Page 53 - Military Technology 12/2018

C4I Forum MT 12/2018 · 51

while retaining the possibility for other inclinations to send appropriate in-
structions to the guidance system. An appropriate inclination also enables If it is necessary to observe areas outside
orbiting altitude and spacing between satellites to be adjusted to achieve the electronic steering range,
a constellation characterised by repeating ground tracks, so that a satellite it is possible to steer
(or a successive satellite in the same constellation) will view a particular the beam mechanically
region from the same angle each time it passes over: this configuration and slave the satellite’s
is effective for imagery analysis based on automated processes to detect rotation appropriately.
changes between images.
A satellite’s technological capabilities depend on many factors, such
as size and type of radar antenna, transmission power, type of signal
and waveform, data processing capabilities and communication capac-
ity. Usually a modern SAR system uses active electronically steered ar-
rays (AESA), consisting of a matrix of transmit/receive modules: timing
differences between the signal of each module are used to form and direct
the radar beam, although in some systems reflector antennas that allow
lighter weight and lower cost are used. The most important advantage for
AESA antennas is the ability to steer the radar beam electronically; the
range over which the beam can be so steered depends on the spacing
between transmit/receive modules. Electronic steering is instantaneous,
allowing the satellite to switch quickly from one area to another of interest,
even when those areas are far apart. If it is necessary to observe are- 5-10Gbps: other similar systems are in their initial testing phases in both
as outside the electronic steering range, it is possible to steer the beam Europe and Japan.
mechanically and slave the satellite’s rotation appropriately. Mechanical Of course, modern space-based SAR provides 3D images, because
steering is much slower than electronic and the radar may not be able to standard 2D images can be difficult to interpret due to component signa-
operate while the satellite is being rotated. In the case of reflector anten- tures from the varying heights of buildings, structures, terrain character-
nas, radar steering would be more difficult and using this kind of anten- istics and similar content being projected into the scene. There is there-
na the satellite’s ability to observe widely separated areas of interest is fore currently considerable emphasis on studies of new algorithms and
somewhat reduced. Another important feature that supports selection of software for surveillance, identification and sensing, as well as novel ap-
an AESA antenna is the capability to defeating electronic jamming and in proaches to persistence surveillance, situational awareness and wide-area
cancelling ground and sea clutter. In fact, the characteristics of AESA an- imaging, with particular regard to the following fields:
tennae facilitate the use of advanced signal-processing procedures, such • Laser sources;
as space-time adaptive processing, which is able to filter and eliminate • Beam pointing technologies;
ground or sea clutter. Of course, these techniques are not possible with • Vibrometric imaging;
reflector antennae. • Spectral and polarimetric imaging;
Modern SAR transmit very complicated waveforms, which provide the • Unconventional imaging techniques;
possibility to modulate and vary frequency and waves for improvement • Image reconstruction from partial or incomplete data; and
of radar performance. This is a fundamental capability that reinforces the • Technologies for defeating camouflage, concealment and deception.
method by which radar signals propagate to the Earth’s surface. At fre-
quencies above 15GHz a radar signal undergoes significant attenuation In maritime surveillance Inverse SAR (ISAR) systems are used to detect,
in atmosphere, with an associated decrease in strength. At frequencies image and classify surface ships and other objects in all weather condi-
below 1GHz the signal may incur refraction problems, with changes in di- tions. ISAR technology uses the relative movements of the target rather
rection and phase distortions. For these reasons, all existing and planned than the emitter to create the image. Due to the different radar reflection
space-based SAR operate in the region between 1-12GHz. In general, characteristics of sea, hull, superstructure and masts, ships usually stand
higher frequencies (which correspond to shorter wavelengths), allow for out in ISAR images as they move and there can be enough radar infor-
better image resolution and greater signal to noise ratio (SNR - a measure mation derived from ship motion to also allow determination of the type
of the quality of the signal). Since high resolution is of primary importance of ship being observed. For this purpose, automatic ship classification
to space-based SAR, current designs use a frequency range of 10-12GHz. systems have been designed and developed for the classification of na-
Furthermore, it is possible to apply a specific pulse compression tech- val targets. Current systems use both neural networks and conventional
nique to improve spatial resolution of the radar. The range in which the processing techniques to determine the ship class of a target from the
pulse varies (the radar bandwidth) is directly connected to the radar’s res- ISAR imagery acquired by satellites. The image intensity is proportional to
olution and, at least within certain limits, to higher bandwidth which corre- the returned signal strength and processing consists of target segmenta-
sponds to finer resolution. But resolution cannot be improved ad infinitum tion, clutter rejection, centring, feature-vector extraction and consequent
by simply increasing the bandwidth: in fact, the necessity for high-res- classification.
olution, high-rate analogue-to-digital converters for processing returning Modern space-based SAR is intended to serve both military and civilian
pulse limits how high the radar bandwidth can be. In this regard, particular users and is able to offer higher-resolution images than have previously
techniques regarding processing or waveform compression can provide been available. These systems provide important capabilities for detection
finer resolution imagery without the need for faster analogue-to-digital and surveillance that are not possible using traditional airborne platforms,
converters. such as continuous access to important areas and rapid surveillance of
Space-based SAR generate a huge volume of ‘row’ data. Given the large target areas. Single-channel SAR imaging is evolving, with advanced
complexity of the processing algorithms necessary to convert this data multi-aperture SAR concepts now under development. The spatial diver-
into usable intelligence information, much of the data processing is per- sity of multiple parallel reception channels is being used to discriminate
formed on the ground and the rate at which data is downlinked to the moving targets from the stationary background and to determine exact
Earth – the communications bandwidth – is necessarily very high: an topographies within the target area. The flexible programming potential
estimated downlink rate of 1 billion bits per second (1Gbps) is necessary available with this new generation of space-based SAR, together with very
to avoid unacceptable delays in data processing. The Tracking and Data advanced array signal processing algorithms, allows for new capabilities
Relay Satellite System (TDRSS) developed by NASA offers an example of and overcomes the limitation of conventional single-channel SAR. Future
the high downlink rates available today. This system includes 20 Ku-band space-based SAR, with multiple parallel channels and waveform diversity,
communications channels, each of which is capable of transmitting data will improve some characteristics such as fully adaptive digital beamform-
at 300 million bps, and six Ka-band communications channels, each of ing in azimuth and elevation, with a remarkable increase in SNR and hence
which can transmit data at 800 million. Those rates, although remarka- coverage as well as the ability to dramatically increase image resolution.
ble, are not sufficient to avoid delays in processing downlink space-based
SAR data. To improve these downlink capabilities, laser communication
links are now in development. Specifically, the US Air Force is develop- Engineer Paolo Quaranta is Deputy Head of the R&D Department of Advanced
Aeronautical Technologies of the Italian CNR (National Research Council). In this
ing the Transformational Satellite Communication System (TSAT), which position he is leading research programmes in the field of technological application
uses laser communications links capable of providing downlink rates of for military aircraft.