value14• They are calibrated by using thepeak carrier values of continuous signals modulated100% by a sinusoidal tone of 400 Hz.

5.4 Vhf Peak MeterAn instrument for the measurement of vhf

atmospherics was set up consisting of essentiallya dipole antenna,' a balun to match the antennato the coaxial feeder, a vhf receiver tuned to 63·5MHz with a pass band of 5 MHz and an outputunitI5• The output unit had been designed tomeasure the peak field strength of vhf atmosphericsas the dial reading of a microammeter.

5.5 Lightning Flash Counters

Radiation field-actuated, lightning flash countersoperating at 1'6 and 3 MHz with different thresholds

4

were constructed16 and used extensively. in studieson noise burst rates and characteristics of thunder­storms. A simple, cheap, portable and reliableinstrument for the collection of data on the numberof lightning flashes of all types per unit time overunit area of the earth's surface, called lightningflash density gauge, was recently fabricated andused to record flash densities17.

5.6 Pulse Rate Counter

A pulse rate counter has been developed to countthe number of pulses in a noise burst crossing acertain amplitude threshold and simultaneouslyto record the burst duration corresponding to thepulses. The instrument has been designed18 toreceive noise pulses at 3 MHz with a bandwidthof 3·3 kHz/6 dB.

5.7 Atmospheric Noise Burst Generator

This instrument19 employs Zener diodes andgas tubes as sources of noise and multi vibratorsfor the simulation of pulses in the atmosphericnoise bursts. The artificial noise bursts generatedusing this instrument exhibit all the observedcharacteristics of natural noise bursts over the fre­quency range 0'1-10 MHz.

6. Noise BurstsIn this section, the results of various studies

conducted on noise bursts using the instrumentsdescribed in section 5 are discussed.

6.1 Amplitude Parameters

6.1.1 Lf measurements - Quasi-peak levels ofnoise bursts were measured at Bangalore at 105and 280 kHz with a receiver bandwidth of 4 kHz/6dB. The distribution of these values has beenextensively investigated over 'short terms', i.e.periods of 5 min at different hours of the day andon different days of the seasons, and was foundto be log-normal with a standard deviation of about4 dB. The mean value is generally found to beabout 3 dB below the noise burst level. The long­term characteristics of noise burst levels, obtainedfrom the short-term values for a selected half hourof all the days of a season, indicate systematicvariation in certain time blocks20.

Quasi-peak and peak values of noise bursts weremeasured at Bangalore at 30 and 60 kHz als021.There is a broad agreement between the peak valuesobtained in the measurements and those given byCCIR.

6.1.2 Mf and hf measurements - Extensive mea­surements were made in mf and hf bands thatformed later the basis to provide data in thesebands. Investigations on atmospheric noise inter­ference were conducted at poona in both the bandsand measurements were made at several spot fre­quencie,·22-27. The short-term and long-term distri­butions of quasi-peak values of the noise were studiedat 3·0 and 4·5 MHz \vithin a bandwidth of 6 kHz/6dB at Bangalore28·3o. A study of the characteristicsof the noise in the 1·5-3·0 MHz band revealed thatthe noise levels show systematic variation between12-20 hrs 1ST, while the distribution forthe time block 12-16 hrs can be represented by asingle log-normal distribution; for the time blocks16-20 and 20-24 hrs they are heterogeneous31. Goodcorrelation was observed between half-hourly noise

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AIYA et al;; ATMOSPHERIC RADIO NOISE MEASUREMENTS IN INDIA

burst levels at 1·6 and 2·9 MHz and it was foundthat' noise level at one frequency can be predictedirom that at another with a standard error estimate-of about 3 dB. However, the correlation decreasesas the frequency separation increases32. Measure­ments were made to investigate the effect of rtceiverbandwidth on the quasi-peak values and time para­meters of noise bursts in the frequency range 0,1-9MHz33. The quasi-peak value varies as the squareToot of bandwidth from 0·3 to 6 kHz. There isno effect on time parameters of the noise bun'tsand for both amplitude and time parameters, the-distribution which is log-normal does not changewith bandwidth, and the standard deviation isalso unaffected.

By the simultaneous collection of data on noisebursts for the peak. average, or rms value andthe quasi-peak value in a suitably randomizedway at different frequencies and statistically ana­lysing them, the ratio of each of the differentparameters to the quasi-peak value was deter­mined13,14. In this way the most appropriate<:onversion factors for different frequency ranges forall the three amplitude parameters were evaluated.

Peak and quasi-peak values were also measuredsimultaneously for the same noise burst using<:alibrated cathode ray oscillograph (CRO) andnoise meter, at 3 MHz with 3 kHzj6 dB bandwidth.The ratio of both the values was in broad agreementwith the earlier measurements13, but was foundto increase by about 3 dB for 8 kHzj6 dB band­width at the same frequency34,

6.1.3 Vhf measurements - Atmospheric radio noisein the vhf band received practically no attentionat high latitude regions possibly due to the pre­sence of a high level man-made noise. CCIRreport No. 322B also does not furnish information inthis band. The noise, however, assumes impor­tance in countries in the tropical region due tothe intense activity of thunderstorms and existenceof low man-made noise because of the relativelyunderdeveloped nature of these countries.

In view of this, measurements were made atBangalore, on the frequf'Ilcy of occurrence andthe magnitude of the noise in its continuous andburst forms at 63·5 MHz in a pass band of 5 MHzj3 dB, and data were furnished35,36. The dataprovide, for the first time, useful guidelines onwhich assessment of atmospheric noise can bemade for use in planning vhf communication andradio-astronomy systems.6.2 Ti~e Para~eters

The time parameters of relevance in the studyof noise bursts are the burst duration and theburst repetition rate. The interval between thetime when a burst becomes audible to the timewhen it becomes inaudible represents the burstduration. The number of bursts received perminute represents the burst rate. Tape recordersand level recorders have been employed in thestudy of time parameters. Such measurementscovered a wide range of frequencies from 1£ to hfbands and measurements were carried out overlong periods of time.

It has been found that both these parametersare random and follow a log-normal law. Themedian duration of noise bursts has been estimatedto be about 500 msec with a standard deviation

of 6 dB37. When the activity of a, thundercloudis around its peak, the mean value of. the durationof a burst has been observed to be,about 200 msec.At other times it is larger3B,39. The mean durationof a noise burst is observed to exhibit a systematicvariation with the growth and decay of the activityof a thundercloud, and found to follow the cyclicvariation corresponding to the growth and decayof cells in thunderclouds40.

The burst duration measurements carried outusing the pulse rate counter -give a median valueof about 200 msec and a standard deviation of 6dBu. Since the measurements were carried outafter slicing the burst at a certain amplitude level,the value of 200 msec for the burst duration cor­responds to the peaks in the noise bursts. Thisconfirms Aiya's observationll that the maximumamplitude changes, including the most violent ofsuch changes, are confined to only about 0·2 secpart of the burst.

Apart from magnetic tape and level recorders,lightning flash counters and noise burst field strengthmeters also have been used in the study of theburst repetition rate, which is a direct index ofthe growth and decay of the activity of a thunder­cloud. In an active thundercloud, a decayingconvection cell gives rise to a new cell and thisprocess continues until the thundercloud itselfdecays. The growth and decay of cells thus pro­duce a cyclic variation in the burst repetition ,rate.For the greater portion of the activity, the burstrate exceeds the value of 1 and often reaches thevalue of 40. The number of active cells recordedper thunderstorm has a mean value of 5 with acell life of about 30 min42,43. However, the meanoccupation time in per cent and noise burst ratewere found to be highly correlated, despite theduration variation, thus allowing prediction ofthe former for a given noise burst rate40.

Another parameter of interest in the study oftime parameters of noise bursts is the burst repeti­tion period. This refers to the time interval be­tween successive bursts becoming audible. Themagnetic tape recorder technique has been extendedfor the repetition period measurements. This para­meter has also been found to follow a log-normallaw in the 1£, mf and hf bands. The median valuehas been estimated to be about 1-4 see with astandard deviation of 4 dB37.

6.3 Structure of Noise Btlrsts

6.3.1 Primary pulses from lightning flashes­Actual radiations from a lightning flash occur asa number of primary pulses of random amplitude,duration and time interval between two successivepulses. The duration of such pul~es is of the orderof microseconds or very much less. Occasionallysuch pulses occur in bunches for periods of a milli­second or less. It is such bunching which probablycorresponds to the K-type of' discharges, steppedleaders, etc. When such pulsive radiation passesthrough a receiver tuned to a spot frequency andbandwidth, the waveform gets modified. Thebunched primary pulses get integrated becauseof the receiver bandwidth and become pulses ofthe order of milli"econds duration in the noisebursts. These pulses in noi~e bursts detemunethe clusters of errors in digital communicationthrough the burst error channel.

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TABLE 1 - PULSE CHARACTERISTICSOFATMOSPHERICNOISE BURSTS AT 3 MHz WITH

3·3 kHz/6 dB PASS BAND

(Distribution of all parameters listed is log-normal)

7. Results

The instruments described in section 5 weredeveloped for specific purposes and were designed torealize the objective to the maximum possibleextent. Their limitations, if any, have been dis­cussed in the papers describing them and ariseprincipally from the l:mJations of the availabletechnology. Improvements should always bepossible. In order to el:m:nate the effect of suchlimitations on the results and their interpretation,the maximum possible precautions were taken bothin the design of the exreriments and the interpre­tation of data.

Conclusions of significance arising out of thetwo decades of continued experimental work areof relevance to three major are3.S, viz. characteristicsof the random processes, characteristics of tropicalthunderstorms and characteristics of atmosphericradio noise over the land m3.SS of India. In whatfollows, a brief summary of the results pertainingto each of these areas is furnished.

6.3.3 Pulse rate counter studies - The pulse rate­counter developed to count the number of pulsesin a noise burst cros ;ing a particular amplitudethreshold and simultaneously to record the durationcorresponding to these pubes was previously des­cribed1S,41. The instrument has been used atBangalore (12°58'N, 77°35'£) to investigate thepulse characteristics of no so Dursts at 3 MHz witha receiver bandwidth of 3·3 kHzj6 dB.

The parameters of no:se bursts studied were:(i) number of pulses, (ii) burst duration, and(iii) pulse rate normal:zed to 100 msec duration of theburst in an amplitude range of 6 and 12 dB fromthe peak value of the burst. All the three para­meters are found to follow log-normal distributionrather than a Poisson distribution which wouldbe expected for the random events. The corres­ponding median values and the standard deviations41are given in Table 1.

The results presented in Table 1 pertain to studiesof locally active thunderclouds during their peakactivity /and the duration of a burst as mentionedcorresponds to the time interval between the firstand the last pulse of the magnitude indicated. Theteis always some noise in a burst before and aftersuch pulses.

Parameter MedianStandardvalue

deviationdB

No. of pulses in an amplitude

209,0range of 6 dB from the peak Duration

200 msec6·0Pulse

rateinnoisebursts 107·0normalized to 100 msec

No. of pulses in an amplitude

458·0range of 12 dB from the peak Duration

270 msec6·0Pulse

rateinnoisebursts 176·5normalized to 100 msec

Fig. 1 - Oscillogram of a typical radio noise burst asrecorded at 3 MHz with a receiver bandwidth of 3·3 kHz

(on a time base, 50 msec/cm)

INDIAN J. RADIO SPACE PHYS" VOL. 1, MARCH 1972

6'.3.2 eRG studies - CRO records of several noisebursts due to narrow band atmospherics were ob­tained using Tektronix oscilloscoj:'e type 545 anda qualitative study of waveform was carriedout at several frequencies. A narrow band atmo­spheric is found to consist of several pulses of ampli­tude ranging from a few microvolts to tenths ofa volt. The time of recurrence of the pulses andthe total number of pulses appearing in a noiseburst are random phenomena. At If and below,the pulses al1pearing in a noise burst are few andhave relatively large amJlitudes. The principalsource of vlf and If em;ssion from lightning flasheshas been traced to return strokes in the groundflashes and K-ty;:,e of discharges in cloud and airflashes. At mf and hf bands more pulses aprearin a noise burst and correspondingly the reakamJlitudes attained by these pulses decrease.The waveform of a typical noise burst41, as recordedat 3 MHz with a receiver bandwidth of 3·3 kHz I

6 dB on a CRO screen, is reproduced in Fig. 1. Astudy of these waveforms has shown that all typesof flashes are equally effective in radiating noisepulses in the mf and hf bands. Collection of dataon the number of pulses at a given frequency apdbandwidth is of importance in predicting the rer­farmance of data communication systems.

Prel:minary measurements on the number ofpulses in a "noise burst were carried out using aCR019. However, counting of the exact number ofpulses in a burst directly from the CRO recordswas found to be very difficult. Whenever severalpulses occurred very closely in a burst, the overalltrace appeared as a continuous patch and indi­vidual pulses could not be distinguished. Therefore,CRO records do not seem to be useful mean8 ofcollecting such data in the mf and hf bands. Highre80lution pulse rate counters are well suited incounting the pulses in a burst over a specific thres­hold level.

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AI"fA et /II;: ATMOSPHERIC RADIO NOISE MEASUREMENTS IN INDIA

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continoous throughout the activity- of1l~thmider,..cloud. But, the flashes {)ccurring are.~ difierW­types, the largest number 'beingihe cl.o1ad(·flashes~many of whichcannot be seen byvisualollservatio1l.,Whenever thunder is beatd, there arealwa.ys:someground" strokes. If seasonal averaging is done.it is found that about 10% of aU the,.ftashes areground flashest5. This percentage .is very,smallcompared to the observed percentages in tem­perate regions. The median value of the peak tateof flashing during the activity of athundettloudis 8/min. But, rates of flashing as high as over4O/minhave been occasionally'observed, The me..dian value of the duration of a flash is, 500 msec;However, this value gets-reduced to about 200msec during the peak activity of a thundercloud.The noise power radiated by a typicaliiashhasbeen theoretically evaluated and the resriltscom­pared with the measured median values"",The,eis agreement between the two15."The noise fielii'strength varies with the stage of" activi~ -of athundercloud and becomes maximal when .•theactivity has reached its peak30.-Therandom van .•ations of the integrated value of the radiation rfieldsarising from a flash when measured atone predeter­mined distance lie within a range of 20 aD.' "

Within a flash, K-type discharges 'baveibeeJlreported by workers investigating lightning,,-dis­charge. These are found to correspond/"to -'thepulses within a noise burst which·lie wit-bi»6 ;dBof the peak amplitude pulseu.7.3 Noise Data for India

In the frequency range 0·3-30 MHz, tbenQi~data required for the different types 'of,tatito'cl)Dt'::muni~ation services have been furnish~foI' rtbeentire land mass of India46,41. This is donethrotigha set of three data sheets and a map of Indi-ar.giVingthe days of local activity in the different regionsdu ring the four seasons of the year. The firStsheet gives the data for voice commUnication.The second one gives the conversion of these data.for data communicationand the other details neeeS+

sary. The,third s"!leetfUrnish~stl}.e·desirable~to-noise ratios. :The prpce<lurefcw·agi~iIg). .:t_appropriate ,noise level Jor, a desired;- - ,­of .time for','ensuring satisfactQry',Mmgct

has been des9rib~d aqd. the _r~<tu~:_grams have been prepated!l8;_The_b~ca 1 kHz bandwidth Itt 1 MHz widt 1

conver~ionfa~!Qrsf.o.rJ.oth~::...ff~~l).~i"widths. 'Since thepJ;0C6~rt'!:;tl:dQJlted~iDt.1~-'_lt­nishit;J-g'of t~ese ,~ata is .b'£ .sckntiif~;fn,~~~t.~

desST}b~9-~ne~Y ,~n-t~.e :pex~;~~t:~gra£fi::,",'Hlf -",.~, CommunIcatIon engIneers expect' at Ie~~.oof the time satisfactory service in -a::sh()rt,(~.p'!l

of .time, say 5 min. Hence, t~e~Y~h~~_~tAvnoise present for 10% of thet1mEl_rs.m~~J~studied. The mean value of this"noise asa SO] •

of interference to voice communication'i's,"the noise level (N). It can be convertedi' .average,or peak amplitudes by the use61sassessed conversion factors. The short­acteristic of N is log-normaland the 'trlftand standard deviation are adequate.-"1bution of the median value of N over a long periodof time like a time block is also log-normal. Hence,long term characteristics are adequately represented,by the median value and standard deviation. Data

1,1 Rmtdom BwtltlThe duration of a noise burst, the time interval

between, successi~e noise bursts, the amplitude-of a noise burst, and the number of noise burstsreceived per ininute' at a certairi frequency followthe log-normaldistribution. The number of pulseswithin a certain range of amplitude in a n9ise burstand the time interval between successive pulsesalso follow the log-normal distribution.

The lifetime of a thundercloud, the lifetime ofa convection cell in a thundercloud, the number-of such cells developed during the lifetime of athundercloud, the duration of a lightning flash,the time interval between successive flashes, thepeak rate of flashing in a lightning discharge andthe distance up to which thunder can be heardfollow the log-normal distribution.

A Poisson distribution is a normal expectationfor at least some of the random events. Therefore,the conclusions drawn have been carefully re­checked. The log-normal law is an approximaterepresentation and not a perfectly accurate one.But, the accuracy realized is of the order of 90%.This should be deemed adequate for most engineer­ing evaluations.'1.2 Thunderstorms

Some of the conclusions which follow from thecurrent investigations on the characteristics oftropical thunderstorms have been described anddiscussed earlier38,u. During spring and autumn,storms develop, grow and decay ordinarily duringthe afternoons and last for 3-4 hr. But duringthe regular monSoon period, viz. summer, theyare usually long drawn out. In any such typicalthunderstorm over the land mass, it appears thatseveral thunderclouds in the different regions ofthe large area involved become active but duringdifferent periods of the day. Consequently, thereis activity from about local noon till the time ofsunrise the following day. The decay in activityis rather sharp. Thunderstorm days occur inbunches, the most probable number. being 4.thunderstorms over the sea appear similar butfor one distinct characteristic, viz. a peak of acti­vity between 16 and 17 hrs LMT followed by asharp fall immediately thereafter.

Recent round the clock studies of noise fieldstrengths during local storms show a cyclic varia­tion of this parameter and the measurements ofthe time interval between successive crests andtroughs give a lifetime for a thundercloud whichis about the same as that obtained by othermethods21. This confirms the previous conclusiondrawnindirectly that a thunderstormactivity as sucharises from the activity of several thunderclouds.

The lifetime of a convection cell has a medianvalue of about 30 min. This is the same as thatreported from' experiments in temperate regions.But, the median value of the number of such cellsdeveloping during the activity of a thundercloudis 5. The median duration of the life of an activethundercloud is about 3 hr. This is very muchhigher than the values reported from temperatureregions. The distance probability distribution ofthunder heard is log-normal with a median valueof 9 km15•

The following conclusions emerge regarding thelightning discharge. The lightning discharge is

INDIAN J. RADIO SPACE PHYS., 'VOL. 1. MARCH 1972

are furnished separately for days of local activityand for other days. For data communication,this information is supplemented with the dataon the peak amplitude pulses in a noise burst.

Atmospheric noise in India as examined thisway consists of noise bursts arising from the localor near sources. The distant sources and otherinterference contribute to a background whichhas been evaluated and furnished. A surprisingresult confirmed by actual experiments is the factthat there is nO atmospheric noise during the hours08-12 hrs LMT. Similarly, on days other than thedays of local activity, there is no atmospheric noiseduring 12-16 hrs LMT.

For the vhf band, 30-300 MHz, atmosphericnoise data15 are furnished at tOO MHz and a passband of 1 kHz. Values for other frequencies andbandwidths can be deduced by applying the inversefrequency and the square-root bandwidth laws.The rms value of the equivalent fluctuation noiseat 100 MHz and 1 kHz pass band, present for 10%of the time, for about an hour at a time at leastonce on a thunderstorm day between local noonand midnight is 20 ± 10 dB above 1 p.V1m,

Vhf noise radiation from lightning flashes cantravel to distances of the order of 500 km via thesporadic E-Iayer or by troposcatter. This is esti­mated to give rise to continuous noise of the orderof 0·1 p.V/m at 100 MHz and 1 kHz pass band.This noise will be present even in remote placesduring the thunderstorm seasonS and has to bereckoned with in the planning of radio-astronomywork.

8. Conclusion

The lightning flash is the source of atmosphericradio noise. The geographical location of theland mass of India makes it one of the mostsuitable countries in the world for scientific studiesof atmospheric noise in terms of the lightning flash.Such studies over a period of over two decadeshave furnished a wealth of valuable information.They have enabled the furnishing of noise datain a simplified but rational way. They have ledto results of value on thunderstorms and lightning.A judicious use of the instruments developed canlead to the collection of further valuable informa­tion on thunderstorms and lightning.

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