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Passive sonar is discreet and does not disturb marine life because it listens only to the sounds emitted by noisemakers, which send out acoustic signals that travel through the ocean to be picked up by the sonar's antennae. These antennas are disturbed by noise from the vessel and the environment. The signals received by the antenna sensors are processed by appropriate algorithms, the outputs of which are displayed so that an operator can decide, with the aid of audio listening, whether detection is worthwhile. The article describes each stage of the information flow, from the sound source to the operator. This is also the order of the terms in the sonar equation used to estimate range. Three examples from real situations at sea demonstrate the use of this formalism.
Lidar observation has benefited from the technological advances in recent decades and can now be used for operational purposes. It can track the evolution of atmospheric aerosols with high vertical resolution, thereby improving the knowledge of their impact on societal issues. It is also a promising complement to existing observations, such as those made from spaceborne instruments, and to predictive modeling. Coupled with forecasting models, it strengthens resilience in the face of tomorrow's major climatic challenges by enabling more effective anticipation of extreme weather events.
Although indiscreet and energy consumer, active sonar is widely used especially in the military field in order to detect submarines. The first examples presented are equipment (Asdic) from the Second World War and their successors. Following examples are the modern low-frequency active sonar and networks of sonobuoys dropped from aircraft. The presentation follows, step by step, the path of the emitted pulse when reflected on an obstacle or a target. After the return path, the echo is received by an array, processed by algorithms whose outputs are displayed to an operator. This sequence is also one of the terms of the active sonar equation. Examples from real situations at sea show the use of this equation for calculation of detection range.
A radio link between the ground and a satellite passes through the atmosphere of the Earth. In the atmosphere, two areas can impact the propagation of waves: the troposphere and the ionosphere. The troposphere refers to the low layers of the atmosphere where meteorological phenomena occur. These phenomena have a significant influence on the Earth-space propagation. Generally, this influence tends to increase with the frequency of the wave. The ionosphere is a region of the high atmosphere where compounds are partially ionized by solar radiation. This ionization phenomenon has also an impact on the propagation of waves. Reversely to what occurs in the troposphere, the ionospheric effects increase at low frequencies of below 1 GHz and decrease with frequency.
Radar is main sensor to achieve efficiently maritime and coastal surveillance. The article presents different architecture choices adapted to the environment, platform and missions. It deals with transmission architectures - centralized or distributed upon an active antenna -, solutions for exploring the area with the antenna beam - mechanical, electronic scanning upon one or two axes -, frequency choice, transmitted wave generation, reception and pulse compression. It then discusses the specific techniques involved in optimizing detection in view of the particularities regarding signals backscattered from the sea surface. The article concludes with the foreseeable evolutions of the associated technologies.
An object or event localized in space is, together with its attribute data, geographic information. The Geographic Information Systems (GIS) allow for obtaining, managing, using and transmitting such information by treating the graphic aspect of the object but also its semantic content. The geographic information can have varied origins, objects are localizable by nature, others by association of similar or different themes. The application domains of GISs are vast and cover territorial development up to geomarketing. From these analyses models or simulations are derived which make of GISs and their components (materials, software, personnel, data) a territorial decision tool.
Hyperfrequency systems include external sources of noise, received by the antenna with the signal, and internal sources of noise. All these noises can be measured separately, modeled and introduced into simulators in order to forecast the performances. Behavior forecasts are presented for oscillators, receivers, transmitters, antennas, as well as for different types of materials. Noise is sometimes at the origin of new applications such as radiation or jamming in order to cancel out undesired signals.
This article deals with Radar Cross Section (RCS) measurements in anechoic chambers. The first part introduces the basic concepts of RCS and the measurement setups allowing this type of experimentation in France. The design of such an experimental setup in an anechoic chamber is then explained. The RCS measurement protocol and the associated data processing are detailed. The last part concerns the radar imaging of a target from RCS measurements, which allows the analysis of the different contributing factors influencing the RCS.
The use of geographical information has grown considerably with the development of computer tools. In companies or administrations, geographic information systems (GIS) allow the representation of the spatial environment from basic geometric shapes (polygons, vectors, meshes...). The GIS thus exploit software tools, data to be processed, computer servers but also technical know-how. This article proposes to present to the engineers the potentialities offered by GIS and the associated computer tools. Thus, examples of GIS exploitation are presented on various topics.
The main purpose for Maritime and Coastal surveillance radars is ships detection. In this second part, the paper starts by depicting the signals involved in the detection process: the features of maritime targets and the properties of the sea clutter which is the main parasitic signal disturbing the detection. We then present the signal processing techniques allowing detection probability improvement while keeping a low false alarm in maritime environment. The end of the paper is dedicated to a presentation of the main trade-off in maritime radar system engineering allowing the reader to understand the influence of technical choices on reachable performances.
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