IEEE - Aerospace and Electronic Systems - September 2021 - 10

Space-Based Global Maritime Surveillance. Part I: Satellite Technologies
Figure 1.
Processing ofimages acquired by space-based sensors and AIS data with the state-of-the-art artificial intelligence and data fusion techniques
allows extraction of information on the current maritime situation, such as detected ships and their category (e.g., oil tankers, fishing boats,
passenger ships), maritime traffic routes, oil spills, anomalies (e.g., trans-shipments), and ship trajectories. In particular, the extracted information
can help to detect and prevent piracy, human trafficking, and environmental threats such as oil spills.
between 160 and 2,000 km.Apolar orbit is a particular LEO in
which a satellite passes above or nearly above both poles ofthe
Earth on each revolution. The portion of the Earth's surface
that a space-based sensor sees while moving along its orbit is
referred to as its swath. Typical satellite swaths in LEO range
from tens to hundreds of kilometres, and for this reason they
can only observe and communicate with a limited region of
the Earth at a time. This means that constellations of satellites
are needed to provide continuous coverage and guarantee a
lower revisit time, that is the time interval between two consecutive
times that a sensor observes the same region. Sensors
mounted on constellations of at least four satellites in polar
orbits, e.g., COSMO-SkyMed, are able to illuminate the same
area of interest multiple times a day above the poles, and at
least twice a day above the Equator. Satellites in lower regions
ofLEO suffer from fast orbital decay, requiring either periodic
rebooting to maintain a stable orbit, or launching replacement
satellites when old ones re-enter. Space-based sensors for EO
require the acquired images to be transferred to specific ground
stations as soon as they are in their field ofview (FOV). Therefore,
the density and the geographical distribution of the
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ground stations have a direct impact on the latency at which
the acquired images are available to the end-user. In order to
reduce the latency ofthe communication with ground stations,
constellations of interconnected satellites can be used as tactical
assets to transfer the information acquired by satellites to
the terrestrial stations almost in real-time. In this context, the
European Data Relay System (EDRS), an ESA project within
the Advanced Research in Telecommunications Systems
(ARTES) program, uses two geostationary data relay satellites
to provide links to satellites in LEO, thus, enabling real-time
communications between spacecraft and terrestrial stations.
In the following sections, we provide a comprehensive
overview on the main current space-based sensor technologies
that can be exploited for MS. For each technology,
we present the main advantages and limitations, when
used in the scope ofMS.
SAT-AIS
In 2002, the International Maritime Organization (IMO)
Safety of Life at Sea (SOLAS) convention [9] included a
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IEEE - Aerospace and Electronic Systems - September 2021

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