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Backscatter lidars for aerosols and clouds are widely used in surface network for atmospheric applications i.e. pollution, meteorology, climate. Instrument simulators are presented as tool required for instrument design and realization. Then, the main characteristics of the atmospheric medium and the standard techniques for backscatter lidar signal inversion are described in sight for the applications.
This article presents the lidar, with reference to instrument techniques, atmospheric spectroscopy and signals. These are presented in a reader-friendly way to define actual needs and meet objectives. While maintaining an innovative research activity, since the 2000s the lidar community has broadened out to embrace new activities in networking and space missions.
In many technological fields (telecommunication, remote sensing, geolocalisation, industrial control, seismology), the useful information is not directly accessible as it is buried in the observed signal; this issue requires the development of hidden information methods. The Kalman filter, based upon a linear state model puts into equation the evolution of the useful signal and its relationship to the signal measured from a series of incomplete or noisy measurements. This article introduces elements of statistical estimations where the variable or the process to be estimated are hidden. It describes the dynamic state model, composed of the equation of the state process that the Kalman filter tries to estimate and the measuring process.
The history of stealth is fairly recent but has experienced a significant development over the past decades leading to the creation of several generations of stealth aircrafts and notably the Lockheed F22 and the F22 Raptor.Stealth aircrafts are conceived in order to deflect or absorb waves and send back to the radar a very attenuated signal. Stealth is based upon several basic principles linked to existing electromagnetic phenomena. The radar equivalent surface of the object must be reduced to the minimum so that it is detected by radars as late as possible.The design of a stealth aircraft is subjected to rules in terms of shape, cavities and choice of absorbing materials. Since these developments, stealth has been successfully applied to missiles, drones and warships.
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.
The principles of the Light Detection and Ranging (LiDAR) sensor when applied to intelligent vehicles are presented in this article. LiDAR provide 3D information on their immediate environment by emitting a laser beam that reflects in objects nearby, allowing for the measurement of their distance. In this article the principles are presented to describe the way the perceived data is generated, it includes examples of the mainstream LiDAR. The processing of the acquired data using different perception algorithms is included to provide an understanding of the techniques used to classify and track the objects of interests. The purpose is to convey to the reader the basic principles of this sensor that is not only used as part of autonomous vehicles but also in driving assistance functions.
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.
Space telecommunications technologies have developed rapidly. Built on the successes achieved with polar geostationary satellites, they are now moving towards the market for scrolling satellites, deployed in low or very low layer constellations.These less expensive satellites have the advantage of carrying signals with a shorter latency time in correspondence with 5G and 6G mobile telephone systems and scientific information networks. The proliferation of projects raises fears of difficulties in the management of all satellites. The profitability of the projects can only be ensured if safety and regulatory measures are respected.The governance of Space has become obvious.
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