  We demonstrate that extremely rapid and weak periodic and non-periodic
signals can easily be detected by using the autocorrelation of intensity as a
function of time. We use standard radio-astronomical observations that have
artificial periodic and non-periodic signals generated by the electronics of
terrestrial origin. The autocorrelation detects weak signals that have small
amplitudes because it averages over long integration times. Another advantage
is that it allows a direct visualization of the shape of the signals, while it
is difficult to see the shape with a Fourier transform. Although Fourier
transforms can also detect periodic signals, a novelty of this work is that we
demonstrate another major advantage of the autocorrelation, that it can detect
non-periodic signals while the Fourier transform cannot. Another major novelty
of our work is that we use electric fields taken in a standard format with
standard instrumentation at a radio observatory and therefore no specialized
instrumentation is needed. Because the electric fields are sampled every 15.625
nanoseconds, they therefore allow detection of very rapid time variations.
Notwithstanding the long integration times, the autocorrelation detects very
rapid intensity variations as a function of time. The autocorrelation could
also detect messages from Extraterrestrial Intelligence as non-periodic
signals.
