  Maritime surveillance (MS) is crucial for search and rescue operations,
fishery monitoring, pollution control, law enforcement, migration monitoring,
and national security policies. Since the early days of seafaring, MS has been
a critical task for providing security in human coexistence. Several
generations of sensors providing detailed maritime information have become
available for large offshore areas in real time: maritime radar sensors in the
1950s and the automatic identification system (AIS) in the 1990s among them.
However, ground-based maritime radars and AIS data do not always provide a
comprehensive and seamless coverage of the entire maritime space. Therefore,
the exploitation of space-based sensor technologies installed on satellites
orbiting around the Earth, such as satellite AIS data, synthetic aperture
radar, optical sensors, and global navigation satellite systems reflectometry,
becomes crucial for MS and to complement the existing terrestrial technologies.
In the first part of this work, we provide an overview of the main available
space-based sensors technologies and present the advantages and limitations of
each technology in the scope of MS. The second part, related to artificial
intelligence, signal processing and data fusion techniques, is provided in a
companion paper, titled: "Space-based Global Maritime Surveillance. Part II:
Artificial Intelligence and Data Fusion Techniques" [1].
