IEEE - Aerospace and Electronic Systems - August 2023 - 50

An Overview of Radar Operation in the Presence of Diminishing Spectrum
Table 1.
Radar Frequency Selection Considerations
Factor
Range Resolution
Frequency
Type
Design Rationale
IBW Resolve closely spaced targets, minimize distributed clutter
RCS, facilitate radar image quality and target discrimination
Electronic Protection OBW or
IBW
Atmospheric
Propagation
Clutter Phenomenology OBW and
IBW
Target Response
Materials Penetration
Platform Mounting
Technology and
Manufacturing
Readiness
OBW and
IBW
Mode Implementation OBW and
IBW
Frequency variability used to avoid in-band interference or
spread interference power
OBW Manage propagation loss to control radar operating range and
geometry
Select operating frequency to affect clutter reflectivity or IBW to
reduce the RCS of distributed clutter by reducing clutter cell
area
Optimize frequency selection to enhance target signature and
IBW to resolve target features
OBW Select frequency to penetrate or reflect from materials
Different modes favor different operating frequencies
OBW Limited payload capacity favors higher frequency systems to
provide adequate antenna gain and suitable beamwidth
OBW and
IBW
Compromise based on RF hardware availability and datahandling
capacity
Many radars employ pulse compression waveforms
[1], [2], [3]. Pulse compression allows the radar to
achieve higher average power while maintaining the
range resolution of a much shorter pulse. The radar
increases duty cycle by transmitting a longer pulse for a
given pulse repetition interval (PRI), but encodes the
pulse with a waveform with bandwidth B. The resulting
range resolution is dR ¼ c=ð2BÞ [1]. Figure 1 shows
radar resolution as a function of bandwidth, along with
ground range resolution for several grazing angles cg,
where grazing angle is the angle that subtends the radar
line of site and a tangent to the Earth's surface. As
seen, finer resolution in slant range translates to finer
ground range resolution. Since the RCS of distributed
clutter is, sc ¼ s0Ac, where s0 is clutter backscatter per
unit area and Ac
is the clutter cell area, finer ground
range resolution leads to smaller cell area and thus
reduced clutter signature. With increasing bandwidth,
the radar can collect a high range resolution (HRR) profile
of the target to facilitate target discrimination [15].
To discern target features, HRRs may use bandwidths
in excess of several hundred megahertz to achieve range
resolution finer than one third of a meter. Stretch processing
is sometimes used to mitigate the computational
burden of pulse compressing high bandwidth waveforms
[16].
50
Radar systems strive to approach noise-limited performance
through various interference mitigation methods.
Electronic protection (EP) refers to various methods to
assure radar performance in the presence of RFI, whether
the RFI is intentional or not. Frequency agility and waveform
selection play an important role in EP [14]. With sufficient
OBW, the radar can hop to a new frequency upon
sensing in-band interference, thus avoiding RFI. Conversely,
wide IBW allows the radar to spread its energy,
Figure 1.
Range resolution in slant and ground planes.
IEEE A&E SYSTEMS MAGAZINE
AUGUST 2023
IEEE - Aerospace and Electronic Systems - August 2023

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