The influence of large-scale TIDs on the bearings of geographically spaced HF transmissions

The influence of large-scale TIDs on the bearings of geographically spaced HF transmissions B. L. TEDD,‘* H. J. STRANGEWAYS?and T. B. JOKES’ ’ Physics Department, Leicester University, Leicester LEI 7RH. U.K., and ’ Department of Physics, University of Sheffteld,Sheffield S? 7KH. U.K. (Receiued injinoljorm 22 August 1983) Abstract-- An investigation is made of the adequacy of the simple corrugated reflector model to simulate the effectsof large-scale travelling ionospheric disturbances (TIDs) on the bearings of HF transmissions.Model results are compared with the quasi-periodic variations of bearings measured simultaneously for a number of geographically spaced transmission paths and with results obtained from 3D ray tracing Ptudies incorporating a more realistic TID model (the ‘wave’ model). Although thebearingerrorsignatureis.atsome times(particularlyduring theday).similar~o that predicted by the corrugated reflector model, ray tracing calculations with the ‘wave’ model give generally better results and predict bearing error signatures which correspond much better to some of those observed in the experimental data than those predicted by the simpler model. However, the prediction accurac! of borh models is found to be limited by temporal variation of both the TID waveform and the ambient ionosphere.

2. COMPUTATIONAL

I. INTRODUCHON

Large-scale travelling ionospheric disturbances (TIDs) produce gradients in electron density transverse to the direction of propagation of an HF radiowave, which give rise to oITgreat-circle propagation. The resulting bearingerrorscan be verylarge( * S.O’)compared with the instrumental error (- O.S”) and are therefore of great importance in practical HF direction finding. Large-scale TIDs are the ionospheric manifestation of atmospheric gravity waves (AGWs) whose sources are generally related to aurora1 disturbances during magneticstorms(GEo~~Es, 1968; FRANCIS,1975).They have quasi-periods of about 30 min4 h, horizontal speedsof30@-IOOOms-’ in a north to south direction in the northern hemisphere and horizontal wavelengths greater than 1000 km. The influence of a TID on the bearing (azimuthal angle of arrival) of a one-hop HF signal can be simulated by means of the corrugated reflector model (WALTON and BAILEY, 1976; LYON,1979). In this paper, the adequacy of the model to simulate the effects of large-scale TlDs is investigated by comparing the computed results with both experimental observations and results obtained from a 3D ray tracing model. -_-. __ * Present address : Radiophysics Laboratory, Thayer School of Engineering, Dartmouth College. New Hampshire, IJ.S.A. t To whom reprint requests should be addressed.

2.1. The corrugated

AND EXPERIVESTIL

wflecror

DETAILS

model

A comprehensive description of the corrugated reflector model has been given b!- LYOS (1979). Reflection of an HF wave is assumed to take place at a sinusoidally corrugated horizontal surface of constant electron density (see Fig. I) which is given by -_=h+Acos

r%

1

LiJ

where (_u,4’.Z)is an orthogonal co-ordinate system with the origin at the transmitter. 5 is the height above the earth’s surface, .Y represents horizontal distance in the wave propagation direction, and 11.i and A are the height, (horizontal) wavelength and amplitude of the model TID, respectively. The horizontal motion of the disturbance is taken into account bv a phase factor. The location of the reflection point and the corresponding value of bearing error (defined as the difference between the computed and true bearings of the transmitter, measured clockwise) are calculated from the path geometry and the reflection condition. by means of an iterative technique. 2.2. JD ray trucittg itnwtigtrtic~t~ A more realistic treatment ol the TID-HF wave interaction which accounts for both the refraction and reflection of a radiowave through a perturbed 109

L. TEUD. H. J.

H.

I IO

and T. I3. JONW

STRANGEWAYS

2.3. Experinwntal Bearing direction

ohserwtions

measurements

were

two concentric The

with

beam

forming

determining

of 24

located in southern England.

network

and the algorithm

iscontrolled

to monitor

transmissions

finder

which was

the bearings of ;I number ol

(generally

was automatically

Fig. 1. Geometry of the corrugated reflector model.

1l/O3 minicomputer

by a PDP

programmed HF

for

the bearing has been described elsewhere

(HOCKLEY, 1973; GETHINC;. 1978). The direction I

a

array of

rings (150 and SO m diameter)

elevated feed monopoles, h

performed

finder consisting of a Wullenweber

3--8) sequentially.

transferred

[)ata

floppy disk. In this

to

way bearing data could be collected for long periods described

GEORGES

ionosphere

has been

by

STEPHENSON

(I 969). The ‘wave’ model has been em(e.g. TREHARNE,

ployed in a number of investigations 1972). Ray paths are calculated

and

through

a disturbed

(days) without

any operator

program. The electron density, N, is given by

were

identified

centred

is the ambient

spherical

perturbation

electron

polar

(2)

density

co-ordinates

produced by the TID,

and

by rapid

1981 and I6 large-scale TID in the

induced bearing

where N,

bearing

records.

fluctuations

in the bearing

(time scales of less than

/I is the

interference

between

components

ens

z.(

transmissions it can mode was one-

hop F.

[z]Tx(1:‘)f3) II.-0

t’ +

{

by wave

ol’ P multi-modcd

however be assumed that the dominant

x

TID-

measurcmcnts

I min) produced

signal. For most ofthe monitored

/i = d cxp - [(r .- R,, - -‘,,):‘H]’

events

The

errors weresometimes partly obscured

in earth-

given by

are indi-

Some 700 h of data were acquired between October 1979 and March

+/I)

The loca-

monitored

cated in Fig. 2.

ionosphere by means olthe JONES( 1966) 3D ray tracing

:X = N&,n,fj)*(l

intervention.

tions of some of the transmitters

_2 +

.‘:--. P.

3. MODEL RESULTS AND CO~If’.~RISON WITH EXPERIILIENTAL DAT.\

where R, is the earth’s radius

3.1. Bearing m-or magnitude

I() is the height of the maximum H is the wave amplitude

wave amplitude Parameters

‘scale height’

i, and i, are the horizontal

and vertical wavelengths

6 is the wave perturbation

amplitude.

large-scale

representative

TID

paths have been input

of a

ol the charitcteristics

and of the monitored

transmission

into the c~,rrugared

rcllector

model. The model results, which are presented in Fig. 3. Theperturbation horizontal ward

hasthefollowingcharacteristics:(a)a

sinusoidal

time dependence:

sense of phase propagation;

variation

of amplitude

~,,givingan

(c) a Gaussian

with height centred on altitude

incrcaseofthe

below zn and attenuation to south propagation

(b) a down-

waveamplitude

illustrate

the

geometry

on the computed

influence

The error magnitude

of

with height

direction.

path length dependence obtained

for

the

in the experimental

bearing

Luxembourg

the Wertachtal

computing

shape, but differ in their

between

two

fixed

and the cumbersome

algorithm,

points

on

Nevertheless. a few calculations and compared corrugated

with

that traces a ray

the earth’s

have been performed

the results obtained

from

reflector model. Single hop ordinary

ray paths are calculated

which

terminate

ground range of 5 km from the stipulated location.

surface. the

within transmitter

a

error

data

is

time signatures

(R = 633

km)

and

(R = 1050 km) paths ‘ire similar

in

peak to peak magnitudes

(approximately

6’ and 3.5 ‘, respectively). Thcsc rrans-

mission

have almost

paths

ings and their

mode

path at !hc

shorter path lengths (see Fig. 3a). An csamplc of this shown in Fig. 4. The

The ‘wave’ model is less suitable than the corrugated

path

increases with decreasing

reflector model for extensive modelling. due to the large nature ol’ the homing-in

transmission

length R, the increase being most appreciable

above this height :(d)a north

resources required

the

beacing error magnitude.

respective

close together that the TID

equal

great-&dc

hcar-

mid-points

are suniciently

amplitude

can hc assumed

to change little as it crosses the two pnth~. .This li~ll~ws from

R~c,r~uos~,

and

MATSUSHITA ( 1975) and S-II:RI.IU;ci d. (lY71),

the

who

determined

theoretical

works

the latitudinal

of

dependence

ol‘ large-scale

+ I; Madrl d

Fig. 2. Locations of some of the trxxmitt~rs

monitored.

_^_..----R-123C’km

c-c--I___-.--

1200 1500 ‘A-2biiF-?ioc Path lengrt,.R ikmi

ROC

Fig. .3. InIluence of the transmission path geometry on thz computed brarin.g error m+nitud< IwrrugtiI~d reflector model) :(~)function ofpath length, R(I) = 90 . A := 40km. i =z WtX1)(1 km)(b) function ofarimuth xngl~ 11between the TtD direction and the HF’ path (II = 700 km. .-I -- 40 km. j :- 3(H!O kml.

B. L. ‘TEDD.H. J.

112

STRANGEWAYS

and

T. B. JONFS

Great circle 5.

I

Luxembourg

6.0900

MHZ.

bearing = 109.3

I

R=633

I

KM

I

1

-25 2: d C

-1.0

ho Wertochtol

P ._

-50 200

Great

11.7200 MHZ.

I

I

I 20.5

21 0

circle

bearinp:ll3.6

I 215 4

Time

R=1050

KM

I

I

I

I 22.0

225

I 2.30

I 23:

0

in hours (UT1

Fig. 4. Bearing data recorded on 31 March 1981. Each datum point denotes an 8 s average of the HF bearing measuredat90sintervals.Thehorizontaldashed linerepresentsthetruegreat-circle bearingofthetransmitter at the direction finder. Bearings that lieoutside the ordinate range are represented by a w(wild). An .Ydenotes a timefor which thedirectionfinderfaikd toobtainabearing.Asmoothlineisdrawn through thebearingpoints to highlight the variation of interest.

AGWs and their ionospheric responses, respectively. Only small changes in TID amplitude were shown to occur over distances of many hundreds of kilometers. whereas the reflection points of the Luxembourg and Wertachtal transmitters (assuming one-hop F propagation) are separated by less than 150 km in the northsouth direction. Any variation of TID amplitude with height would also influence the bearing error magnitude if the two reflection heights are unequal, however, these differ by only a few tens of kilometers and consequently the TID amplitude will be essentially the same at both. Thus the observed dilference in bearing error between the two paths must result from the difference in transmission path lengths. The height of the reflection surface h can also be varied in the corrugated reflector model to illustrate the dependence of bearing error magnitude on the signal reflection height. This can also show the dependence on the signal frequency for a fixed path, since the reflection height increases with increasing frequency. The model results, in thiscase,aremoreapproximateas(unlike the wave model) the TID amplitude is taken to be the same at each height whereas it can vary significantly with height. A maximum in the amplitude of large-scale TIDs generally occurs within a height range of 2060 km in the infiection region of the electron density profile below the f’ layer peak, and the amplitude is smaller at heights both close to and well below the F layer peak (FRANCIS,1973; MORGAN and BALLARD,1978). This will result in signals reflected at quite different altitudes

experiencing significantly dilferent lateral path deviation effects. The dependence of bearing error magnitude on the azimuthal orientation of the signal path relative to the TIDdirection Uis illustrated in Fig. 3b. The largest and smallest bearing errors are obtained for (i = 90; and O“, respectively. The data recorded on 26 -March 1981, presented in Fig. 5, illustrate this dependence. The TIDinduced bearing errors are much larger on the east-west 10.15 MHz Prague path during the period 18.30-20.30 UT than observed on the south-north 9.57 MHz Madrid path between 19.00 and 21.00 UT. Both transmissions have similar frequencies and path lengths and the reflection points are sufliciently close that the TID amplitude can be assumed to change little as the disturbance travels between them. The observed bearing error magnitudes are consistent with the equatorward velocity calculated for the disturbance from cross-correlation of the bearing time series. The influence of the transmission path geometry on the magnitude of the TlD induced bearin errors has also been investigated by means of the ‘wave model. Similar qualitative results are obtained to those for the corrugated reflector model, but quantitativedifferences exist due to the vertical amplitude lariation of the disturbance which is included in the ‘icave’ model. Simple geometric elfects are often obscured in the bearing data. however. due to other ditlerences between monitored transmissions, such ~5 transmitter frequency.

.l‘he bearings of geographically

Prague

Great circle bearing-92.3

IO.1250 MU.?.

113

spaced H F trunsmissions

I?=1220

KM

a”--

50,

9 5700

Madrld

Great circle bearing: 185.3

MHZ

R=l333

KM

Fip. 5. Hearing data recorded on 26 M:trch

19X I

----

,

_Great Luxembourg 6.0900

circle

MHZ

Other factors also intluence the bearing error. The lateral path deviation of an H F radio wave depends not only on the horizontal

gradients ofelec:ron

density but

Large TID-induced sometimes

observed

bearing

error

magnitudes

tor the Moscow

whereas only small errors would be predicted

becau~c

iR = 2.586 kmr.

also on the proximity of the signal frequency to the path

of its long

MLJ~(TI~H~~~INx.

probable

explanation

multihop

signal is the dominant

scale TID

extends over ;1 very large geographic:rl

be produced approached. variation shown

lY5X).Verylargebrarinperrorscan

as the layer

penetration

condition

is

An example of this effect is the bearing

for the Luxembourg

in Fig. 6. The

signal on I3 May

1980.

large scatter in the bearings

transmission

it can introduce or more

same direction.

frequency being close to the path MI! F.

erior.

I.:.

Lwmbourg

6 3900

MHZ r

Great cwcle bearing = 109.3

Rr633

Time in “ours (UT)

path

is that. during

points,

mode. Since a large-

which,

*

I

;rrc‘i!.

elfect ;~t t\vSr

if in rouphl!

will add to produce

KM

.I

thcs~ time2;. Lo

a lateral path deviation

reflection

between 23.45 and 00.30 IJT results from the signal

arc

transmission.

IIN

a large bearing

I

x-

114

B. L. TWD,

H. J. S~~RANOEWAYS and T. B.

Fig. 7. Influence of the relative direction of propagation of a TID to thatofanHFwaveon thebearingerror timesignature (corrugated re&ctof model).

3.2. Bearing error time signature

The bearing error time signature predicted by the corrugated reflector model exhibits a characteristic saw-tooth shape which depends on the azimuthal orientation of the signal path relative to the TID direction (WALTON and BAILEY,1976). This behaviour is illustrated in Fig. 7 for model parameters chosen to emphasise the shape dependence on the value of the phaseangleO.Similarresultsareobtainedfor themodel parameters employed in Fig. 3, but there the effect is smaller. Reciprocity considerations dictate that if the direction of motion of the TID is changed by 180”, then the saw-tooth pattern of the bearing variation wiil

JONES

be laterally reversed {mirror image in the ordinate). A qualitative assessment of TID direction should therefore be possible by comparing these model results with the experimental bearing signatures. Several examples of the saw-tooth shape can be seen in the bearing records, e.g. the TID event of 31 March 1981 shown in Fig 4 and for the Moscow record of I7 May 1980 shown in Fig 8. Holvever, for many of the 16 TID events studied, the saw-tcoth variation was not clear, suggesting that other effects were also influencing the bearing error signature. In order to check the accuracy of the corrugated reflector model, simulations have also been performed for the ‘wave’ model for an ionosphere disturbed by a large-scale TID of quasi-period of about I h. The computed bearing error time signatures are found to depart significantly from a purely sinusoidal or sawtooth shape, as shown in Fig. 9. They compromise a sharp maximum (peak) followed by a broad minimum (trough), or vice-versa depending on the azimuthal orientation of the transmission path relative to the TID direction. The peak-trough variation is superimposed on an underlying saw-tooth variation which has a similar form to that obtained for the corrugated reflector model. Many examples of this type of bearing error variation are present in the experimental bearing data. For example. 3 north-south propagating disturbance is indicated from the bearings of the Prague record of Fig. 5; this is in agreement with the TID direction calculated for this event from crosscorrelation between the bearing error variation on different transmission paths. Another example of the

_..~.’

._, __ ._......_- - .._--vT_*_-_

I

-10.0

-

9.0

d

1 9.5

I

IO 0

i 10.5

iJ

I .d--.I

1.o

II 5

Time in hours (UT)

Fig. 8. Bearing data recorded on I7

May 1980.

I2 c

125

:

_.-I

I30

The hearings of geographically

2

t

!a

spaced HF transmissions

D East-west transmtssion path o West -east

tronsmlsston

path

-2t

I

-3L Fig. 9. The bearing error variation calculated h! means of the ‘wan’ model. The ambiem ionosphere was represented by an z-Chapman profile with critIcal frequency IO MHz, peak height 315 km and scale height 62 km. The ray tracing model employed a dipole magnetic field and the refractive index was calculated from thr Appleton-Hartree equation with collisions included. The TID characteristics employed were >., = 1000km, & = 200 km, B = 0.5; I,, = 300 km, H = 100 km and a north-south TID propagation direction. Rays were computed for a frequency of 10 MHz and a transmission path length of 1000 km.

peak-trough signature is exhibited by the TID of Fig. 8. although in this case the expected form is less clear. The bearing error time signatures are often quite different from one event to another. This variation can partly be resolved by differences found when different TID parameters are employed in the two models described above. The relative slopes of the two sides of the characteristic saw-tooth time signature, obtained from the corrugated reflector model, depend on the amplitude to wavelength ratio of the reflection surface (LYON, 1979). Likewise, varying the parameters of the ‘wave’ perturbation significantly affects the nature of the peak--trough variation. For example, the symmetry of the bearing variation about the time axis depends on the ratio of the vertical to horizontal wavelength tic. clTectively on the period of the disturbance). Significant differences are also observed in the rime signatures present in the bearings of transmissions monitored during the same TTD event. The very large wavelengths of large-scale TlDs suggest that their waveforms should be preserved over great horizontal distances and the TID-induced bearing errors measured on the spaced paths should be highly correlated. As would be expected, the degree of correlation generally decreases with increasing signal reflection point separation but sometimes poor correlationisobservedeven between thebearingerrors for closely spaced paths. An example of this is the poor correlation Prague

between the bearings

transmissions

on

1I

of the Vienna

and

May 1980, which arc

reproduced in Fig. 10. Assuming one-hop f; propagation, the respective signal reflection points have a horizontal separation of less than 200 km. Various factors must act to reduce the correlation of TID-induced bearing errors even on closely-spaced iransmission paths. These are now discussed. A major limitation of the models described in Section Z is the assumption that a large-scale TID comprises a single spectral component. In the real ionosphere, AG Ws of dinerent periods and wavelengths. generated in the aurora1 o\aI.superimpose to give the appearance of a disturbance whose period increases with decreasing altitude and with increasing horizontal distance from the source ( RKI~MOK~)and MATsustri,rA, 1975). The enect of this dispersive hehaviour makes it difficult to calculate bearing error accurately. since the signal reflection points can differ in both height and latitude from one transmission path to another. with resulting differences in quasi-period of the disturbing waveform. Differences in quasi-period arc generally quite small, due to the relatively close signal reflection points compaied with the distance over which signilicant dispersion occurs. Nevertheless. unequal quasi-periods are observed on different paths in the experimental data. The mid-latitude AGW response 1~) an isolated aurora1 substorm consists of a single impulse or sinusoid. but the ionospheric response can depart signiticantly from this simple form due to the time variation of the ambient F’ layer (P(IRTI:R and T~‘As.

8. L. TEIII), II. J.

I16

Vienna

6 % .c-

P .._ ;i $

Prague

and T.

IO.1185 MHZ.

Great circle bearing: 100.7

IO. I.250 MHZ.

I Great circle bearingz92.3

I

I

-50i

50

STKANXWAYS

B. JONF-F R:l393

I

.A

KM

1

R=1220

KM

Time in hours (UT1

Fig. 10. Bearing data

recorded cm I I May

lY741. Man) af !he TID events occurred during the dusk transition period. a time when the height and cornpositIon of the K iayer change rapidly over a period of ;I fe\i hours. Several of the events also occurred during the early stages of very large geomapnetic substorms, during which very large changes in the ambient ionosphere occur (KAM. 1973). During these conditions. significant changes can be produced in both the ionospheric response to an AGW disturbance and in ionospheric reelection height. Conversely, the ambient ionospherechanges much less rapidly near midday. This can explain the close resemblance oi the TID waveform to the computed time signatures for the bearing observations presented in Fig. 8 recorded between 9.30 a.m. and 12.30 p.m. TID induced bearing errors are often superimposed on large bearing errors produced by systematic ionospheric tilts iSITs). SIT erects are caused by the spatial and temporal variations of the regular ionosphere and are very transmission path dependent. thus reducing the correlation of the TID-induced hearing errors on the spaced paths. The SIT effects present in the Luaembourg record of Fig. 5 consist ofa decrease in bearing with time. upon which the TID error is superimposed. The SIT eIl’ects are not as marked in the hearings of the other two transmission paths monitored. Medium-scale TlDs, if present, can also influence the correlation of the bearings recorded on adjacent paths. due to their much larger dispersion with distance. The quiteJilTerenl waveforms present in the bearing records of the two transmissions reproduced in Fig. IO can be explained in this way. The large scale TID present in the Vienna bearings is probably also present in the bearings of the Prague

1980

path, but the bearing of the latter transmission affected by an additional medium-scale TI D.

is also

Computations performed with the corrugated reHector model clearly demonsrrate the influence of’ the transmission path geometry as wcli as the TID parameters on the magnitude and time t.:lristion ol bearing errors at HF. This behnciolur is c~~nlirmed b! means of a more realistic TID model cmpioyed in conjunction with a 3D ray-tracing program. The computer simu!ations have been compsrcd \+11h the characteristics of I6 large-scale TID L:kcnls. e;lch observed in the bearing records ofa number of spaced transmission paths. Despite the very long \va\elength of large scale TIDs. the time sigtxtiurcs present in the bearings ofspaced paths can differ significantly in their magnitude and shape. This behaviour cannot be explained solely in terms of path geomc!r! and TID magnitude. so that account needs to bt t.tkcn of the propagation characteristics of large scale 1’IDs. the ambient ionospheric conditions and the nalur~ of the HF direction finding technique. ConsequentI!. the corrugated reflector model has only limited applicability in practice due to the complexit! of the Ill: radiowave-TID interaction. Although. Ihc ‘wave model is more realistic. this can also bil to csplain satisfactorily the path dcpendcnce of the large scale TID-induced bearing errclrs in many cases. B.L.T. is grateful to the SIIRC‘ l’t~r;I C’ase .4rknorlcdgrmenrs Studentship. Financial support for this research rr~)grani from the MOD is acknowledged. The authors wish tg1 th;mk the Government Communications tlradquarter. ior providing equipment and fnr assistance in using it.

The Bearings of geographically spaced HF transmissions

117

REFERENCES

FRANCISS. H. FUNCIS S. H. GEORGEZZ T. M. GEORGES T. M. and STEPHENSON J. J. GETHINGP. J. D.

1973 1975 1968 1969 1978

H~CKLEYJ. JON= R. M. KANER. P. LYONG. F. MORGANM. G. and BALLARDK. A. PORTERH. S. and TUAN T. F. RICHMOND A. D. and MATSUSHITA S. STERLINGD. L., H~~KE W. H. and COHENR. TITHERIDGE J. E. TREHARNE R. F. WALTONE. K. and BAILEYA. D.

1973 1966 1973 1979 1978 1974 1975 1971 1958 1972 1976

J. geophys. Res. 78,2278. J. atnws. terr. Phys. 37, 1011. J. atnws. terr. Phys. 30,735. Radio Sci. 4,679. Radio direction-finding and the resolution of multicomportent wavefields. Peter Penegrinus Ltd. Radio electron. Engng 43,475. ESSA Technical Reoort IER 17-ITSA 17. J. ammos.terr. Phys.jS, 1953. J. atmos. terr. Phys. 41.5. 1. geophys. Res. 83,574l. J. atrnos. terr. Phys. 36, 135. J. geophys. Res. 80,2839. J. geophys. Res. 76,3777. J. atmos. terr. Phvs. 13, 17. AGARD Conf. Proc. Ho. 115. Radio Sci. 11, 175.