agreement with other investigators over other than North Atlantic
paths.
With NPM-Austin (5,800 lm) as a base for predicting field
strengths for NPM-Thule (7,600 km), the latter is low by 6 or 8 db.
This is an apparent contradiction of most similar measurements which
give greater attenuations over east-west paths, as compared with north -
south paths.
However, the auroral zone was not crossed in the course
of those measurements.
CONCLUSIONS AND RECOMMENDATIONS
The first AFOAT~1 close-in (320 km) recordings of the electro-
Magnetic pulse emitted at the time of detonation of a large nuclear
device (megaton range) were made during the Castle operations.
Varia-
tions in field strengths, measured with presumably identical equipment
at the various locations, are not explainable. Waveforms were recorded
at distances up to 12,000 km. Beyond about 2,000 to 4,000 km, close-in
detail disappears. With proper corrections for path, terrain, ionoSpheric conditions, time of day, etc., it may be possible to make a
fair estimate of yield as a function of field strength received. How
ever, corrections to be made are imperfectly known. Frequency analysis
of waveforms, together with other characteristics, may offer some as-
sistance.
Reception and identification of nuclear-device pulses, when time
of detonation is known to a millisecond, is relatively easy; doing the
same thing on a 24-hour basis, when the detonation time is not known,
is much more difficult. This means that more information is needed on
techniques of discrimination and much of this can be learned by studying naturally-occurring atmospherics.
Studies of the various parameters affecting VLF propagation should
be emphasized, and this should encompass a well-coordinated theoretical
and experimental approach. Studies in VLF propagation to date have
largely been concerned with communications frequencies where transmis-
sions are repeatable. Further work along this tack in the lower portions of the band is worthwhile. In addition, further studies of
naturally—occurring atmospherics should be conducted, because natural
flashes have many similarities to the man-made type. The sferics
studies should be extended to the low-audio frequencies and into the
megacycle range~---both being areas where limited work has been done so
far.
39
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