60 Hz Hum
The power grid radiates VLF radio waves at frequencies
of 60 hertz and the harmonic multiples of that number. The sound is a background
hum that is stronger the closer the receiver is to the lines and also stronger
if the line voltage is higher. On the spectrogram, 60 Hz hum appears as
a line or series of lines running horizontally near the bottom of the spectrogram.
LORAN
LORAN navigation signals can be heard in the VLF range.
The sound is that of a rapid clicking. On the spectrogram, a series of closely
spaced dots appear in evenly spaced horizontal rows.
Russian ALPHA Navigation Signals
The three Russain ALPHA navigation transmitters each transmit
a distinct set of tones that repeat. The frequency of the transmissions
is between 13 kHz and 15 kHz, but sometimes they are heard at lower frequencies.
On the spectrogram, ALPHA signals appear as a series of horizontal dashes.
OMEGA
OMEGA was a navigation signal operated by the US Coast
Guard (in the United States) and other countries around the world. In the
frequency range above 10 kilohertz, the signal could be heard as a repeating
series of tones. On the spectrogram, OMEGA show up as a series of horizontal
dashes of about one-second duration. Each of the seven OMEGA stations transmitted
a unique pattern of four tones that repeated every ten seconds. OMEGA ceased
operation in September 1997, done in by the advent of global positioning
system (GPS) technology. Those of us who have studied VLF miss OMEGA a lot.
- -*- OMEGA (35k)
SEPAC
In the spring of 1992, INSPIRE participated in the SEPAC
(Space Experiments with Particle ACcelerators) experiment carried on board
the Shuttle Atlantis as part of the ATLAS 1 mission (STS-45). SEPAC consisted of an electron gun mounted on a pallet in the payload bay which
could shoot a modulated beam of electrons into space. INSPIRE attempted
to record the resulting VLF waves created by turning the electron gun off
and on at various frequencies. The sound files are simulated examples of
the expected SEPAC radio signals. During the missions the signals were not
detected on the surface of the earth although they were detected by a receiver
on Atlantis. It was tentatively determined that the power output of the
SEPAC instrument may not have been sufficient to be detected on earth.
-
-*- Expected waves from SEPAC (131k)
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