ots propagation measurements during thunderstorms

102 pp. 102-110

OTS propagation measurements during thunderstorms

Neil J. M c E W A N M. A., Ph.D. Lecturer *

Artur P. ALVES Engenheiro Electrotecnico, Research Student * *

Hing Wai P O O N Higher Diploma, Electronic Eng. (Hong Kong), Research Student *

Asoka W. D I S S A N A Y A K E B. Sc., M. Sc., Ph.D., Research Fellow *

Abstract

OTS propagation measurements are described which comprise a complete determination o f the incoming polarizations for both linearly and circularly polarized beacons. Auxiliary equipment includes radars, E-field sensors and a microphone array. General trends in the propagation data are discussed and two basic models are proposed for the observed correlations between electrostatic f ield and cross-polarization..4 detailed analysis, with radar data, is now given for one thunder- s torm..4 method for the prediction o f the linear cross- polarization from the circular is shown to work well for this ice-dominated event. Fast lightning-induced, cross-polar jumps are described, and by relating them to acoustic thunder location data, the physical models for particle alignment are discussed.

Key words : Wave propagation, Centimetric wave, Satellite communication, Cross-polarization, Correlation, Electric field, Thunderstorm, Experimental study.

M E S U R E S D E S P R O P A G A T I O N S P E N D A N T D E S ORAGES

A L'AIDE D U SATELLITE OTS

sation rectiligne ~ partir de celle observ~e en polarisation circulaire convient bien ?tce ph~nombne domind par des particules de glace. Des sauts rapides entre polarisations croisdes sont provoqu~s par les ~clairs. En les identifiant parmi les donn~es de localisation acoustique, on discute des modbles physiques d'aligne- ment des particules.

Mots el~s : Propagation onde, Onde centim6trique, T616- communication par satellite, Transpolarisation, Corr61ation, Champ 61ectrostatique, Orage, Etude exp6rimentale.

Contents

1. Introduction.

2. Measurements and equipment.

3. Electric f ield correlations.

4. General observations on propagation.

5. Thunderstorm event.

6. Conclusions.

References (10 ref.).

1. I N T R O D U C T I O N

Analyse

On ddcrit des mesures de propagation avec le satellite OTS comprenant la ddtermination complbte des polarisations recues, pour les balises ?t polarisation rectiligne ou circulaire. L'dquipement auxiliaire com- prend deux radars, deux capteurs de champ dlectrique et un rdseau de microphones. On analyse les caract~- ristiques g~n~rales de la propagation, en prdsentant deux modbles pour des corrdlations entre le champ dlectrostatique et la transpolarisation. Un orage est analys~ en d~tail d partir des donn~es des radars. Une mdthode de pr~vision de la transpolarisation en polari-

The OTS propagat ion experiments at Bradford University are directed to the following areas :

(1) at tenuation and cross-polarization stat is t ics;

(2) radar prediction of a t t e n u a t i o n ; at tenuation and reflectivity properties of rain and bright band ;

(3) nature, size and shape of cross polarizing particles ;

(4) their al ignment m e c h a n i s m s ;

(5) relationships with a tmospher ic electrici ty;

(6) linear and circular polar izat ion compar i sons ; detailed behaviour of cross polar signals including phase and rates of change.

* Postgraduate School of Electrical and Electronic Engineering. University of Bradford, Bradford, West Yorkshire BD7 1DP (UK). ** Lecturer on leave from Faculdade de Engenharia, Universidade do Porto (Portugal).

ANN. T~L~COMMUNIC. 36, n ~ I-2, 1981 I/9

N.J . MCEWAN. -- OTS PROPAGATION MEASUREMENTS DURING THUNDERSTORMS 103

In this paper we discuss briefly the equipment, general trends observed in our OTS propagation data, and correlations between electric field and cross polarization. We then present two new results which were obtained from a particular thunderstorm :

(a) the successful prediction of linear cross polarization from circular in ice dominated conditions, using absolute circular cross polar phase d a t a ;

(b) observation of lightning-induced abrupt changes in cross-polarization, made in conjunction with an acoustic thunder location technique.

2. MEASUREMENTS AND EQUIPMENT

Figure 1 summarises the measurements and equipment layout. We give a brief discussion of various points arising from it.

2.1. Receivers.

The TM, 11.785 GHz linearly-polarized beacon, and the BI, 11.575 GHz circularly-polarized beacon, of OTS are both received on the single 3.6 m Cassegrain antenna using a dual linearly-polarized orthomode transducer. The quantities measured are copolar (nor- mal) and cross-polar (orthogonal) component ampli- tudes for both beacons. It is also important to note that we have an accurate absolute measurement of the relative phase between copolar and cross-polar compo-

nents, i.e. the incoming polarization states are fully determined for both beacons.

The TM receiver is conventional, using phase- locked down conversion to 20 kHz and coherent detection. The BI receiver is unusual in that the incoming nominally circularly-polarized beacon signal is received first as linear orthogonal components, and that matrixing is done at the final IF of 20 kHz to give two hands of circular polarization. To give the necessary gain and phase stability, each channel is con- tinuously stabilised by injection of an RF calibration signal. It is 590 Hz away from B1, and --~ 23 dB below its clear weather level, and is extracted immediately before the final matrix. Fuller description is given in [1].

The residual cross-polar levels are ,-, 39 dB for TM and > 35 dB for B1. At our site, the TM polarization appears 15 ~ counter-clockwise from horizontal, looking towards the satellite.

2.2. Cross-polar calibration of receivers.

The coherently detected in-phase and quadrature components of the TM (linear) cross-polar signal are filtered first with a 1 ms time constant, then again with a 1 s time constant before conversion to amplitude and phase for chart recorder and computer logging. The cross-polar channel is calibrated by a pha~or plot method which is interesting. The cross- polar signal is presented as a phasor display and the antenna feed is rotated through a known angle. The phasor traces out a straight line which is horizontal (Fig. 2) if the receiver phase offset is set correctly.

2/9

M!cropnone Re'note I I I On Sde I fL - ~ E F~eld I "-1 E F'.eld | t I I

Arrcly Probe I / Probe I 11 Precision I F~xed I / J ~ m

-7 t ~ ,r, ~ / Rod~r I

.u~t~p',exe~ - - M \ .2 \ # I!--I ' x ADC -- i } 8 Trock I ; / I" I

r .6m / ond ~ I~ I /

. . . . ~ FM Topo h ~_ I I OTS I

I I ; t I I Receivers I ~ " " ' - < __Computer I [ - -- i ~ _ l I < ] -TV

I [ i Elevation I

ppi Rodar I ChQrt ! OTS Adoptive

~{ognetic Recorders CGncel~at ion Tope Deck Receiver

. . . . . . . . . . . . R_e_c_or_d.ng_. E.~.p_rp_ept . . . . . . . . . . . . . . . . . . . . . . . . . .

FIG. 1. - - Equipment for ors experiment at Bradford University (Oxenhope Moor Site).

Equipements utilisds p~ur I'exp~rience OTS ~ 1' Universitd de Bradford (site de Oxenhope Moor).

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104 N. J . M C E W A N . - OTS P R O P A G A T I O N M E A S U R E M E N T S D U R I N G T H U N D E R S T O R M S

Ouodralure

C

~ e r r o r

A ~ "ross polar In Phose

~cross-polar phGsor tip locus-for o feed rototion FIG. 2. - - Phasor diagram for calibration

of TM cross-polar channel.

Diagramme amplitude-phase utilis~ pour calibrer la voie polarisation crois~e du signal TM.

The point " B " of min imum residual is located very accurately and an accurate calibration of XPD and the residual XPD at " B " is obtained.

The B1 receiver works with two channels having good linearity and au tomat ic stabilisation. To check the calibration, it suffices to produce artificial cross- polarization by switching in a known at tenuat ion in one channel entering the 20 kHz matrix. An interesting point is that a phase shift of 30 ~ is introduced before the cross-polar coherent detector, so that the cross- polar phase is referred to vertical, i.e. an in-phase cross-polar componen t denotes a polarization ellipse elongated in a vertical direction (30 ~ ---- 2 x 15 ~ see previous section).

2.3. Fixed antenna radar.

This is a r ranged to provide reflectivity values over 62 range gates extending f rom 0 to 22.3 km along the slant path. The peak power is 3 kW, the frequency 9.4 GHz , and the antenna diameter 1.2 m. The radar features au tomat ic triggering of data logging by presence Of echoes, a continuous automat ic calibration system, and PIN diode X/g switching giving a very short dead- t ime range. It also has a heated feed which eliminates wet radome losses [2]. Video averaging t ime is 1.25 s and the 62 gates are logged in l0 s.

2.4. Electric field sensors.

Each consists o f a pointed electrode at a height of about 5 m. The a tmospher ic electrostatic fields produce a point discharge current (up to ,-- 20 ~A) which is logged. Correlat ions with satellite cross- polarization are observed, and in some events the field and cross-polar peaks coincide. In other events there may be no obvious correlation or a correlation

ANN. TELECOMMUNIC., 36, n ~ 1-2, 1981

with a time offset between the traces. Some examples are discussed later. At present only data f rom the on-site sensor can be presented.

2.5. Fast analogue recordings.

Abrupt changes in cross-polar level, coincident with lightning strokes, were observed in earlier observations with ATS-6. These were the first indications of an impor tant connection between atmospheric electricity and microwave propagation, and were interpreted as an indication of electrostatic alignment of ice particles. The t ime scale of the changes was --, 0.1 s [3].

Knowledge of the precise time scale and structure of such changes is interesting, both f rom the point of view of a system designer concerned with cross-polarization and cancellation systems, and of the physics involved. T o enable such events seen with OTS to be recorded, we have an 8-track FM tape recorder capable of recording with 300 Hz bandwidth for a full day. Four tracks are presently used to record the in-phase and quadra ture components for T M and B1 beacons, these being smoothed with only a 1 ms time constant after coherent detection.

A fifth channel is used to record the a tmospheric electric field sensor output. We have found that lightning activity within a range of at least 15 km produces an unmistakeable pattern of spikes on the electric field record, so we no longer need to use the photoelectric sensor previously used to detect lightning. Abrupt changes have been seen using OTS and some are discussed later. We have also commonly found ordinary cross-polar events showing step changes with a time scale of ,~ 10 s (quite unlike the time structure of fades) for which the FM recorder was very useful.

2.6. Microphone array.

The precise mechanism of the abrupt changes in cross-polarization caused by lightning strokes is unclear, beyond the basic idea of electrostatic particle realignment. To obtain further data on this, it was decided to a t tempt range and bearing measurements of thunder, so that the point of origin of the electrostatic changes would be known. We have used an array of three microphones (Fig. l) arranged in an equilateral triangle of side 90 m. Their signals were recorded in 300 Hz bandwidth on the three remaining tracks of the analogue recorder, after non-linear amplification to enhance dynamic range. The microphones were in shallow pits to reduce wind noise. They are an electret type, o f known sensitivity, with a flat response down to ,~ l Hz. Several thunder recordings have been obtained.

3/'9

N. J . M C E W A N . - OTS P R O P A G A T I O N MEASUREMENTS D U R I N G T H U N D E R S T O R M S 1 0 5

2.7. Other experiments.

Figure 1 shows a 3.0 m antenna which is being used in a new experiment on adaptive cancellation of cross- polarization, but data f rom this are not yet available. It also shows a small scanning radar which produces a map of the gross structure of precipitation in the area, by producing azimuth PPI scans at a sequence of preset elevations. Interpretat ion of recently obtained PPI maps is now in progress.

H~GH CROSS P O L ~ I S A T I O N ~

/ /ran

/. . . . . . ./ / FADE / l ~ - - - - ~ ' /

3. E L E C T R I C F I E L D C O R R E L A T I O N S

Abrupt changes in cross-polarization coincident with lightning strokes are conclusive evidence that cross-polarizing particles can be aligned by electrostatic fields. This suggests that cross-polarization might correlate with potential gradient at the ground. However it is thought that the electrostatic fields are generated by charges on ice particles, and also that the anomalous cross-polarization is caused by ice particles. Hence there might be correlations simply because both phenomena are associated with ice, regardless of al ignment effects. We shall discuss the observed correlation, after considering the pre- dictions of a particular model.

3.1. Model 1 : Cross-polarizing structure itself generates E-field.

The most obvious model is of a moving cell (Fig. 3) having a dipole charge structure aloft. We postulate that the structure is frozen, i.e. that all observed temporal changes are due to translation. We need make no assumption abou t whether alignment is occurring, or whether the cross-polarizing particles are also charge carriers.

I f the cell moves towards the receiving site we expect this sequence of events (Fig. 3) : cross- polarization followed by fade, followed by strong negative electric field. The field may exhibit a reversal, but should be strongest and negative after the fade. (For a dipole structure, because it is reflected in the ground, there is a reversal distance outside which the potential gradient at the ground is positive.) I f the cell moves across, rather than along, the path, there may be no obvious correlation.

/ W-~ / Sr~O ~GArtVE PO~i D ~

FIG. 3. - - Model 1 for electric field effects.

Moddle I pour les effets du champ ~lectrique.

seen, a thunderstorm, which gave a fairly plausible fit to the pattern.

With OTS, we have observed cross-polar events showing no field activity (above the threshold of our point discharge sensor), and others with both present but no clear pattern. In many events, however, there was an obvious correlation with a characteristic type of behaviour. We found that there were coincident burs t s of cross-polar activity and of field activity. Within each burst, there was no obvious correlation between the detailed variations of cross- polarizat ion and of electric field. The remarkable feature was that there was often a very close agreement in the t iming of the beginning and end of the bursts. G o o d examples are given in Figure 4. I t is also worth

1 0 ~ f f . ~ n ~ Cop~ar LeVel(riB)

r 0 - - _ _ z . . . . . i , i r I i / ,2 i4 L ine r XpoLar Lever ( ~ )

FIG. 4. - - Event on 8/8/78 showing coincident bursts of cross-polar and E-field activity.

Ev~nement du 8/8/78 oh se sont produites des variations brutales simultandes du champ E et du signal contrapolaire.

3.2. Observed correlation.

It is interesting that in the OTS data gathered over nearly two years there appear to be no convincing examples of this expected pattern. During earlier observations with ATS-6 at 20 G H z one event was

pointing out that, with ATS-6, two events were seen showing simple coincident peaks in E-field and cross-polar level. One of these events is described in [3].

The problem with explaining this type of event is that the cross-polarization is thought to be occurring in ice particles a long way, perhaps l0 km, f rom the

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106 N. J . M C E W A N . - OTS P R O P A G A T I O N MEASUREMENTS D U R I N G T H U N D E R S T O R M S

receiving site. It is hard to explain coincident variations at the site and 10 km away in terms of translating cells. I f we invoke a large circular ice cell moving across the path, there is no obvious reason why the on-site field sensor would reach its, threshold and start indicating at the same time as the edge of the cell crossed the satellite path. I f the cell is small the problem is worse since the cross-polarization starts as the cell crosses the path, but the distance of the charge dipole from the site, which would deter- mine the on-site field, is stationary at this time for transverse motion.

3.3. Mode l 2 : Electric field generated remotely (Fig. 5).

A possible alternative model for these correlations is that the field changes are due to true time evolution

antenna setteltite path

1 poin t discharge field probe

cross-pokar is ing :.. particles

. . . . . - . = , . : . . . ' . ? ' : ..:....:.-.: . . . : . . . . . . . .

�9 ;.':~:".'i'~"" remo'~e s t r u c t u r e �9 .~;;!..~.:' g ~ i ~ ~ e E ~d

FIG. 5. - - Model 2 for electric field effects.

Moddle 2 pour les effets du champ dlectrique.

peaking between 6 and 7 dB, and 3 peaking between 5 and 6 dB, have been observed in 1,5 year of continuous data, with 6 dB being exceeded for less than l min.

By far the largest part of severe cross-polarization at this site was of the anomalous, ice-induced type. In these events, if a significant fade was present it tended to occur in the latter half of, or even after, the duration of the cross-polarization. This was consistent with cells approaching from the south- west so that the cell top crossed the link before the wet lower part. (At our site OTS was at elevation 27.6 ~ and azimuth 165.3 ~ east.) Availability of an absolute calibration of cross-polar relative phases has confirmed the prediction [4, 5] of near-quadrature phase for linear polarization in anomalous events. In circular polarization, cross-polar phase for anomalous cross-polarization is a direct indication of the principal plane directions of the ice medium. It has been generally found that the principal planes are near polarization angles 0 ~ and 90 ~ as is expected for a populat ion of horizontal ice plates, or of ice needles lying in a horizontal plane with random azimuth angles. Some noticeable departures do occur as in the event discussed next.

5. T H U N D E R S T O R M EVENT

rather than just translation, and that there is a remote, active cell generating a variable E field which is seen by both the on-site field sensor and the cross-polarizing particles which are aligned by it. We may suppose that the particles are randomly oriented, hence non- cross-polarizing, until a field is present. It is thought that the charging mechanism in electrically active cells is a positive feedback process. The fairly well- defined onset of cross-polarization and point-discharge activity could indicate the time at which the charging process begins.

It must be emphasized that these models are entirely speculative at present. We hope to investigate them further by using fixed and scanning radar data. Nevertheless, there are undoubtedly real correlations and these leave much to explain.

4. GENERAL OBSERVATIONS ON P R O P A G A T I O N

Since the data are not yet exhaustively analysed, we comment briefly on a few generally observed trends. Firstly we note that the OTS data continue to confirm the impression that the receiving site is subject to much less than average fading. One fade

The event now described was an unexpected winter thunderstorm which occurred when the ground air temperature was around 8~ above average for the time of year, on 3 December 1979. Figure 6 is a general timeplot of the event. The fade at the beginning occurred when hail was falling. Nearly all the cross- polarizations was anomalous, with very little fading. The electric field/cross-polar variations conform roughly to the coincident burst pattern, though in this case the electric field trace showed many spikes and reversals due to lightning. The accuracy of Figure 6 has been improved by a careful vectorial subtraction of the instrumental residual cross-polar levels seen in clear weather.

Plots (e) and (f) in Figure 6 show reflectivity obtained from the fixed-antenna radar integrated over range gates 15 to 62 (5.4 km to 22.3 km range) and 23 to 62 (8.3 km to 22.3 km). There is an obvious correlation with the bursts of anomalous cross- polarization. If range gates before number 15 are included in the integrated reflectivity, the resulting curve rapidly ceases to bear any resemblance to the cross-polar plot but closely resembles the fade plot. In fact, in going from plot (f) to plot (e), i.e. adding in gates 15 to 22, we improve the correlation with cross-polarization in the second half of the event but introduce also an extra peak at the beginning which is very large in relation to any cross-polarization

ANN. TI~L~COMMUNIC., 36, n ~ 1-2, 1981 5/9

N.J. MCEWAN. -- OTS PROPAGATION MEASUREMENTS DURING THUNDERSTORMS 107

a Fade lOB) t ~ sc~e 3ran -- - - -- 0 U ~

:i T 7

~o

-5o

cc,o~

o ~

d EteFtrlc fie:d dlschQrge current

~ In~egra~t~ ~ f I~ ~ i~ r 5 ~ ~o Z2 3 ~m (~z f ~da)

f

' , J ~ j ' ,

. .

f ~r~egtG~ed RML~tivity. 83 to 223Km: z)" (dB)

FIG. 6. - - General time plot of thunderstorm on 3/12/79. Lightning strokes are numbered on Electric Field Trace. Strokes 1-4 occurred before propagation effects started, but appear

in Fig. 10.

Enregistrement de rorage du 3/12/79. Les #clairs sont compt~s et indiqu~s sur la vole du champ dlectrique. Les dclairs 1-4 se sont produits avant le d~but des effets sur la propagation mais

ils sont indus dans la figure 10.

that it produces. I t is inferred that in gates 15 to 22 (~, 5 to : ~ 8 km) large particles (e.g. hail) and/or melting particles, giving relatively little cross-polarization, and smaller particles which are strongly cross- polarizing, were each present at various times. An interesting feature seen in plot (f) is the appearance of three peaks x, y, z lining up with peaks seen in Iinear, but not circular, cross-polar level. As discussed later, the depar ture between linear and circular cross- polar plots at this point was due to a change in the mean al ignment angle of the cross-polarizing medium. It is likely that the radar was here seeing the appearance of groups of particles which were differently aligned f rom those in the 5 to 8 km range.

The linear (but not circular) cross-polar level is very sensitive to mean alignment in this par t of the plot. The general deduction f rom the radar data is that most o f the cross-polarization in this event arose f rom particles lying at slant ranges between 5 and 12 km. Though there was no well defined bright band in this event, except possibly around gates 4 and 5, it is safe to assume these particles were ice because they caused little fading.

Figure 7 a and b show cumulative plots o f the two cross-polar phasors for the entire event. I t is emphasized that phase has an absolute physical meaning in these plots because of the special methods used to calibrate the receivers. For an event dominated, as this was by differential phase shift effects, the linear (TM) cross-polar phase was near - - 90 ~ as expected.

:lro, t u r e

%

�9 I n I ~ t se

0 u a r o t u re

' ~ 1 7 6 1 7 6 *

~ I

" " . I ~ ~ " b % " ~ ~

; b~ �9

FIG. 7. - - Cumulative cross-polar phasor plots for linear (left) and circular polarization. The linear quadrature component is inverted to save space. Radius scale is linear and gives XPD,

not signal level.

Phaseurs des polarisations crois~es pour les polarisations lindaire et circulaire (gauche}. La composante en quadrature de la polarisation lindaire a dtd invers~e pour r~duire la figure. L'dehelle radiale est linkaire et donne le XPD et non pas le niveau du signal.

I f the differential phase shift medium has a l inear principal plane of polar izat ion at angle 0 clockwise f rom vertical as seen looking towards the satellite, and if this plane experiences less phase delay than the or thogonal plane, then the B I circular cross- polar phase should be - - 90 ~ + 2 0 (N.B. in all plots of the T M cross-polar phasors, the quadra tu re component is shown inverted as this saves space). It is seen that, for mos t o f the time there is a principal plane (with less phase delay) near vertical, but tha t at certain times it moves as much as 30 ~ anticlockwise. A model for this is suggested presently.

5.1. Prediction of linear cross-polarization from circular.

I f the med ium is dominated by differential phase shift, and homogeneous in alignment, it can be described by only two parameters , a differential phase shift W and a principal plane angle 0. As noted, the circular cross-polar phase gives a sensitive measure of 0 while tF can be inferred just f rom its ampli tude. The inferred 0 and tF can be used to predict the cross- polar ampl i tude and phase that would be expected for the linear T M beacon. The predicted phase gives little informat ion as it is very close to quadrature . The predicted ampl i tude is plotted in Figure 6 b and for most of the event it is seen to agree r emarkab ly well with observation. I t is particularly interesting that successful prediction is achieved around x, y, z and other points where the linear and circular cross- polarization variat ions differ f rom each other because the principal planes have moved anticlockwise, passing through the ,-~ 15 ~ c c w polarization angle o f the TM link. Full advantage has here been taken of the

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108 N.J . MCEWAN. -- OTS PROPAGATION MEASUREMENTS DURING THUNDERSTORMS

absolute B1 cross-polar phase, and accurate ampli- tudes, offered by our calibration methods. This is believed to be the first prediction of l inear from c i rcu lar using absolute phase information. Where departures between predicted and observed occur, they may be due to complex particle alignment or to the presence of slight differential attenuation. This requires further analysis. There may be a larger departure at the beginning of the event due to hail lingering on the antenna.

Loci showing the time evolution of the phasors during selected periods are given in Figure 8. The deviation of circular cross-polar phase f rom quadrature in these loci account for the dips in linear cross- polar level around x, y, z and t in Figure 6.

Quo |rature

"/" ~ d B

I In phas~

Qua Jralure

~ t

In phase

QuOlrQture

x

Y

In l:~os e

FJo. 8. - - Cross-polar phasor plots (XPD) correspondent to the time intervals between points (a) E-F ; (b) C-D ; (c) A-B marked on Fig. 6. Linear quadrature component is again

inverted.

Evolution des phaseurs de polarisations croisdes (XPD) corres- pondant aux intervalles de temps entre les points (a) E-F; (b) C-D ; (c) A-B indiqu6s figure 6. La comlzosante lineaire en

quadrature est de nouveau inversde.

Qua Iroture Qua Jrot ure -20dB -20riB

,BI

o Predicted T~

In phase

Stroke n ~ 13

- 32d B

In phase

n- ~ It,

nature Qua -20dB

:lnature -20dB

t f _ _ --32dB

In phase

rr 16 n~17

FIG. 9. - - Cross-polar phasors before and after lightning induced jumps. The exact path between the beginning and end points is not defined. Lightning strokes numbered as in Fig. 6. Linear

quadrature component is again inverted.

Phaseurs de polarisation crois~e avant et aprds les sauts provoqu~s par les coups de tonnerre. Le chemin exact d'un point d I'autre n'est pas bien connu. Les dclairs sont num~rotds de la m~me manidre que dans la figure 6. La composante lin~aire en quadrature

est de nouveau invers6e.

Using the b roadband analogue recording, the rise times of the jumps were found to be ,--0.3 s for stroke 13 and ,-~ 0.25 s for the others. Rise times of this order have been found for other OTS events. Values were around 0.1 - - 0.2 s in the ATS-6 event [3]. It is curious that slower jumps have not been seen. It may indicate that a critical field change is needed to produce the effect at all, and that once this level is reached the effect is inevitably fast. Cross-polar step changes on a scale of abou t 10 s are seen in many events but these do not correlate with electrostatic events and are probably bulk effects. A fast recording system is desirable to distinguish them.

5.2. Abrupt changes in cross-polarization.

Of the many lightning strokes seen on the E-field trace, just four produced large abrupt changes in cross-polarization. Those strokes are numbered 13, 14, 16 and 17 in Figure 6. Figure 9 shows the tracks followed by the cross-polar phasors during these jumps. I t also shows the linear cross-polar jumps predicted as before f rom the circular ones, and agreeing well with observation.

Three out of four jumps showed a sudden rise in cross-polarization and this now seems to be a general trend. A preponderance of rising jumps was found in the first example of the effect seen with ATS-6 [3], and in other OTS events we cannot present here. This remains unexplained, though it is consistent with ice needles lining up with axes vertical b e f o r e a stroke. I t is interesting ( though unexplained) that in this event all the cross-polar j umps occurred for strokes at which the change in E-field was of negative sign.

5.3. Acoustic location of thunder.

Using the microphone array, it has been possible to locate many of the thunderclaps. Figure 10 is the resulting PPI map. Some height information is also available but not used here. Events 13, 14 and 17 are thought to have been cloud-to-ground strokes while 16 was probably cloud-to-cloud, as deduced f rom a different sound signature and E-field pattern. The map shows a north-easterly movement of the activity which ended with stroke 17 quite near the receiving site. The movemen t is consistent with the south-westerly wind direction. The stroke locations show possible correlations with the behaviour of cross- polar jumps as discussed below. Since the cross- polarizing region was at slant ranges between 5 and 12 km, we deduce f rom Figure 10 that abrupt cross-polar changes can be produced by electrostatic jumps originating at least 5 km away f rom the cross- polarizing particles. A remarkable feature of the

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N. J . MCEWA N . -- OTS P R O P A G A T I O N MEASUREMENTS D U R I N G T H U N D E R S T O R M S 109

o 5

N ~

__~ site o14 o 17 ~ 7

6 1013 o 9 \ "16 ~ ~ \

\ 1kin

o 12

o l

FIG. 10. - - Maps of acoustically located lightning strokes. Numbers as in Fig. 6.

Localisation acoustique des coups de tonnerre. Num~ros identiques h ceux de la fig. 6.

f rom the south-west. To complete this model we have to suppose that horizontally-aligned ice plates were also present, since for needles aligned as described, there would be excess phase lag for the principal plane nearer vertical. We must then suppose that the needle cross-polarizing effect decreased at a jump, as would happen if the elevation of the needle axes reduced (e.g. an on e n d vertical alignment collapsing into horizontal in a weakened field). The data strongly suggest that o u t - o f - h o r i z o n t a l ice needle (or plate) alignment occurs in thunderstorms, but the Hawor th model [6, 7] may well be correct for most events.

A final comment about this s torm is that it started with a (probably hail induced) fade, even though it was approaching from the south-west. It was an atypical event for this site, being a frontal thunderstorm occurring in winter.

phasor plots of the jumps is that in three out of four, the c ircu lar phasors move almost entirely along the quadrature axis. This is hard to explain in our earlier model [6, 7] which postulates that ice needles are aerodynamically aligned with their axes horizontal, and the azimuth angle of the axis is determined by the horizontal component of the electrostatic field. With this model much larger jumps in circular cross-polar phase are expected if the azimuth angle changes.

I f we retain the idea of re-aligning needles, we must suppose instead that they can show out-of-horizontal alignment. If, however, they jump from a vertical to a horizontal alignment, it remains to be explained why the azimuth angle of the axes in the horizontal state should still give near-quadrature phase after the jump. A possible explanation is as follows : the azimuth of the needle axes was lined up with the horizontal component o f the E-field (A stronger hypothesis, though not needed for this explanation, is that the needle axis actually lay along the E-field lines). We propose that there was a charge centre close to the located thunder positions, which moved north-east over the receiving site. The horizontal field component seen by the particles would have been in a radial direction f rom the charge centre. At the time of the jumps, the charge centre (Fig. 10) was near the site, so the azimuth of alignment was nearly coincident with that of the satellite path, giving near quadrature phase. This model makes a prediction : up to the time of stroke 17, there should have been a downward drift o f circular cross-polar phase as the charge centre moved north-east. This was borne out to some extent, since there was such a drift from about stroke 13 to a few minutes after stroke 17. Stroke 17, the most north-easterly, also showed the largest deviation from the simple quadrature movement. The phase behaviour after this may well have been confused by further charge structures moving over

6. C O N C L U S I O N S

The observations described here are probably the most comprehensive characterisation of satellite-earth microwave propagation yet attempted. They are noteworthy for the auxiliary instrumentation used and for the complete measurement o f the incoming polarizations.

The basic concepts of satellite-earth propagat ion seem, from these measurements, to be fairly well understood. As has been further confirmed by radar data presented here, ice crystals are a major factor in a cross-polarization at this site. The ice medium can be treated quite effectively as a differential phase shift medium whose principal planes (eigen-polarizations) are linear and tend to be near polarization angles 0 ~ and 90 ~ with excess phase lag in the horizontal polarization. Deviations of angle up to 30 ~ sometimes occur, however. The correct prediction of near-quadrature anomalous cross-polar phase in linear polarization, and the successful prediction of linear cross-polarization from circular, confirm the validity of the basic theory. Similar general conclusions [8] about principal plane angles have been reached in the COMSTAR satellite propagat ion experiment, where a complete 19 G H z transmission matrix has been determined by polarization switching.

Ice crystals can be aligned by electrostatic fields, but these fields are themselves generated by charges carried on ice particles. Observed correlations between cross-polarization and electrostatic field were discussed and two basic models proposed. More work can be done to clarify the relationship between cross-polarizing and charge-carrying particles.

We reported here the first observation of a thunderstorm made with a satellite receiver and with instru- ments to locate lightning strokes. It was confirmed that particles can be re-aligned at lightning strokes

8/9 ANN. T I ~ L ~ C O M M U N I C . , 36, n ~ 1-2, 1981

110 N.J . McEWAN. - OTS PROPAGATION MEASUREMENTS DURING THUNDERSTORMS

in t imes o f 0.2 - - 0.3 s. I t was d e d u c e d t ha t the par - t icles can be a t leas t 5 k m f rom the l igh tn ing channel when this h a p p e n s , a n d tha t out-of-horizontal par t ic le a l i g n m e n t m u s t be occurr ing. U s i n g the t h u n d e r l oca t i on d a t a , we m a d e a p laus ib le a l i g n m e n t mode l in which the par t i c le axes wou ld l ine up in a z i m u t h wi th the h o r i z o n t a l c o m p o n e n t o f the e lec t ros ta t ic field.

F i n a l l y i t is w o r t h c o m m e n t i n g on the re la t ive i m p o r t a n c e to sys tems engineer ing o f ice a n d ra in effects. O u r obse rva t i ons con t r a s t wi th those a t some U S A sites [9, 10], where ice effects a p p e a r in f requen t a n d re la t ive ly sl ight, a n d the wors t XPD'S occur in deep r a in fades. W e ra re ly see deep fades , b u t ice effects have given XPD'S o f 20 dB a t 11.56 H z (and 13.5 dB a t 20 G H z wi th ATS-6).

I t m a y be n o t e d tha t there a re some satel l i te s i tua- t ions where c r o s s - p o l a r level, no t c r o s s - p o l a r i so la t ion , is the key sys tems factor . A n e x a m p l e is where the c r o s s - p o l a r s ignal p r o d u c e d on an up l ink to one

satel l i te a p p e a r s as in te r fe rence on a nea rby sa te l l i te . I t is a lso pos s ib l e fo r c ro s s -po l a r i za t i on on a n u p l ink to be f o l l o w e d b y f ad ing on a different d o w n link. Ra in g iv ing the same c ros s -po la r i so l a t i on as ice will p r o d u c e m u c h lower c ro s s -po l a r s ignal level. There are a l so cases where the de le ter ious effect o f c ross - ta lk m a y be i n d e p e n d e n t o f fade, an e x a m p l e be ing vis ible p a t t e r n i n g p r o d u c e d on FM TV signals due to c ross - ta lk . F o r these reasons we feel it shou ld no t be a s s u m e d w i t h o u t fu r the r ana lys is t ha t ice effects a re o f l i t t le i m p o r t a n c e in systems engineer ing.

A CKNO W L E D G E M E N T S .

Support f rom the UK Science Research Council f o r propagation research at Bradford University is grate- ful ly acknowledged. A. P. Alves is supported by Insti- tuto Nacional de lnvestigacao Cientifica, Portugal.

Manuscrit requ le 10 aofft 1980.

R E F E R E N C E S

[1] ALVES (A. P.), McEWAN (N. J.). Arbitrary Polarization microwave receiver applied to OTS reception. Electron Lett., UK (27th Sept. 1979), 15, n ~ 20, pp. 657-658.

[2] McEwAN (N. J.), CHEUNG (T. M.), DISSANAYAKE (A. W.), WATSON (P. A.). A self calibrating 9.4 GHz meteorological radar. Proceedings of a one-day workshop sponsored by ESA, held at Technical University of Graz, Austria, on November 1978. Technical University of Graz Publication.

[3] HAWORTH (D. P.), McEWAN (N. J.), WATSON (P.A.). Relationship between atmospheric electricity and microwave radio propagation. Nature, UK (21st April 1977), 266, 703.

[4] McEWAN (N. J.). Phase of crosspolarized signals on microwave satellite links. Electron Lett., UK (4th August 1977), 13, n ~ 16, pp. 489-491.

[5] EVANS (B. G.), HOLT (A. R.). Cross-polarization phase due to ice crystals on microwave satellite paths. Electron Lett. UK (27th Oct. 1977), 13, n ~ 22, pp. 664-666.

[6] HAWORTH (D. P.), WATSON (P. A.), McEWAN (N.J.). Model for the effect of Electric Fields on satellite-earth microwave radio propagation. Electron Lett., UK (15th Sept. 1977), 13, n ~ 19, pp. 562-567.

[7] HAWORTH (D. P.), McEWAN (N.J.), WATSON (P. A.). Cross-polarization for linearly and circularly polarized waves propagating throtigh a population of ice particles on satellite-earth paths..Electron Lett., UK (10th Nov. 1977), 13, n ~ 23, pp. 7~3-704.

[8] ARNOLD (H. W.), Cox (D. C.). Dependence of depolarization on incident polarization for 19 GHz satellite signals. Bell Syst. tech. J. USA (1978), 57, pp. 3267-3275.

[9] Cox (D. C.). Depolarization of radio waves by atmospheric hydrometeors in Earth-Space paths : a review. Review Paper presented at URSI Commission of Open Symposium. ~ Effects of the lower atmosphere on Radio Propagation at Frequencies above 10 GHz >>, held at Lennoxville, Quebec, Canada (26-30 May 1980). To be published in Radio Science.

ll0l BOSTIAN (C. W.), PRATT (T.), TSOLAKIS (A.). Ice depolarization at 11.7 and 19 GHz on slant paths in Virginia, USA. Proceedings of URSI Commission F Open Symposium. ~ Effects of the lower atmosphere on radio propagation at frequencies above 10 GHz >>, held at Lennoxville, Quebec, Canada (26-30 May 1980).

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ots propagation measurements during thunderstorms

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