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Phase height measurements of the E-region
Journal of A~~p~~~and Terrsstrisl Physics, 1871. Vol. 5.3, pp. 723-729. Pergsn~on Press.
Printed In Northern f&and
Phase height measurements of the E-region D. H. SCHRADERand G. I,. HOWER Department of Electrical Engineering, Washington State University, Pullman, Wash. 99163, U.S.A. (&ce~v~ 22 September 1970; in retied form 16 November1970)
Abstrset-A phsee-p&h sounderwith spaced receiversand not requiring ultra-stable oscillators is described. Measurementsof the morning variation in the E-laps height using this equipment are reported. From these rne~~rnen~ made in March 1968 a scale height of about 7 km is cakul&ed. 1.
INTR~DuOTI~~~
A PEASE-PATH sounder has been constructed and operated at Washington State University (WSU) to investigate wave motions in the E-region over Pullman, Washington. This sounder transmits a radio pulse vertioelly upwards and the echo returned from the ionosphere is received by three radio receivers with spacings shown in Fig. 1. The phase angle of the received signal at each receiver is compared to that of the transmitted signal and the time variations of these phase angles are recorded. These vtlriations are proportional to the variations in the phase-path traversed by the signal. Changes of the electron density in the lower ionosphere and movements of the reflection level will cause phi-pith v~~&tions. Phase-path sounders have been in use for some time (FINDLBY, 1951; W~rrs and DAVIES,1960). The WSU system, however, has several advantages over others previously reported. Since this sounder does not require long term phase stability between the tmnsmitter signal source and the receiver local oscillator, it is relatively inexpensive to construct and operate. Since it uses a pulse, the F-layer and E-layer signttls can be separated and measured independently. Finally the transmitter land receiver may be separated by any distance up to that over which a ground wave may propagate between them. The phase-path sounder measurements are designed to detect wave motions by means of layer height variations. Three simultaneous measurements of the phasepath length sre made. The purpose of having three spaced me~uremen~ is to allow the separation of phi-pith vsrirttions caused by solar flares and spatially random phase-path changes from those caused by wave motions, as well as to allow the determination of the direction of horizontal phase propagation of pressure wsves in the ionosphere. One can separate these different effects because the effects c&used by solar flares occur simultaneously at the three receiving points while those due to pressure wsves should be separated in time due to the propagation of the waves in the horizontal direction. The basic method by which the reflected radio signal phase is measured is to compare the phase of the sky wave component of a high power pulse with the phase of the ground wave component of a low power pulse that is phase locked to the high power pulse. The high power pulse is short enough and the transmitter is close enough to the receiver so that the ground wave component of the high power pulse has propagated beyond the receiver by the time that the sky wave component 723
124
D. H. SCERADER
and Q. L. HOWER
Fig. 1. Layout of transmitter-receiverlocations.
reachm the receiver. In the meantime, the transmitter has started to transmit a phase locked, low power pulse at a slightly different frequency which arrives at the receiver coincident with the arrival of the sky wave component of the high power pulse. These two signals heterodyne cautig a eignal at the di%ence frequency whose phase angle varies directly with the phase angle of the sky wave component of the high power pulse since the propagation path of the ground wave component of the low power pulse is highly phase stable. 2. EQUIPMENT The system uses a transmitting and three receiving stations, none of which are at the same location. The tmmitter transmits at two frequencies and at two different power levels requiring two separate final amplifier atages. The eignals driving these two stagea are derived from a common source con&&g of a crystal oscillator 80 that the two different signals can be phm locked. The high power pulse is at a frequency of 2,166 MHz and a power of 1 kW and lasts for approx 0.4 maec. Since thie pulse ia used &8 a synchronization pulse for the receiving stations and since it must not be longer than the propagation time from the ground to the E-layer and back, it has a short turn-off time. After the high power pulse has ceti, the transmitter tranemits a pulse at 2.166 MHz and about 10 W which la&s for a few milliseconds. This sequence is repeated at a rate of 10 times per second. The tranemitting antenna is a pair of crossed, inverted-V antennas which are fed 90” out of phase in order to produce a circularly polarized wave. Deep&e careful construction and adjustment, measurements made with circularly polarized receivers indicate that both magneto-ionic modes of propagation are excited in the ionosphere by the transmitter. However, there is rejection of the unwanted extraordinary mode and, in addition, the added attenuation of this mode during the daytime has helped to reduce the problem in phase measurement caused by magneto-ionic fading.
Phase height measUrementaof the E-region RECEIVER
726
A
TIME - MINUTES
Fig. 2. Example of f&n records.
The receiving system consists of two receivers, an osc~os~o~ and a recording camera. One receiver is relatively insensitive and is used to detect the ground wave of the high power pulse. The output of this receiver is used to synchronize the oscilloscope and to generate a blanking pulse for the main receiver. The main receiver is turned off by the blanking pulse during the high power ground wave pulse. It then receives and ampl&s the combined signal of the sky wave eminent of the high power pulse and the ground wave component of the lower power pulse. The beat frequency between these two signals is detected, amplified and used to intensity modulate the oscilloscope. A recording camera with its shutter constantly open is placed to view the face of the oscilloscope. The film in this camera is drawn uniformly past the aperture. S~chroni~ation of the film records taken at the three separate sites is obtained by turning the transmitter off for 1 set every 30 sec. At 30-min intervals the transmitter was keyed off and on with the call letters. A sample of one of the 6lm records is shown in Fig. 2. The records produced are the same type as reported by FINDLAY(1951) and more recently by FRASER and VINCENT(1970). The group of four lines which are slightly tilted with respect to the horizontal are caused by the heterodyne note between the E-layer reflected signal and the ground wave component of the low power pulse. Thus, the slope of these lines is proportional to the rate at which the phase path of the sky wave pulse is changing. The time resolution allows phase data to be read from the f?lm every quarter of a second if required. The vertical distance on the film between two sloping lines is equivalent to 0-I msec. When one of these lines moves downward by this distance, the phase-path length has changed by 2~ rad. The accuracy with which the phase measurement can be made is about one radian. Assuming that the radio signal travels at the speed of light over its entire phase path, a change of one radian would correspond to an increase or deorease of the E-layer reflection height by about 7 m. 3. DATA AXAIXSIS To estimate the effect of the electron density in the region below 85 km we assume quasi-longitudinal propagation and an electron density profile similar to that given by BELROSE(1966). The result is given by s
=krds
1‘v
-grad.
726
D. H. SCHRADEB
aad G. L. HOWER
where A# is the change in phase caused by the presence of the electrons, cu is the radian frequency of the radio wave, c is the velooity of light and p is the real part of the refractive index. The result corresponds to a possible effect on measured Elayer reflection height of about 90 m. We see that the entire ionization below 85 km causes a phase change only about 10 times the minimum detectable phase change. There are many interesting features to the phase-path changes observed. For instance, we have observed the early morning variation in the E-layer reflection height accurately. Figure 3 is a plot of the reflection height vs. time averaged over 3 days: 26 February, 28 February and 6 March, all in 1970, and a similar plot from data recorded 27 February, 1 March, 2 Ivfarch and 3 iNarch, all in 1968. The third curve (solid line) was derived theoretically by the method discussed below.
O-1970 X-1968
2 -7 . -8 0800
0830
0900
0930
TIME
Fig. 3. EarIy morning veriation of B-layer phase height.
Although we cannot determine the absolute layer height directly, it is possible to estimate the scale height, H, from the curvature of the ph~e-path length variation as shown in Fig. 3 and from the Chapman theory of layer formation. BUTCHER (1970) has also estimated the scale height from phase me~uremen~ of a radio wave reflected at oblique incidence from the E-layer. The frequency of the radio wave was 2.6 MHz and the transmitter was located at Rugby, England. In that work the E-layer height for the sun directly overhead (k,) was assumed to be 112 km. In the following discussion no assumption is made concerning h,, but the scale height is found as a function of different assumed values of the maximum electron density, N,, which would occur with the sun directly overhead. To explain this calculation we examine the relation between electron density, N, normalized height, z = (h. - h&/H and solar zenith angle, x, (DAVIES, 1965) N = N, exp [I - 2 - se0 2 exp (-2)]/2
where h, is the reflection height and a recombination-tie
loss process has been
Phase height measurement.@of the Lc-region
727
assumed. For reflection of a radio wave at 2.165 MHz we require an eleotron density of N = M762 x 1O11m-8. Rewriting the above expression we have 2 +ln[I
-2ln
($3
-z]
=ln(secX).
This is a relation between z and the variable u = In (set x) with a parameter IV,. This relation has been plotted in Fig. 4 for various values of the parameter, I?,,. As can be seen from Fig. 4 the plots are almost linear and therefore are easily made. We then calculate In (set x) as a function of time of day for 7 March from data given in the Ephemeris (1968). The vertical lines in Fig. 4 MW at the values of In (set x) as cakdated for the times shown. The intersection of the vertical lines with the graphs of the reIation between z and u yields the value of z for that time and that assumed value of N,. With these curves and with the measured values of change in the phase-path length, we may estimate H for various assumed values of IV,. To explain thii
0.9
0.8 0.7 0.6 0.5 N + 0.4 5 0.3 3 0.2 Fi 0.1 5 0.0 g -0.1 N-0.2 21 r-o.3 q-o.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 1.0
1.2 1.4 LntSecX)
1.6
Fig. 4. Nomogram for determination of wale height.
D. H. SCHRADER and G. L. HOWER
728
procedure let Ah represent the measured variation in reflection in Fig. 3 for the 1968 data. Then the reflection height (h) is
height as plotted
h = Ah + h, = H, + h, OC
Ah = H, + (h, - h,) where h, and h, are undetermined and unspecified constants. If, for example, we take N, to be 6:5 x 10” then from Fig. 4 we find at 0730 that z is -0.08 and from Fig. 3 we find that Ah is zero. At 0930, z is -1.148 from Fig. 4 and Ah is -7586 m from Fig. 3. After substituting these two sets of values of Ah and z into the equation above, we determine H to be 7100 m. This procedure was followed using the times 0730-0830, 0830-0930 and 0730-0930 for the following values of N,: 1.36 x loll, 3 x lo”, 4.6 x 10” and 5.5 x 10” rn3. The results are shown in Table 1 below. Tablo 1
YIXW ..--
Time Ysiw
1968 1968 1968
073~0830 0830-0930 0730-0930
1970 1970 1970
0730-0830 0830-0930 0730-0930
.4vo:L 1.35 x 10” me3 -. --__--
lVo =: 3 x 10” mm3 6600 6670 6630
4690 4200 4550
7520 5410 6750
N, = 4.5 x 1Wm-3 ..~._ _. 0940 7000 6960
N, = 5.6 x 10” rnw9 --._
._
7110 7090 7100 8100 5560 7230
The agreement between the phase height curve resulting from the 1968 measurements and the theoretical curves is best for a scale height of 7100 m and an assumed N, of 5.5 x 10” n-3 as seen in the table above and as seen by the closeness of the theoretical curve to the measured data in Fig. 3. This method seems reasonably accurate for determining the scale height, however it is not very sensitive to variations in N,. The 1968 data in the table show that a 7 per cent change in scale height corresponds to a 80 per cent change in N,. Consequently we cannot claim any accuracy for the estimated value of N,. The 1970 results show a greater curvature than the theoretical curves and thus do not agree with the theoretical model as well. As shown in the table, a scale height of about 4500 m corresponding to a N, of 1.35 x 10” m-3 is as close as we can come. However, at this low a value of N, there is an appreciable difference between the phase velocity in the medium near to the point of reflection and the phase velocity in free space. In that case the change in reflection height is not simply related to the change in phase delay. Hence we estimate the scale height of the K-layer to have been about 7 km when the 1968 data were measured. This value of scale height is to be compared with t.he value of 6.55 km derived from similar measurements during 15-28 March 1965 by BUTCHER (1970). The WSU system has also been able to detect solar flare effects and instances of apparent wave motions. The latter observations will be discussed in a future paper.
PM
height measurementrrof the E-region
729
Ackno&dge?#8elat8--Re8ear&supported by the Atmospheric Sciences Section, National Science Foundation, NSF Grante GA377 and GA10996. Mr. R. A. BUREAU contributed significantly to the development of the phase-path sounder.
REFERENCES The American Ephemeris and Nautical Almanac BELROSEJ. S.
1908
Emxx~a
E. C. DAVIES K.
1970 1965
FINDLAY J. W. FEUER 0. J. end Vmxm~ R. A. WhxTs J. M. and DAVIEE3 K.
1951 1970 1960
1966
U.S. Government Printing Office, Washington D.C. Ph&ca of the &Y’t~8 &b$x?r hnee+re, p. 53. Prentice-Hall, New Jersey. J. Atwwq&. Tew. P&8. $& 97. ~On08~hM’iC Radio Pvopaqatim, U.S. Gov. ernment Printing Office, Washington D.C. J. Atmqh, Tew. Phy8.1, 353. J. Atmmph. Terr. P&8. aa, 169I. J. g$O$&8. Rea, 6!& 2295.
Printed In Northern f&and
Phase height measurements of the E-region D. H. SCHRADERand G. I,. HOWER Department of Electrical Engineering, Washington State University, Pullman, Wash. 99163, U.S.A. (&ce~v~ 22 September 1970; in retied form 16 November1970)
Abstrset-A phsee-p&h sounderwith spaced receiversand not requiring ultra-stable oscillators is described. Measurementsof the morning variation in the E-laps height using this equipment are reported. From these rne~~rnen~ made in March 1968 a scale height of about 7 km is cakul&ed. 1.
INTR~DuOTI~~~
A PEASE-PATH sounder has been constructed and operated at Washington State University (WSU) to investigate wave motions in the E-region over Pullman, Washington. This sounder transmits a radio pulse vertioelly upwards and the echo returned from the ionosphere is received by three radio receivers with spacings shown in Fig. 1. The phase angle of the received signal at each receiver is compared to that of the transmitted signal and the time variations of these phase angles are recorded. These vtlriations are proportional to the variations in the phase-path traversed by the signal. Changes of the electron density in the lower ionosphere and movements of the reflection level will cause phi-pith v~~&tions. Phase-path sounders have been in use for some time (FINDLBY, 1951; W~rrs and DAVIES,1960). The WSU system, however, has several advantages over others previously reported. Since this sounder does not require long term phase stability between the tmnsmitter signal source and the receiver local oscillator, it is relatively inexpensive to construct and operate. Since it uses a pulse, the F-layer and E-layer signttls can be separated and measured independently. Finally the transmitter land receiver may be separated by any distance up to that over which a ground wave may propagate between them. The phase-path sounder measurements are designed to detect wave motions by means of layer height variations. Three simultaneous measurements of the phasepath length sre made. The purpose of having three spaced me~uremen~ is to allow the separation of phi-pith vsrirttions caused by solar flares and spatially random phase-path changes from those caused by wave motions, as well as to allow the determination of the direction of horizontal phase propagation of pressure wsves in the ionosphere. One can separate these different effects because the effects c&used by solar flares occur simultaneously at the three receiving points while those due to pressure wsves should be separated in time due to the propagation of the waves in the horizontal direction. The basic method by which the reflected radio signal phase is measured is to compare the phase of the sky wave component of a high power pulse with the phase of the ground wave component of a low power pulse that is phase locked to the high power pulse. The high power pulse is short enough and the transmitter is close enough to the receiver so that the ground wave component of the high power pulse has propagated beyond the receiver by the time that the sky wave component 723
124
D. H. SCERADER
and Q. L. HOWER
Fig. 1. Layout of transmitter-receiverlocations.
reachm the receiver. In the meantime, the transmitter has started to transmit a phase locked, low power pulse at a slightly different frequency which arrives at the receiver coincident with the arrival of the sky wave component of the high power pulse. These two signals heterodyne cautig a eignal at the di%ence frequency whose phase angle varies directly with the phase angle of the sky wave component of the high power pulse since the propagation path of the ground wave component of the low power pulse is highly phase stable. 2. EQUIPMENT The system uses a transmitting and three receiving stations, none of which are at the same location. The tmmitter transmits at two frequencies and at two different power levels requiring two separate final amplifier atages. The eignals driving these two stagea are derived from a common source con&&g of a crystal oscillator 80 that the two different signals can be phm locked. The high power pulse is at a frequency of 2,166 MHz and a power of 1 kW and lasts for approx 0.4 maec. Since thie pulse ia used &8 a synchronization pulse for the receiving stations and since it must not be longer than the propagation time from the ground to the E-layer and back, it has a short turn-off time. After the high power pulse has ceti, the transmitter tranemits a pulse at 2.166 MHz and about 10 W which la&s for a few milliseconds. This sequence is repeated at a rate of 10 times per second. The tranemitting antenna is a pair of crossed, inverted-V antennas which are fed 90” out of phase in order to produce a circularly polarized wave. Deep&e careful construction and adjustment, measurements made with circularly polarized receivers indicate that both magneto-ionic modes of propagation are excited in the ionosphere by the transmitter. However, there is rejection of the unwanted extraordinary mode and, in addition, the added attenuation of this mode during the daytime has helped to reduce the problem in phase measurement caused by magneto-ionic fading.
Phase height measUrementaof the E-region RECEIVER
726
A
TIME - MINUTES
Fig. 2. Example of f&n records.
The receiving system consists of two receivers, an osc~os~o~ and a recording camera. One receiver is relatively insensitive and is used to detect the ground wave of the high power pulse. The output of this receiver is used to synchronize the oscilloscope and to generate a blanking pulse for the main receiver. The main receiver is turned off by the blanking pulse during the high power ground wave pulse. It then receives and ampl&s the combined signal of the sky wave eminent of the high power pulse and the ground wave component of the lower power pulse. The beat frequency between these two signals is detected, amplified and used to intensity modulate the oscilloscope. A recording camera with its shutter constantly open is placed to view the face of the oscilloscope. The film in this camera is drawn uniformly past the aperture. S~chroni~ation of the film records taken at the three separate sites is obtained by turning the transmitter off for 1 set every 30 sec. At 30-min intervals the transmitter was keyed off and on with the call letters. A sample of one of the 6lm records is shown in Fig. 2. The records produced are the same type as reported by FINDLAY(1951) and more recently by FRASER and VINCENT(1970). The group of four lines which are slightly tilted with respect to the horizontal are caused by the heterodyne note between the E-layer reflected signal and the ground wave component of the low power pulse. Thus, the slope of these lines is proportional to the rate at which the phase path of the sky wave pulse is changing. The time resolution allows phase data to be read from the f?lm every quarter of a second if required. The vertical distance on the film between two sloping lines is equivalent to 0-I msec. When one of these lines moves downward by this distance, the phase-path length has changed by 2~ rad. The accuracy with which the phase measurement can be made is about one radian. Assuming that the radio signal travels at the speed of light over its entire phase path, a change of one radian would correspond to an increase or deorease of the E-layer reflection height by about 7 m. 3. DATA AXAIXSIS To estimate the effect of the electron density in the region below 85 km we assume quasi-longitudinal propagation and an electron density profile similar to that given by BELROSE(1966). The result is given by s
=krds
1‘v
-grad.
726
D. H. SCHRADEB
aad G. L. HOWER
where A# is the change in phase caused by the presence of the electrons, cu is the radian frequency of the radio wave, c is the velooity of light and p is the real part of the refractive index. The result corresponds to a possible effect on measured Elayer reflection height of about 90 m. We see that the entire ionization below 85 km causes a phase change only about 10 times the minimum detectable phase change. There are many interesting features to the phase-path changes observed. For instance, we have observed the early morning variation in the E-layer reflection height accurately. Figure 3 is a plot of the reflection height vs. time averaged over 3 days: 26 February, 28 February and 6 March, all in 1970, and a similar plot from data recorded 27 February, 1 March, 2 Ivfarch and 3 iNarch, all in 1968. The third curve (solid line) was derived theoretically by the method discussed below.
O-1970 X-1968
2 -7 . -8 0800
0830
0900
0930
TIME
Fig. 3. EarIy morning veriation of B-layer phase height.
Although we cannot determine the absolute layer height directly, it is possible to estimate the scale height, H, from the curvature of the ph~e-path length variation as shown in Fig. 3 and from the Chapman theory of layer formation. BUTCHER (1970) has also estimated the scale height from phase me~uremen~ of a radio wave reflected at oblique incidence from the E-layer. The frequency of the radio wave was 2.6 MHz and the transmitter was located at Rugby, England. In that work the E-layer height for the sun directly overhead (k,) was assumed to be 112 km. In the following discussion no assumption is made concerning h,, but the scale height is found as a function of different assumed values of the maximum electron density, N,, which would occur with the sun directly overhead. To explain this calculation we examine the relation between electron density, N, normalized height, z = (h. - h&/H and solar zenith angle, x, (DAVIES, 1965) N = N, exp [I - 2 - se0 2 exp (-2)]/2
where h, is the reflection height and a recombination-tie
loss process has been
Phase height measurement.@of the Lc-region
727
assumed. For reflection of a radio wave at 2.165 MHz we require an eleotron density of N = M762 x 1O11m-8. Rewriting the above expression we have 2 +ln[I
-2ln
($3
-z]
=ln(secX).
This is a relation between z and the variable u = In (set x) with a parameter IV,. This relation has been plotted in Fig. 4 for various values of the parameter, I?,,. As can be seen from Fig. 4 the plots are almost linear and therefore are easily made. We then calculate In (set x) as a function of time of day for 7 March from data given in the Ephemeris (1968). The vertical lines in Fig. 4 MW at the values of In (set x) as cakdated for the times shown. The intersection of the vertical lines with the graphs of the reIation between z and u yields the value of z for that time and that assumed value of N,. With these curves and with the measured values of change in the phase-path length, we may estimate H for various assumed values of IV,. To explain thii
0.9
0.8 0.7 0.6 0.5 N + 0.4 5 0.3 3 0.2 Fi 0.1 5 0.0 g -0.1 N-0.2 21 r-o.3 q-o.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 1.0
1.2 1.4 LntSecX)
1.6
Fig. 4. Nomogram for determination of wale height.
D. H. SCHRADER and G. L. HOWER
728
procedure let Ah represent the measured variation in reflection in Fig. 3 for the 1968 data. Then the reflection height (h) is
height as plotted
h = Ah + h, = H, + h, OC
Ah = H, + (h, - h,) where h, and h, are undetermined and unspecified constants. If, for example, we take N, to be 6:5 x 10” then from Fig. 4 we find at 0730 that z is -0.08 and from Fig. 3 we find that Ah is zero. At 0930, z is -1.148 from Fig. 4 and Ah is -7586 m from Fig. 3. After substituting these two sets of values of Ah and z into the equation above, we determine H to be 7100 m. This procedure was followed using the times 0730-0830, 0830-0930 and 0730-0930 for the following values of N,: 1.36 x loll, 3 x lo”, 4.6 x 10” and 5.5 x 10” rn3. The results are shown in Table 1 below. Tablo 1
YIXW ..--
Time Ysiw
1968 1968 1968
073~0830 0830-0930 0730-0930
1970 1970 1970
0730-0830 0830-0930 0730-0930
.4vo:L 1.35 x 10” me3 -. --__--
lVo =: 3 x 10” mm3 6600 6670 6630
4690 4200 4550
7520 5410 6750
N, = 4.5 x 1Wm-3 ..~._ _. 0940 7000 6960
N, = 5.6 x 10” rnw9 --._
._
7110 7090 7100 8100 5560 7230
The agreement between the phase height curve resulting from the 1968 measurements and the theoretical curves is best for a scale height of 7100 m and an assumed N, of 5.5 x 10” n-3 as seen in the table above and as seen by the closeness of the theoretical curve to the measured data in Fig. 3. This method seems reasonably accurate for determining the scale height, however it is not very sensitive to variations in N,. The 1968 data in the table show that a 7 per cent change in scale height corresponds to a 80 per cent change in N,. Consequently we cannot claim any accuracy for the estimated value of N,. The 1970 results show a greater curvature than the theoretical curves and thus do not agree with the theoretical model as well. As shown in the table, a scale height of about 4500 m corresponding to a N, of 1.35 x 10” m-3 is as close as we can come. However, at this low a value of N, there is an appreciable difference between the phase velocity in the medium near to the point of reflection and the phase velocity in free space. In that case the change in reflection height is not simply related to the change in phase delay. Hence we estimate the scale height of the K-layer to have been about 7 km when the 1968 data were measured. This value of scale height is to be compared with t.he value of 6.55 km derived from similar measurements during 15-28 March 1965 by BUTCHER (1970). The WSU system has also been able to detect solar flare effects and instances of apparent wave motions. The latter observations will be discussed in a future paper.
PM
height measurementrrof the E-region
729
Ackno&dge?#8elat8--Re8ear&supported by the Atmospheric Sciences Section, National Science Foundation, NSF Grante GA377 and GA10996. Mr. R. A. BUREAU contributed significantly to the development of the phase-path sounder.
REFERENCES The American Ephemeris and Nautical Almanac BELROSEJ. S.
1908
Emxx~a
E. C. DAVIES K.
1970 1965
FINDLAY J. W. FEUER 0. J. end Vmxm~ R. A. WhxTs J. M. and DAVIEE3 K.
1951 1970 1960
1966
U.S. Government Printing Office, Washington D.C. Ph&ca of the &Y’t~8 &b$x?r hnee+re, p. 53. Prentice-Hall, New Jersey. J. Atwwq&. Tew. P&8. $& 97. ~On08~hM’iC Radio Pvopaqatim, U.S. Gov. ernment Printing Office, Washington D.C. J. Atmqh, Tew. Phy8.1, 353. J. Atmmph. Terr. P&8. aa, 169I. J. g$O$&8. Rea, 6!& 2295.