sky wave give an ionospheric layer height of about 90 lam.
Some records
(not included in this report) show as many as five sky waves, but of
course with less energy for each reflection, and they also indicate an
ionospheric layer height of about 90 km.
characteristic first negative-going pulse;
ALJ close-in records show the
The close-in records from Shot 6
(23 km away) appear to have spurious components imposed. This could be
pick-up on lead-ins or through the earth. Other experiments at comparable distances at the Nevada Test Site have all involved detonations
almost two orders of magnitude lower in energy.
The changes in waveform due to the filtering effects of the ionosphere (absorption of the higher frequency components) and interference
between different sky-wave modes is quite apparent as the broad-band
pulse is recorded at greater distances; the pulse loses character and
presents a damped sine wave appearance, Broad-band waveforms at the
far stations, in general, mean about 6 to 100 kc, which encompasses the
greatest portion of the energy available.
FourierAnalyses.
The National Bureau of Standards has
Fourier analyses of two of the Castle shots (References 11,12).
transform
©
performed
The
-2rrft
°
has been computed by using the values of fleld strength tabulated as a
function of time and performing the integration numerically on the SEAC.
The magnitude of g(f) (the square root of the sum of the squares of the
real and imaginary parts) for the Shot 2 pulse, as received at some NBS
sites, has been plotted as a function of frequency in Mgure 13.
In order to evaluate absorption due to earth and ionospheric losses,
a distance factor is employed. For short distances this is the inverse
of the distance, but for longer distances, the earth's curvature is
taken into account, assuming that the ionosphere and the earth act as a
wave guide. From Reference 10, the factor 1s
/SIN2
Where:
|
!
(2)
0 = Distance from the origin, (lm).
R = Earth's radius, (km).
Figure 14 is a plot of the Shot 2 pulse, but the spectral intensity C, which is proportional to the amplitude of the received field
strength per cycle of bandwidth is miltiplied by ~/SiINg, Figure 15
is a similar plot for Shot 3.
deduced from Mgures 14 and 15,
Some propagation characteristics may be
For Shot 3, 126 kt, the peak frequen-
cies of Figure 15 decrease from about 18 ke for Eniwetok (320 iam) to
ll ke for Ft. Belvoir, Virginia (11,530 km). However, in the case of
Shot 2,‘ with a "ei4 of 10.5 Mt, the peak frequency of Figure 14 re=
25
a