We show that pulse compression techniques which use sets of more than two complementary sequences can be useful in HF radar probing of the D region of the ionosphere. The sets we propose here have an important advantage over the standard complementary pair of codes: they can eliminate E or F region multiple-hop echoes which interfere with D region echoes, when relatively short radar interpulse periods are used. Consequently, both altitude resolution and radar sensitivity can be substantially improved over previous experiments by coding the transmitter pulses and by using shorter interpulse periods. Sets of complementary sequences also perform coherent integration of the received signals and can eliminate the instrumental dc bias. The technique has been tested at Arecibo in an experiment in which we measure the spectra of partially reflected D region echoes. In this experiment, sophisticated processing hardware is used to decode, to perform coherent integration, and to compute–simultaneously and on-line–the spectra of echoes from 256 altitudes in the 0- to 230-km range.
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