IEEE - Aerospace and Electronic Systems - January 2023 - 56

Passive Radar: Past, Present, and Future Challenges
Figure 3.
Operative scenario for ground-based transmitters and main PR processing blocks.
Broadcast services transmitters, including groundbased
both analog and digital audio/video broadcasting
services, are suitable for long-range surveillance of aerial
and maritime targets. Moving toward higher frequency
and wider bandwidth signals (such as those of satellite
transmitters or base stations for networking services), PR
systems can be employed for shorter range monitoring
applications to detect small drones, vehicles, and people
or for remote areas surveillance.
In this article, PR is considered from the perspective
given by the class of exploited transmitters of opportunity:
for each class (ground-based transmitters, satellite-based
transmitters, and wireless network services), strengths and
weaknesses are discussed and results for different fields of
application illustrated.
GROUND-BASED TRANSMITTERS OF OPPORTUNITY
FOR BROADCAST SERVICES
Terrestrial transmitters of opportunity for broadcast services
have been the most popular andwidely used in PR technology
so far. In fact, the extensive coverage and great continuity
offered by broadcast transmitters, e.g., FM radio, DAB, or
DVB-T, make these IOs especially suitable for long-range
surveillance applications, which require high reliability and
robustness to the conditions ofthe operating environment. For
these reasons, PR technology relying on such IOs has been
the subject ofmany research activities and lead to the development
ofan effective signal processing scheme.
A typical operative scenario is sketched in Figure 3. Here,
the passive system employs a reference antenna steered
toward the transmitter acquiring the direct signal (which acts
as the reference signal for matched filtering) and multiple surveillance
antennas pointed in the direction to be surveyed
56
receiving the reflected signal (due to the reflection ofthe transmitted
signal from the target). Along with the sketch of the
considered scenario, Figure 3 shows a block diagram of the
typical signal processing scheme foreseen in a PR system. To
facilitate the understanding of the following sections even for
a nonexpert reader, in the following, we briefly describe the
main processing stages; however, we refer the interested
reader to the referenced technical literature for detailed
explanations and additional insights.
Direct signal interference (DSI) and clutter cancellation:
This stage is aimed at removing from each
surveillance signal the fraction of the direct signal
emitted by the exploited IO (dashed red lines in
Figure 3), as well as additional interferences and
multipath contributions coming from the stationary
scene. An effective DSI removal is crucial to avoid
masking effects on the low-power target echoes. A
common way to perform this stage is to subtract
from the signal collected from each surveillance
antenna the undesired interference signal, estimated
by summing up delayed and weighted replicas
of the direct signal, [21], [22], [23], typically
acquired by the reference antenna.
Range-velocity map evaluation: Once the interfering
contributions have been removed, the key step in the PR
processing chain is based on the evaluation ofthe crossambiguity
function between the surveillance signal and
the reference signal. This stage approximates a bank of
matched filters (MF) tuned to different Doppler values
of interest and a potential target echo appears as a peak
at a given bin of the resulting bistatic range-velocity
map. The number ofemployedDoppler filters is proportional
to the selected coherent processing interval (CPI),
which is usually quite large in PR, to extract the lowpower
target echoes from the competing disturbance as
IEEE A&E SYSTEMS MAGAZINE
JANUARY 2023
IEEE - Aerospace and Electronic Systems - January 2023

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