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The spaced-receiver technique
Jmmd
ofAtmo~phe&
and Terrtmid
Pergamon
Physics. Vol. 56, No. 7. pp. 831-834, 1994 Copyright 0 1994 Ekvier Scienoe Ltd Printed in Great Britain. AU rights twxvcd 0021-91691’94 $6.00 + 0.00
The Spaced-Receiver Technique B. H. BRKJGS Departmentof Physics and MathematicalPhysics, University of Adelaide, Adelaide, SA, 5005, Australia
Abstract-Sir Granville Beynon made significant contributions to the spaced-receiver technique for the measurementof winds in the ionosphere, and the study of atmospheric irregularities. This work is reviewed, and its relevance to recent work is discussed.
1. INTRODUCI’ION The
use of spaced receivers in conjunction with ionosondes and atmospheric radars dates back to the pioneering work of PAWSEY (1935). There have always been two aspects to the work - the determination of winds, and the determination of parameters related to atmospheric irregularities. The first systematic observations were made at Cambridge by MITFW(1949). He used a triangle of three aerials to determine the horizontal winds in the ionosphere from the time displacements of the fading of the echoes as observed at the three points. Shortly afterwards it was pointed out that allowance must be made for the possibility that the pattern moving over the ground may change as it moves, due to the presence of turbulence as well as drift. Allowance must also be made for the possibility that the structures in the pattern may be elongated in a preferred direction. Both these effects were taken into account, and corrected for, by a theory which later became known as “full correlation analysis” (BRIGGS, PHILLIPSand &INN, 1950; PHILLIPS and SPENCER, 1955). The output parameters of this analysis are (i) the true velocity; (ii) the apparent (i.e.
831
uncorrected) velocity; (iii) a parameter describing the importance of random changes; and (iv) the parameters of a “characteristic ellipse” which describes the spatial properties of the pattern formed over the ground by the down~ming waves. The radius of this ellipse in any direction gives the separation of two aerials for which the correlation of the fading time series would be 0.5. The orientation of the major axis gives the mean direction of elongation of the irregularities in the pattern. 2.
OB~VA~ONS AT SWANSRA UmG M.F. BROADCAST TRAN-
OBLIQUE
Sir Granville carried out work using the spacedreceiver technique at Swansea in 1957-8 (BEYNON and GOODWIN,1958). Instead of the usual vertical incidence pulse system, use was made of t~smissions from distant medium wave broadcast Signal transmitters at Stuttgart and Vilnius. strengths were recorded using three spaced aerials. As the equivalent vertical incidence frequencies were 190 kHz and 96 kHz respectively, the observations related to the lower E and D regions.
B. H. BRIGGS
832
The main results were published in 1967 (BEYNON 1967). I will not comment on this work further, since G.L. Goodwin is providing a separate note on his recollections of the observations. It is interesting that the same method was later used by Sprenger and Schminder at Ktihlungsbom and Collm (e.g. SPRENGER and SCHMINDER, 1967) and that the work there has continued until the present time. and GOODWIN,
3.
WORK WITH DATA FROM HALLEY BAY, ANTABCTICA
During the International Quiet Sun Year (IQSY) (1964-5) a spaced-aerial system was installed at Halley Bay, Antarctica. It operated in conjunction with a vertical incidence pulse sounder on frequencies of 1.O MHz, 2.2 MHz and 3.6 MHz. Separate receiving arrays with appropriate spacings were used for the different frequencies and in all cases these consisted of a square array (side approximately k/2) with four aerials, together with an additional fifth aerial at the centre of the square. As four receiving channels were available, this system offered the possibility of comparing results obtained simultaneously from right-angled triangles The fading was with different orientations. recorded on 35 mm film and also as varying frequencies on magnetic tape. The latter could be used for analysis by computer, using the full correlation analysis. This work led to an important paper entitled: “Ionospheric drift measurements on adjacent aerial 1969). (BEYNON and WRIGHT, arrays” Comparisons of results from adjacent triangles with different orientations often showed serious discrepancies in the direction and magnitude of the drifts. In particular, the velocity had a tendency to be perpendicular to the hypotenuse of the triangle. (It should be noted that the comparison triangles were right-angled, and had their hypotenuses at right angles.) This result was still obtained when full correlation analysis was employed. Taking into account the geometry of the aerial array, the authors showed that the results could be explained if the maximum amplitude contours of the moving pattern were curved rather than straight. Furthermore, these curved fronts often seemed to be expanding or contracting as they passed over the array. They state: “Over a 14 month period (November 1964 - December 1965)
out of a total of about 3500 drift measurements, some 47 per cent showed significant discrepancies corresponding to convex or expanding fronts, 22 per cent discrepancies corresponding to concave or contracting fronts, and the remaining 31 per cent were normal. These proportions apply to both region-E and -F observations”. Somewhat similar results were reported by RAO and RAO(1957) and HARNISCHMACHERand RAWER (1963). In a following paper the authors extended the work to cover the parameters of the characteristic ellipse, as observed on adjacent aerial arrays (BEYNON and WRIGHT, 1969). They found a marked tendency for the major axis of the ellipse to lie along the hypotenuse of the triangle used to determine it. This result, they suggested, could also be explained if contours of maximum amplitude were curved rather than straight.
4. RELATED WORK AT ADELAIDE
Around this time (1969) a large 1 km x 1 km aerial array was being completed at Buckland Park, near Adelaide, and the first observations were being obtained. The array consists of 89 crossed dipoles, and the associated equipment at that time comprised 89 receivers and an 89-channel data logger which enables the 89 fading signals to be recorded on magnetic tape. In addition, the signals were used to control the brightnesses of 89 filament lamps, arranged in the same configuration as the dipoles. A diffusing screen was placed over the lamps to produce a smooth picture of the amplitude pattern over the 1 km x 1 km area of ground. This could be viewed and also recorded on a tine-camera (BRIGGS et al., 1969). Any peculiarities of the pattern such as expanding or contracting rings of increased amplitude could easily have been observed with this system, but none was found. The pattern usually seemed to consist of more or less circular regions of increased amplitude which drifted across the field of view, sometimes changing in size and shape as they Sometimes fringe-like patterns were moved. observed but the fringes were always straight, not CUNWI
(FELGATE and GOLLEY, 1971).
These discrepancies with the observations of BEYNON and WRIGHT and others were never resolved. However, results from the large array at
The spaced-receiver technique
Buckland Park did confirm the observations that the velocity tended to be perpendicular to the hypotenuse of a right-angled triangle of aerials, and that the major axis of the characteristic ellipse tended to be aligned with the hypotenuse. These effects could be tested very thoroughly because a suitable selection from the 89 aerial sites could provide right-angled triangles of many orientations and also of different sizes. Results were reported in a paper by GOLLEYand ROSSITER(1970). They found that the major axis of the ellipse tended to lie along the direction of the hypotenuse but only if the aerial spacing was small. For large triangles there was no such effect. They also found what they considered to be a related effect; derived wind speed increased as the triangle size was increased. This became known as the ‘triangle-size effect’. (This effect had also been observed by KELLEHER, 1966.) The velocity tended to a limiting value which was believed to be the correct one, because it agreed with an inde~ndent method which used all 89 aerials. These results suggest some breakdown in the method if the aerials are spaced too closely. For small spacings the correlation coefficients between the time series will be high, perhaps often close to unity. This led GOLLEYand ROSSITER(1970) to suggest that the effects were instrumental. They pointed out that small differences in the receiver channels would tend to depress the crosscorrelation values, and that this effect would be much more serious if the correiations were close to unity rather than in the region of 0.5, say. To test this they connected 8 receivers to the same aerial and computed the cross-correlation values for all possible combinations of the receivers in pairs. The correlation was never unity, but varied from 0.86 to 0.96. This was surely a salutary exercise, which could well be carried out as a routine test, even with modem equipment. As a result of the work of BEYNONand WRIGHT (1969), GOLLEYand ROSSITER(1970) and others it became normal practice to use equilateral rather than right-angled triangles in spaced-aerial observations, in order to avoid the “hy~tenu~ bias”. Also, the side of the equilateral triangle was usually adjusted so as to give cross-correlations of about 0.5 at zero lag.
833
5. RECENT WORK
We must now emphasize that all the work was carried out using total reflections from the E and F regions. Following the work of FRASER(1965) the use of total reflections was gradually abandoned, and now use is made entirely of partial reflections from the D region. These have the great advantage of giving results over a range of heights from about 60-90 km. Also, while earlier work used only the amplitude of the reflected pulses, it is now normal to use both amplitude and phase, combined into a complex signal. This is often coupled with coherent averaging of a number of successive pulses in order to improve the signal-to-noise ratio. Further, the receiving aerials now often consist of arrays rather than single dipoles or loops, and the technique is used with VHF as well as MF radars. The full correlation analysis can, if desired, be carried out in the frequency domain rather than the time domain (BRIGGSand VINCENT,1992). How do all these changes affect things like the “triangle-size effect” and the “hypotenuse effect”? Surprisingly little work has been done on this, considering its potential importance. MEEK (1990) investigated the triangle-size effect using MF data from Saskatoon, and concluded that the effect did occur with D region partial reflections. He suggested that it could be explained by the presence of noise in the spaced-aerial channels. VAN BAELENet al., (1990), working with the MU radar in Japan on 46.5 MHz, and observing scatter from the troposphere and lower stratosphere, showed that using a small spacing for the receiving array yielded a smaller estimate for the “true” velocity. So the effects studied by BEYNONand WRIGHTin 1964-5 are still with us!
6. CONCLUSION This paper has reviewed only the work carried out by Sir Granville using data from Halley Bay, Antarctica. Its continuing importance has been emphasized. Work using the spaced-aerial technique at A~~s~yth will, I hope, be discussed in another contribution. Important papers arising from the work at Aberystwyth were those of JONES and MAUDE(1965, 1968).
B. H. BRIGGS
834
REFmmNCEs
BEYNONW.J.G.
and GOODWING.L.
1958
J.atmos.tw.Phys.,
13, 180
BEYNONW.J.G.
and GOODWING.L.
1967
J.atmos.te~~.Phys.,
29, 1181
BEYNONW.J.G.
and WRIGHT J.C.
1969
J. atmos. Ierr. Phys., 31, 119
BEYNONW.J.G.
and WRIGHT J.C.
1969
J. atmos. terr. Phys., 31, 593
1969
Nature, London,
BRIGGSB.H., ELFORDW.G., M.G.,
FELGATED.G.,
GOLLEY
223, 1321
ROSSITERD.E. and SMITHJ.W.
BRIGGSB.H.,
PHILLIPSG.J. and SHINND.H.
1950
Proc. Phys. Sot. London, B63, 106
BRIGGSB.H. and V~CENT R.A.
1992
Radio Science, 27, 117
FELGATED.G. and GOLLEY M.G.
1971
J. ammos. terr. Phys., 33, 1353
FRASERG.J.
1965
J. Armos. Sci.,
GOLLEY M.G. and ROSSITERD.E.
1970
J. atmos. terr. Phys., 32, 1215
HARNISCHMACHER E. and RAWER K.
1963
Research on Ionospheric
22, 2 17 Structure
and Motion,
Report AF61 (052) - 81 JON= D. sod MAUDE A.D.
1965
Nature, London, 206, 177
JONESD. and MAUDE A.D.
1968
J. atmos. ferr. Phys., 30, 1487
KELLEHERR.F.
1966
J. atmos. rerr. Phys., 28, 213
MEEK C.E.
1990
Radio Science, 25, 641.
MITRA S.N.
1949
Proc.Iturn.Elect. Engrs. , 96, 441.
PAWSEYJ.L.
1935
Proc. Camb. Phil. Sot., 31, 125
PHILLIPSG.J. and SPENCERM.
1955
Proc. Phys. Sot. London, B68, 481
RAO M.S. and RAO B.R.
1957
J. atmos. terr. Phys., 10, 307
SPRENGERK. and SCHMINDERR.
1967
J. atmos. terr. Phys., 29, 183
VAN BAELENJ.S., TSUDA T., RICHMONDA.D.,
1990
Radio Science, 25, 629
AVERY S.K., KATO S., FIJKAOS. and YAMAMOTO M.
Tech.
ofAtmo~phe&
and Terrtmid
Pergamon
Physics. Vol. 56, No. 7. pp. 831-834, 1994 Copyright 0 1994 Ekvier Scienoe Ltd Printed in Great Britain. AU rights twxvcd 0021-91691’94 $6.00 + 0.00
The Spaced-Receiver Technique B. H. BRKJGS Departmentof Physics and MathematicalPhysics, University of Adelaide, Adelaide, SA, 5005, Australia
Abstract-Sir Granville Beynon made significant contributions to the spaced-receiver technique for the measurementof winds in the ionosphere, and the study of atmospheric irregularities. This work is reviewed, and its relevance to recent work is discussed.
1. INTRODUCI’ION The
use of spaced receivers in conjunction with ionosondes and atmospheric radars dates back to the pioneering work of PAWSEY (1935). There have always been two aspects to the work - the determination of winds, and the determination of parameters related to atmospheric irregularities. The first systematic observations were made at Cambridge by MITFW(1949). He used a triangle of three aerials to determine the horizontal winds in the ionosphere from the time displacements of the fading of the echoes as observed at the three points. Shortly afterwards it was pointed out that allowance must be made for the possibility that the pattern moving over the ground may change as it moves, due to the presence of turbulence as well as drift. Allowance must also be made for the possibility that the structures in the pattern may be elongated in a preferred direction. Both these effects were taken into account, and corrected for, by a theory which later became known as “full correlation analysis” (BRIGGS, PHILLIPSand &INN, 1950; PHILLIPS and SPENCER, 1955). The output parameters of this analysis are (i) the true velocity; (ii) the apparent (i.e.
831
uncorrected) velocity; (iii) a parameter describing the importance of random changes; and (iv) the parameters of a “characteristic ellipse” which describes the spatial properties of the pattern formed over the ground by the down~ming waves. The radius of this ellipse in any direction gives the separation of two aerials for which the correlation of the fading time series would be 0.5. The orientation of the major axis gives the mean direction of elongation of the irregularities in the pattern. 2.
OB~VA~ONS AT SWANSRA UmG M.F. BROADCAST TRAN-
OBLIQUE
Sir Granville carried out work using the spacedreceiver technique at Swansea in 1957-8 (BEYNON and GOODWIN,1958). Instead of the usual vertical incidence pulse system, use was made of t~smissions from distant medium wave broadcast Signal transmitters at Stuttgart and Vilnius. strengths were recorded using three spaced aerials. As the equivalent vertical incidence frequencies were 190 kHz and 96 kHz respectively, the observations related to the lower E and D regions.
B. H. BRIGGS
832
The main results were published in 1967 (BEYNON 1967). I will not comment on this work further, since G.L. Goodwin is providing a separate note on his recollections of the observations. It is interesting that the same method was later used by Sprenger and Schminder at Ktihlungsbom and Collm (e.g. SPRENGER and SCHMINDER, 1967) and that the work there has continued until the present time. and GOODWIN,
3.
WORK WITH DATA FROM HALLEY BAY, ANTABCTICA
During the International Quiet Sun Year (IQSY) (1964-5) a spaced-aerial system was installed at Halley Bay, Antarctica. It operated in conjunction with a vertical incidence pulse sounder on frequencies of 1.O MHz, 2.2 MHz and 3.6 MHz. Separate receiving arrays with appropriate spacings were used for the different frequencies and in all cases these consisted of a square array (side approximately k/2) with four aerials, together with an additional fifth aerial at the centre of the square. As four receiving channels were available, this system offered the possibility of comparing results obtained simultaneously from right-angled triangles The fading was with different orientations. recorded on 35 mm film and also as varying frequencies on magnetic tape. The latter could be used for analysis by computer, using the full correlation analysis. This work led to an important paper entitled: “Ionospheric drift measurements on adjacent aerial 1969). (BEYNON and WRIGHT, arrays” Comparisons of results from adjacent triangles with different orientations often showed serious discrepancies in the direction and magnitude of the drifts. In particular, the velocity had a tendency to be perpendicular to the hypotenuse of the triangle. (It should be noted that the comparison triangles were right-angled, and had their hypotenuses at right angles.) This result was still obtained when full correlation analysis was employed. Taking into account the geometry of the aerial array, the authors showed that the results could be explained if the maximum amplitude contours of the moving pattern were curved rather than straight. Furthermore, these curved fronts often seemed to be expanding or contracting as they passed over the array. They state: “Over a 14 month period (November 1964 - December 1965)
out of a total of about 3500 drift measurements, some 47 per cent showed significant discrepancies corresponding to convex or expanding fronts, 22 per cent discrepancies corresponding to concave or contracting fronts, and the remaining 31 per cent were normal. These proportions apply to both region-E and -F observations”. Somewhat similar results were reported by RAO and RAO(1957) and HARNISCHMACHERand RAWER (1963). In a following paper the authors extended the work to cover the parameters of the characteristic ellipse, as observed on adjacent aerial arrays (BEYNON and WRIGHT, 1969). They found a marked tendency for the major axis of the ellipse to lie along the hypotenuse of the triangle used to determine it. This result, they suggested, could also be explained if contours of maximum amplitude were curved rather than straight.
4. RELATED WORK AT ADELAIDE
Around this time (1969) a large 1 km x 1 km aerial array was being completed at Buckland Park, near Adelaide, and the first observations were being obtained. The array consists of 89 crossed dipoles, and the associated equipment at that time comprised 89 receivers and an 89-channel data logger which enables the 89 fading signals to be recorded on magnetic tape. In addition, the signals were used to control the brightnesses of 89 filament lamps, arranged in the same configuration as the dipoles. A diffusing screen was placed over the lamps to produce a smooth picture of the amplitude pattern over the 1 km x 1 km area of ground. This could be viewed and also recorded on a tine-camera (BRIGGS et al., 1969). Any peculiarities of the pattern such as expanding or contracting rings of increased amplitude could easily have been observed with this system, but none was found. The pattern usually seemed to consist of more or less circular regions of increased amplitude which drifted across the field of view, sometimes changing in size and shape as they Sometimes fringe-like patterns were moved. observed but the fringes were always straight, not CUNWI
(FELGATE and GOLLEY, 1971).
These discrepancies with the observations of BEYNON and WRIGHT and others were never resolved. However, results from the large array at
The spaced-receiver technique
Buckland Park did confirm the observations that the velocity tended to be perpendicular to the hypotenuse of a right-angled triangle of aerials, and that the major axis of the characteristic ellipse tended to be aligned with the hypotenuse. These effects could be tested very thoroughly because a suitable selection from the 89 aerial sites could provide right-angled triangles of many orientations and also of different sizes. Results were reported in a paper by GOLLEYand ROSSITER(1970). They found that the major axis of the ellipse tended to lie along the direction of the hypotenuse but only if the aerial spacing was small. For large triangles there was no such effect. They also found what they considered to be a related effect; derived wind speed increased as the triangle size was increased. This became known as the ‘triangle-size effect’. (This effect had also been observed by KELLEHER, 1966.) The velocity tended to a limiting value which was believed to be the correct one, because it agreed with an inde~ndent method which used all 89 aerials. These results suggest some breakdown in the method if the aerials are spaced too closely. For small spacings the correlation coefficients between the time series will be high, perhaps often close to unity. This led GOLLEYand ROSSITER(1970) to suggest that the effects were instrumental. They pointed out that small differences in the receiver channels would tend to depress the crosscorrelation values, and that this effect would be much more serious if the correiations were close to unity rather than in the region of 0.5, say. To test this they connected 8 receivers to the same aerial and computed the cross-correlation values for all possible combinations of the receivers in pairs. The correlation was never unity, but varied from 0.86 to 0.96. This was surely a salutary exercise, which could well be carried out as a routine test, even with modem equipment. As a result of the work of BEYNONand WRIGHT (1969), GOLLEYand ROSSITER(1970) and others it became normal practice to use equilateral rather than right-angled triangles in spaced-aerial observations, in order to avoid the “hy~tenu~ bias”. Also, the side of the equilateral triangle was usually adjusted so as to give cross-correlations of about 0.5 at zero lag.
833
5. RECENT WORK
We must now emphasize that all the work was carried out using total reflections from the E and F regions. Following the work of FRASER(1965) the use of total reflections was gradually abandoned, and now use is made entirely of partial reflections from the D region. These have the great advantage of giving results over a range of heights from about 60-90 km. Also, while earlier work used only the amplitude of the reflected pulses, it is now normal to use both amplitude and phase, combined into a complex signal. This is often coupled with coherent averaging of a number of successive pulses in order to improve the signal-to-noise ratio. Further, the receiving aerials now often consist of arrays rather than single dipoles or loops, and the technique is used with VHF as well as MF radars. The full correlation analysis can, if desired, be carried out in the frequency domain rather than the time domain (BRIGGSand VINCENT,1992). How do all these changes affect things like the “triangle-size effect” and the “hypotenuse effect”? Surprisingly little work has been done on this, considering its potential importance. MEEK (1990) investigated the triangle-size effect using MF data from Saskatoon, and concluded that the effect did occur with D region partial reflections. He suggested that it could be explained by the presence of noise in the spaced-aerial channels. VAN BAELENet al., (1990), working with the MU radar in Japan on 46.5 MHz, and observing scatter from the troposphere and lower stratosphere, showed that using a small spacing for the receiving array yielded a smaller estimate for the “true” velocity. So the effects studied by BEYNONand WRIGHTin 1964-5 are still with us!
6. CONCLUSION This paper has reviewed only the work carried out by Sir Granville using data from Halley Bay, Antarctica. Its continuing importance has been emphasized. Work using the spaced-aerial technique at A~~s~yth will, I hope, be discussed in another contribution. Important papers arising from the work at Aberystwyth were those of JONES and MAUDE(1965, 1968).
B. H. BRIGGS
834
REFmmNCEs
BEYNONW.J.G.
and GOODWING.L.
1958
J.atmos.tw.Phys.,
13, 180
BEYNONW.J.G.
and GOODWING.L.
1967
J.atmos.te~~.Phys.,
29, 1181
BEYNONW.J.G.
and WRIGHT J.C.
1969
J. atmos. Ierr. Phys., 31, 119
BEYNONW.J.G.
and WRIGHT J.C.
1969
J. atmos. terr. Phys., 31, 593
1969
Nature, London,
BRIGGSB.H., ELFORDW.G., M.G.,
FELGATED.G.,
GOLLEY
223, 1321
ROSSITERD.E. and SMITHJ.W.
BRIGGSB.H.,
PHILLIPSG.J. and SHINND.H.
1950
Proc. Phys. Sot. London, B63, 106
BRIGGSB.H. and V~CENT R.A.
1992
Radio Science, 27, 117
FELGATED.G. and GOLLEY M.G.
1971
J. ammos. terr. Phys., 33, 1353
FRASERG.J.
1965
J. Armos. Sci.,
GOLLEY M.G. and ROSSITERD.E.
1970
J. atmos. terr. Phys., 32, 1215
HARNISCHMACHER E. and RAWER K.
1963
Research on Ionospheric
22, 2 17 Structure
and Motion,
Report AF61 (052) - 81 JON= D. sod MAUDE A.D.
1965
Nature, London, 206, 177
JONESD. and MAUDE A.D.
1968
J. atmos. ferr. Phys., 30, 1487
KELLEHERR.F.
1966
J. atmos. rerr. Phys., 28, 213
MEEK C.E.
1990
Radio Science, 25, 641.
MITRA S.N.
1949
Proc.Iturn.Elect. Engrs. , 96, 441.
PAWSEYJ.L.
1935
Proc. Camb. Phil. Sot., 31, 125
PHILLIPSG.J. and SPENCERM.
1955
Proc. Phys. Sot. London, B68, 481
RAO M.S. and RAO B.R.
1957
J. atmos. terr. Phys., 10, 307
SPRENGERK. and SCHMINDERR.
1967
J. atmos. terr. Phys., 29, 183
VAN BAELENJ.S., TSUDA T., RICHMONDA.D.,
1990
Radio Science, 25, 629
AVERY S.K., KATO S., FIJKAOS. and YAMAMOTO M.
Tech.