5. There is an approximate inverse relationship between yield and
predominant frequency.
6, Pulses received close-in (i.e. approximately 20 km) exhibit
very short rise times (less than a microsecond) in a negative direction
(4.e. the electric field vector is downward),
7.
8.
The pulse is predominantly vertically polarized.
Close-in reception indicates that certain nuclear-weapon char-
acteristics can be determined from pulse fine structure.
9. Even low-yield nuclear detonations can produce a pulse receivable at distances in excess of 1,000 ia.
10. The ground wave is generally not detectable beyond about 1,500
km from the source because the ionospheric sky wave reflections predominate.
ll. A fix of the source of the pulse can be obtained with direction-finding equipment; observed azimuthal errors to date using equip-
ment tuned to 10 ke have been between O and 9 degrees; most errors have
been less than 3 degrees.
12. At distances, the pulse is extended to approximately ten times
its close-in length.
This is the result of multiple arrivals by vari-
ous paths, each characterized by one or more ionospheric reflection,
13. Close-in fine structure disappears during sky wave propagation
to distances.
INSTRUMENTATION
All stations, both close to and at distances from the detonation
points, measured the vertically-polarized component of the emitted
pulse. Close to the explosion, attemators were-sometimes needed to
prevent overloading, while at distances, amplifiers were needed. At
all places, the pulse (in no case greater than 1] msec in length) was
displayed on oscilloscope cathode ray tubes, and recorded photographically with either plate cameras triggered by the pulses or by strip
film cameras started manually just before detonation times.
World-wide Timing. In order to assist in locating the pulse, particularly at distant locations where the energy was sometimes no
greater than the noise, reliance at all stations was placed upon timing
broadcasts from WWV (Washington, D. C.), WWVH (Maui, T. H.), JJY (Tokys)
or GBR (Rugby, England).
To further assure a common time base, both
GBR and WWV were recorded on the same film as a low frequency trans-
mitter such as NSS (Annapolis, Maryland).
This film was used for fur-
ther correlation for instances where fading affected reception of WWV
but did not affect reception of the high-powered low-frequency Navy
traffic. In addition, all stations used locally-generated timing
pulses coordinated with a world time station. The nuclear-device pulse
and timing pips were presented on different beams of a milti-beam oscilloscope and photographed.
local.Timing.
Annex A of Reference 3 has a detailed account of
the National Bureau of Standards (NBS) local timing unit.
A typical
time record from a close-in station is shown in Figure 1 and one from a
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