Aerospace and Electronic Systems Magazine December 2017 - 34

Sense and Adapt Capabilities of 3rd Gen HFSWR

Figure 3.

Plot of radar operating frequencies over an eight-day period for the two operating threads.

Figure 4.

PRISM-Performance statistics over an eight-day period for both contiguous-time-onfrequency and bandwidth.

optimized for the radar mission. During the twilight
and the hours of darkness the D-layer, with its absorptive properties, disappears and signals from distant
users and natural atmospheric discharges received via
sky-wave mode of propagation. Consequently, the
night-time noise level increases by 10 to 15 dB and
the number of external users increases significantly.
A plot of the operating frequencies utilized by
the radar system over an eight-day period in 2015
is presented in Figure 3. The statistics for the eightday period are presented in Figure 4 for both timeon-frequency and bandwidth of operation. It can be
observed that, the majority of the time, the radar remained on the allocated channel for the maximum
time of 20 minutes. The rules of operation permitted
the radar to operate beyond the 20-minute limit in
the event another channel was not available, providing it would change frequency as soon as an alternative became available. There is also a period where
the system could not find an available channel and
was consequently off or operating at the safe haven
frequency. Approximately 15% of the time the radar
was forced to move off the allocated channel, thus
not completing the 20 minutes. It can also be noted
that the system was operating at the maximum bandwidth of 37 kHz for the majority of time.
The pie chart of Figure 4 and the histogram
of Figure 5 show minimal downtime of the radar
throughout this period. As HFSWR systems have
a long integration period (1-4 minutes) associated
with each track update, the downtime has minimal
impact to the target tracking capability of the system.
These figures effectively demonstrate the four constructs of the frequency selection logic and operation
of the radar described earlier.

IMPULSIVE NOISE REJECTION
Interference can be a major impediment to the operational performance of radar systems. A common source
of interference is impulsive noise which may appear
due to regional lighting discharges or local man-made
noise sources. Impulsive noise is characterized as having a short duration and therefore may only affect a
small portion of the time series. However, for processing techniques that integrates signals and transform to
the frequency (Doppler) domain, this will result in elevated noise for the entire Doppler frequency band.
Traditional methods for limiting the signal degradation due to impulsive noise rely on the detection and
removal of corrupted pulses to limit frequency leakage
into the signal band [6], [7]. These techniques typically rely upon time domain envelope detection methods within the bandwidth of operation. The corrupted
pulses are identified though detection of the increased
signal power of the envelope. Thus, the received enve-

Figure 5.

Analysis of the time-off data.

32

IEEE A&E SYSTEMS MAGAZINE

DECEMBER 2017



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