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Ann. Geophys., 20, 1469-1477, 2002
www.ann-geophys.net/20/1469/2002/
© European Geosciences Union 2002
High resolution general purpose D-layer experiment for EISCAT incoherent scatter radars using selected set of random codes
T. Turunen, A. Westman, I. Häggström, and G. Wannberg
EISCAT Scientific Association, P.O.Box 164, SE-981 23 Kiruna, Sweden
Correspondence to: T. Turunen
(tauno.turunen@eiscat.com)
Abstract. The ionospheric D-layer
is a narrow bandwidth radar target often with a very small scattering cross
section. The target autocorrelation function can be obtained by transmitting a
series of relatively short coded pulses and computing the correlation between
data obtained from different pulses. The spatial resolution should be as high
as possible and the spatial side lobes of the codes used should be as small as
possible. However, due to the short pulse repetition period (in the order of
milliseconds) at any instant, the radar receives detectable scattered signals
not only from the pulse illuminating the D-region but also from 3–5
ambiguous-range pulses, which makes it difficult to produce a reliable estimate
near zero lag of the autocorrelation function. A new experimental solution to
this measurement problem, using a selected set of 40-bit random codes with 4
µs elements giving 600 m spatial resolution is presented. The zero lag is
approximated by dividing the pulse into two 20-bit codes and computing the
correlation between those two pulses. The lowest altitudes of the E-layer are
measured by dividing the pulse into 5 pieces of 8 bits, which allows for
computation of 4 lags. In addition, coherent integration of data from four
pulses is used for obtaining separately the autocorrelation function estimate
for the lowest altitudes and in cases when the target contains structures with
a long coherence time. Design details and responses of the experiment are
given, and analysed test data are shown.
Key words. Radio science (signal
processing); Ionosphere (plasma temperature and density; instruments and
techniques)
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