Last year a program of radioelectrical observations of meteoric trails was undertaken by the CNET. A 40-MHz forward-scatter path was used. In August 1965, a new pro- gram was begun with a 30-MHz forward-scatter path and a 30-MHz radar.
An accurate localization of the reflection point on each meteor trail is obtained from an angle-measuring system from which one component of the meteor speed vector can be deduced. These data can be obtained easily for a great number of meteor echoes (some tens/hour).
The forward-scatter path is 1000 km long, in the north-south direction, between the Hague (Netherlands) and Toulon (France).
The bistatic radar transmitting and receiving stations are 30 km apart, in the west-east direction.
Measurement of angle of arrival
To determine the direction D of a meteoric echo, the same equipment is used in both forward-scatter path and radar: the phase difference 4> between the field received by two antennas Ax and A2 is measured. The angle
a'=(A~A2,D)
is deduced from the * measurement by the expression
= —r~ cos a
A
where X is the wavelength and d the distance
1 This work is supported by the Centre National d'Etudes Spatiales (CNES) and the I.Q.S.Y.
'Centre National d'Etudes des Telecommunications (CNET), Issy-lcs-Moulineaux (Seine). France.
Let A XA 2 be perpendicular to the path axis, and A3 be a third antenna, AiA3 being aligned along the path axis. When the latter pair of antennas is used, the angle
can be determined in the same way as a'.
The azimuth a and the elevation 0 of the direction D are given by
cos a'=cos a cos /3, cos /3'=sin a cos 0.
The two waves with phase difference $ enter two receivers Rx and i?2 and are mixed with a wave arising from a common local oscillator. The IF waves thus obtained have the same phase difference *. These signals, Ex cos Qt and E2
cos ($#+$), enter a modulator which yields a voltage proportional to Ex cos <t>. Similarly, after changing one of the phases by T/2, the same signals enter a second modulator which yields a voltage proportional to Ex sin «t>. The voltages Ex cos $ and Ex sin $ are applied to the plates of an oscillograph, producing a polar diagram (Ex, <i>).
The various possible errors in a' and fi' are:
Reading error: $ can be read from a film record with a precision of the order of 2°.
Phase shifts due to variations in a circuit element: it is necessary to verify frequently the zero position of the phase comparator.
Noise superimposed on the echo: the phase comparator introduces an error A4> which gives rise to the errors Aa' and A/3' between 0?3 and 0?7 for a SNR of 30 db, and between 0?2 and 0?4 for a SNR of 25 db.
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92 SYMPOSIUM ON METEOR ORBITS AND DUST
Frequency variations in the transmitter or in a local oscillator: supposing A4>= 1°, it would be necessary for the transmitter and the first receiver oscillator to have a relative stability of the order of 10~7.
Errors due to tropospheric and ionospheric refractions are negligible compared to the preceding errors.
Doppler measurement
The two origins of doppler frequency shifts are doppler effects due to (1) movement of the meteor: its duration is of the order of 0.1 sec—the frequency shift is very high, a few hundred Hz; and (2) winds after the formation of the trail. This shift is much lower than that of (1).
The meteor speed is deduced from the doppler shift A/, by using the relation
?.
AT,•
where y is the scattering angle, \' is the speed ->
vector of the meteor, and TV is the inner normal to the equiphase surface.
Forward-scatter path
The phase difference <f>(0 is measured between the reflected wave and the wave generated by an oscillator. In the cylindrical approximation of the Fresnel ellipsoid we have
where r is the cylinder radius, and Vx is the speed component orthogonal to the axis.
Radar
The phase difference $(/) is measured between the reflected wave and the wave directly transmitted through a ground link. We have
where r is the radar distance, and Vis the speed.
The error in <l> is composed of the errors in (1) the phase of the echo, and (2) the phase of the reference wave. It can be shown that in the measurement of a speed component of the meteorite, the errors that dominate are due to the noise.
The errors in V, are A F , = 2 m/sec for SXR = 10 db, and AV,= l m/sec for SNR = 20 db.
Preliminary experimental results
Plate 1 shows a sample of meteoric echo record.
The scope on the upper left corner gives the amplitudes as a function of time. The two other scopes on the right give polar diagrams of amplitude and angles of arrival also as a function of time: elevation (upper right scope) and azimuth (lower right scope). Time is marked every 10 msec, with a larger mark every 100 msec.
From the elevation and azimuth vulues so obtained, and from an approximate value of altitude, the geographic distribution of the echoes can be determined. Figure 1 shows an example of such a distribution (at 11 a.m.);
it can be seen that the echoes only appear in the eastern half of the region where the antenna diagrams overlap, indicated by a dashed line.
Figure 1 also shows the location of the radar relative to the forward-scatter path.
FIGURE 1.—Geographic distribution of echoes.
EQUIPMENT FOR THE OBSERVATIONS OF METEOR TRAILS DBLCOURT
The velocity component orthogonal to the ->
axis V± is completely determined: the amplitude V± is known from the doppler-effect measure-
->
ment, and the angle 0 between V± and the hor- izontal is deduced from the values of a' and /3'.
Figure 2 shows such a distribution of d as a function of local time, established for a few days of 1964. It can be seen that most of the meteors come from the west in the morning and around midday, and from the east in the evening and around midnight, following the movement of the apex.
Acknowledgment
I am indebted to MM. Giraud, Masseboeuf, Revah, and Spizzichino for their work on the described apparatus.
20 22 24
LOCAL TIME ' 0T0 0 5 ECHO/ HOUR 1 0.9 TO I ECHO/HOUR j I TO 2 ECHO/HOUR
FIGURE 2.—Variation in the direction of meteoric trails as a function of time.
94 SYMPOSIUM ON METEOR ORBITS AND DUST
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