Aerospace and Electronic Systems Magazine March 2018 - 38

Experimental Analysis of a HF Hybrid Sky-Surface Wave Radar

Figure 9.

Typical sea clutter spectrum with the same CPI. (a) Sea clutter of HF
surface wave radar with a frequency of 4.05 MHz. (b) Sea clutter of
HFSSWR with a frequency of 19.008 MHz.

and the extraordinary wave, or Fx(h). The red vertical lines in the
ionogram are radio frequency interference. Figure 8 reveals that
the best operating frequencies for single-mode propagation should
be larger than 12 MHz. Within this frequency range, the Es layer is
the primary propagation layer.
During the experiment, a spectrum analyzer (SA) was connected to 1 of the 16 receiving channels. Frequency occupancy can
be monitored in real time. Combined with the information from
the OIS and SA, the 19.008-MHz frequency was chosen as the
best available frequency in that experiment. Under this frequency,
the grating lobe arises because of the large array spacing. But the
unambiguous angular range [−52°, 52°] is big enough for the area
of interest.
The transmitted waveform is FMCW, with a carry frequency
of 19.008 MHz, a bandwidth of 40 kHz, and a sweep interval of
20 ms. The radar data were sequentially recorded and processed in
units of CPI. Each CPI is 41-s long and consists of 2,048 chirps.
The conventional (matched filter) processing method was used to
obtain the range-Doppler (RD) map.
38

Figure 10.

SDC under different ionosphere conditions. (a) Minor ionosphere contamination. (b) Major ionosphere contamination.

EXPERIMENTAL RESULTS
First, a typical sea clutter spectrum from the HF surface wave radar is
shown to compare with that from HFSSWR. Because we are interested in slow targets, only a fraction of the Doppler frequency is shown
in the two figures. Figure 9a shows typical real sea clutter data of HF
surface wave radar with a frequency of 4.05 MHz. The Bragg peaks
are sharp and concentrate in the theoretical position. Figure 9b shows
a typical spread clutter spectrum of this experiment. The Bragg peaks
are broadened. The two spectrum figures are from different radars
operating at different frequencies and in different ocean areas. By
comparing these two figures, we can have an intuitive impression of
the conditions without and with ionospheric contamination.
Ionosphere is a time-varying and nonstationary medium. The
broadening of the sea clutter varies wildly under different ionosphere conditions. During the experiment, the ionospheric state
changed frequently, which caused difficulties in continuous observation. Next, we show two levels of broadening in 1 h. Figure 10a
shows the SDC under minor ionospheric contamination, in which
each Bragg peak splits into two peaks. Figure 10b shows the SDC

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018



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