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86-454: Copernicus is the long-term European Union Earth observation and monitoring programme. It used to be called GMES (Global Monitoring for Environment and Security) and was established by a Regulation that entered into force in 2014. It is a user-driven programme under civil control. Activities conducted under the programme included the launch of six families of dedicated, EU-owned earth observation satellites and instruments—the so-called Sentinels—and

172-412: A microphone , a video signal representing moving images from a video camera , or a digital signal representing data from a computer . In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency , called the carrier wave because it creates the radio waves that "carry" the information through the air. The information signal is used to modulate

258-418: A resonator , similarly to a tuning fork . The tuned circuit has a natural resonant frequency at which it oscillates. The resonant frequency is set equal to the frequency of the desired radio station. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by

344-552: A space heater or wood fire. The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food. Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures in hyperthermia therapy and to kill cancer cells. However, unlike infrared waves, which are mainly absorbed at

430-509: A transmitter , which is connected to an antenna , which radiates the waves. They are received by another antenna connected to a radio receiver , which processes the received signal. Radio waves are very widely used in modern technology for fixed and mobile radio communication , broadcasting , radar and radio navigation systems, communications satellites , wireless computer networks and many other applications. Different frequencies of radio waves have different propagation characteristics in

516-408: A 2D sinusoid at a given frequency without distortion while minimizing the variance of the noise of the resulting image. The purpose is to compute the spectral estimate efficiently. Spectral estimate is given as where R is the covariance matrix, and a ω 1 , ω 2 ∗ {\displaystyle a_{\omega _{1},\omega _{2}}^{*}}

602-465: A ground station using a Ka band downlink. On 31 August 2016, ESA announced they had discovered that a solar panel on the Copernicus Sentinel-1A satellite had been hit by a millimetre-size particle in orbit on 23 August. Thanks to onboard cameras, ground controllers were able to identify the affected area. The satellite's routine operations didn't seem to be altered by the impact, in part due to

688-461: A human user. The radio waves from many transmitters pass through the air simultaneously without interfering with each other. They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency , measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in the receiver consists of one or more tuned circuits which act like

774-403: A left-hand sense. Right circularly polarized radio waves consist of photons spinning in a right hand sense. Left circularly polarized radio waves consist of photons spinning in a left hand sense. Plane polarized radio waves consist of photons in a quantum superposition of right and left hand spin states. The electric field consists of a superposition of right and left rotating fields, resulting in

860-605: A plane oscillation. Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable propagation properties, stemming from their large wavelength . Radio waves have the ability to pass through the atmosphere in any weather, foliage, and through most building materials. By diffraction , longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength. The study of radio propagation , how radio waves move in free space and over

946-402: A plane perpendicular to the direction of motion. In a horizontally polarized radio wave the electric field oscillates in a horizontal direction. In a vertically polarized wave the electric field oscillates in a vertical direction. In a circularly polarized wave the electric field at any point rotates about the direction of travel, once per cycle. A right circularly polarized wave rotates in

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1032-551: A point ( ω x , ω y {\displaystyle \omega _{x},\omega _{y}} ) is given by: where ϕ ^ E V {\displaystyle {\hat {\phi }}_{EV}} is the amplitude of the image at a point ( ω x , ω y ) {\displaystyle \left(\omega _{x},\omega _{y}\right)} , v i _ {\displaystyle {\underline {v_{i}}}}

1118-552: A point in the SAR image is in alignment to one of the signal subspace eigenvectors which is the peak in image estimate. Thus this method does not accurately represent the scattering intensity at each point, but show the particular points of the image. Backprojection Algorithm has two methods: Time-domain Backprojection and Frequency-domain Backprojection . The time-domain Backprojection has more advantages over frequency-domain and thus,

1204-402: A range of viewing distances. To create a SAR image, successive pulses of radio waves are transmitted to "illuminate" a target scene, and the echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths of a meter down to several millimeters. As the SAR device on board the aircraft or spacecraft moves,

1290-404: A right-hand sense about the direction of travel, while a left circularly polarized wave rotates in the opposite sense. The wave's magnetic field is perpendicular to the electric field, and the electric and magnetic field are oriented in a right-hand sense with respect to the direction of radiation. An antenna emits polarized radio waves, with the polarization determined by the direction of

1376-555: Is a nonparametric covariance-based method, which uses an adaptive matched-filterbank approach and follows two main steps: The adaptive Capon bandpass filter is designed to minimize the power of the filter output, as well as pass the frequencies ( ω 1 , ω 2 {\displaystyle \omega _{1},\omega _{2}} ) without any attenuation, i.e., to satisfy, for each ( ω 1 , ω 2 {\displaystyle \omega _{1},\omega _{2}} ), where R

1462-550: Is a parameter-free sparse signal reconstruction based algorithm. It achieves super-resolution and is robust to highly correlated signals. The name emphasizes its basis on the asymptotically minimum variance (AMV) criterion. It is a powerful tool for the recovery of both the amplitude and frequency characteristics of multiple highly correlated sources in challenging environment (e.g., limited number of snapshots, low signal-to-noise ratio . Applications include synthetic-aperture radar imaging and various source localization. SAMV method

1548-404: Is absorbed within a few meters, so the atmosphere is effectively opaque. In radio communication systems, information is transported across space using radio waves. At the sending end, the information to be sent, in the form of a time-varying electrical signal, is applied to a radio transmitter . The information, called the modulation signal , can be an audio signal representing sound from

1634-728: Is also a special case of the FIR filtering approaches. It is seen that although the APES algorithm gives slightly wider spectral peaks than the Capon method, the former yields more accurate overall spectral estimates than the latter and the FFT method. FFT method is fast and simple but have larger sidelobes. Capon has high resolution but high computational complexity. EV also has high resolution and high computational complexity. APES has higher resolution, faster than capon and EV but high computational complexity. MUSIC method

1720-455: Is capable of achieving resolution higher than some established parametric methods, e.g., MUSIC , especially with highly correlated signals. Computational complexity of the SAMV method is higher due to its iterative procedure. This subspace decomposition method separates the eigenvectors of the autocovariance matrix into those corresponding to signals and to clutter. The amplitude of the image at

1806-453: Is critical to operation of the EV method. The eigenvalue of the R matrix decides whether its corresponding eigenvector corresponds to the clutter or to the signal subspace. The MUSIC method is considered to be a poor performer in SAR applications. This method uses a constant instead of the clutter subspace. In this method, the denominator is equated to zero when a sinusoidal signal corresponding to

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1892-407: Is essentially a realization of the mapping of the mathematical framework through generation of the variants and executing matrix operations. The performance of this implementation may vary from machine to machine, and the objective is to identify on which machine it performs best. The Capon spectral method, also called the minimum-variance method, is a multidimensional array-processing technique. It

1978-442: Is extremely small, from 10 to 10   joules . So the antenna of even a very low power transmitter emits an enormous number of photons every second. Therefore, except for certain molecular electron transition processes such as atoms in a maser emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous classical process, governed by Maxwell's equations . Radio waves in vacuum travel at

2064-538: Is given by Radio wave Radio waves are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum , typically with frequencies below 300 gigahertz (GHz) and wavelengths greater than 1 millimeter ( 3 ⁄ 64 inch), about the diameter of a grain of rice. Radio waves with frequencies above about 1 GHz and wavelengths shorter than 30 centimeters are called microwaves . Like all electromagnetic waves, radio waves in vacuum travel at

2150-448: Is more preferred. The time-domain Backprojection forms images or spectrums by matching the data acquired from the radar and as per what it expects to receive. It can be considered as an ideal matched-filter for synthetic-aperture radar. There is no need of having a different motion compensation step due to its quality of handling non-ideal motion/sampling. It can also be used for various imaging geometries. In GEO-SAR, to focus specially on

2236-458: Is not advantageous to capture a waveform for each of both transmission directions for a given pair of antennas, because those waveforms will be identical. When multiple static antennas are used, the total number of unique echo waveforms that can be captured is where N is the number of unique antenna positions. The antenna stays in a fixed position. It may be orthogonal to the flight path, or it may be squinted slightly forward or backward. When

2322-671: Is not generally suitable for SAR imaging, as whitening the clutter eigenvalues destroys the spatial inhomogeneities associated with terrain clutter or other diffuse scattering in SAR imagery. But it offers higher frequency resolution in the resulting power spectral density (PSD) than the fast Fourier transform (FFT)-based methods. The backprojection algorithm is computationally expensive. It is specifically attractive for sensors that are wideband, wide-angle, and/or have long coherent apertures with substantial off-track motion. SAR requires that echo captures be taken at multiple antenna positions. The more captures taken (at different antenna locations)

2408-593: Is provided by the SAR. SAR images have wide applications in remote sensing and mapping of surfaces of the Earth and other planets. Applications of SAR are numerous. Examples include topography, oceanography, glaciology, geology (for example, terrain discrimination and subsurface imaging). SAR can also be used in forestry to determine forest height, biomass, and deforestation. Volcano and earthquake monitoring use differential interferometry . SAR can also be applied for monitoring civil infrastructure stability such as bridges. SAR

2494-616: Is the coherency matrix and v i _ H {\displaystyle {\underline {v_{i}}}^{\mathsf {H}}} is the Hermitian of the coherency matrix, 1 λ i {\displaystyle {\frac {1}{\lambda _{i}}}} is the inverse of the eigenvalues of the clutter subspace, W ( ω x , ω y ) {\displaystyle W\left(\omega _{x},\omega _{y}\right)} are vectors defined as where ⊗ denotes

2580-392: Is the covariance matrix , h ω 1 , ω 2 ∗ {\displaystyle h_{\omega _{1},\omega _{2}}^{*}} is the complex conjugate transpose of the impulse response of the FIR filter, a ω 1 , ω 2 {\displaystyle a_{\omega _{1},\omega _{2}}}

2666-439: Is the 2D Fourier vector, defined as a ω 1 , ω 2 ≜ a ω 1 ⊗ a ω 2 {\displaystyle a_{\omega _{1},\omega _{2}}\triangleq a_{\omega _{1}}\otimes a_{\omega _{2}}} , ⊗ {\displaystyle \otimes } denotes Kronecker product. Therefore, it passes

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2752-452: Is the 2D complex-conjugate transpose of the Fourier vector. The computation of this equation over all frequencies is time-consuming. It is seen that the forward–backward Capon estimator yields better estimation than the forward-only classical capon approach. The main reason behind this is that while the forward–backward Capon uses both the forward and backward data vectors to obtain the estimate of

2838-426: Is the wavelength of a 1  hertz radio signal. A 1  megahertz radio wave (mid- AM band ) has a wavelength of 299.79 meters (983.6 ft). Like other electromagnetic waves, a radio wave has a property called polarization , which is defined as the direction of the wave's oscillating electric field perpendicular to the direction of motion. A plane-polarized radio wave has an electric field that oscillates in

2924-440: Is typically mounted on a moving platform, such as an aircraft or spacecraft, and has its origins in an advanced form of side looking airborne radar (SLAR). The distance the SAR device travels over a target during the period when the target scene is illuminated creates the large synthetic antenna aperture (the size of the antenna). Typically, the larger the aperture, the higher the image resolution will be, regardless of whether

3010-513: Is used in majority of the spectral estimation algorithms, and there are many fast algorithms for computing the multidimensional discrete Fourier transform. Computational Kronecker-core array algebra is a popular algorithm used as new variant of FFT algorithms for the processing in multidimensional synthetic-aperture radar (SAR) systems. This algorithm uses a study of theoretical properties of input/output data indexing sets and groups of permutations. A branch of finite multi-dimensional linear algebra

3096-411: Is used to identify similarities and differences among various FFT algorithm variants and to create new variants. Each multidimensional DFT computation is expressed in matrix form. The multidimensional DFT matrix, in turn, is disintegrated into a set of factors, called functional primitives, which are individually identified with an underlying software/hardware computational design. The FFT implementation

3182-408: Is useful in environment monitoring such as oil spills, flooding, urban growth, military surveillance: including strategic policy and tactical assessment. SAR can be implemented as inverse SAR by observing a moving target over a substantial time with a stationary antenna. A synthetic-aperture radar is an imaging radar mounted on a moving platform. SAR is a Doppler technique. It is based on

3268-400: Is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones. Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called a Faraday cage . A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than about 1 ⁄ 20 of wavelength of

3354-455: The Kronecker product of the two vectors. MUSIC detects frequencies in a signal by performing an eigen decomposition on the covariance matrix of a data vector of the samples obtained from the samples of the received signal. When all of the eigenvectors are included in the clutter subspace (model order = 0) the EV method becomes identical to the Capon method. Thus the determination of model order

3440-415: The speed of light c {\displaystyle c} . When passing through a material medium, they are slowed depending on the medium's permeability and permittivity . Air is tenuous enough that in the Earth's atmosphere radio waves travel at very nearly the speed of light. The wavelength λ {\displaystyle \lambda } is the distance from one peak (crest) of

3526-428: The speed of light , and in the Earth's atmosphere at a slightly lower speed. Radio waves are generated by charged particles undergoing acceleration , such as time-varying electric currents . Naturally occurring radio waves are emitted by lightning and astronomical objects , and are part of the blackbody radiation emitted by all warm objects. Radio waves are generated artificially by an electronic device called

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3612-482: The "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, with the unit milliwatt per square centimeter (mW/cm ). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in

3698-467: The 1909 Nobel Prize in physics for his radio work. Radio communication began to be used commercially around 1900. The modern term " radio wave " replaced the original name " Hertzian wave " around 1912. Radio waves are radiated by charged particles when they are accelerated . Natural sources of radio waves include radio noise produced by lightning and other natural processes in the Earth's atmosphere, and astronomical radio sources in space such as

3784-424: The Capon method, but more accurate spectral estimates for amplitude in SAR. In the Capon method, although the spectral peaks are narrower than the APES, the sidelobes are higher than that for the APES. As a result, the estimate for the amplitude is expected to be less accurate for the Capon method than for the APES method. The APES method requires about 1.5 times more computation than the Capon method. SAMV method

3870-447: The Earth's atmosphere; long waves can diffract around obstacles like mountains and follow the contour of the Earth ( ground waves ), shorter waves can reflect off the ionosphere and return to Earth beyond the horizon ( skywaves ), while much shorter wavelengths bend or diffract very little and travel on a line of sight , so their propagation distances are limited to the visual horizon. To prevent interference between different users,

3956-423: The SAR image stack are a sampled version of the Fourier transform of reflectivity in elevation direction, but the Fourier transform is irregular. Thus the spectral estimation techniques are used to improve the resolution and reduce speckle compared to the results of conventional Fourier transform SAR imaging techniques. FFT (Fast Fourier Transform i.e., periodogram or matched filter ) is one such method, which

4042-460: The Sun, galaxies and nebulas. All warm objects radiate high frequency radio waves ( microwaves ) as part of their black body radiation . Radio waves are produced artificially by time-varying electric currents , consisting of electrons flowing back and forth in a specially shaped metal conductor called an antenna . An electronic device called a radio transmitter applies oscillating electric current to

4128-503: The antenna aperture travels along the flight path, a signal is transmitted at a rate equal to the pulse repetition frequency (PRF). The lower boundary of the PRF is determined by the Doppler bandwidth of the radar. The backscatter of each of these signals is commutatively added on a pixel-by-pixel basis to attain the fine azimuth resolution desired in radar imagery. The spotlight synthetic aperture

4214-614: The antenna location relative to the target changes with time. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions. This process forms the synthetic antenna aperture and allows the creation of higher-resolution images than would otherwise be possible with a given physical antenna. SAR is capable of high-resolution remote sensing , independent of flight altitude, and independent of weather, as SAR can select frequencies to avoid weather-caused signal attenuation. SAR has day and night imaging capability as illumination

4300-490: The antenna — each object will have its own doppler shift. A precise frequency analysis of the radar reflections will thus allow the construction of a detailed image. In order to realise this concept, electromagnetic waves are transmitted sequentially, the echoes are collected and the system electronics digitizes and stores the data for subsequent processing. As transmission and reception occur at different times, they map to different small positions. The well ordered combination of

4386-411: The antenna's field of view. The 3D processing is done in two stages. The azimuth and range direction are focused for the generation of 2D (azimuth-range) high-resolution images, after which a digital elevation model (DEM) is used to measure the phase differences between complex images, which is determined from different look angles to recover the height information. This height information, along with

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4472-540: The antenna, and the antenna radiates the power as radio waves. Radio waves are received by another antenna attached to a radio receiver . When radio waves strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver. From quantum mechanics , like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged elementary particles called photons . In an antenna transmitting radio waves,

4558-421: The aperture is physical (a large antenna) or synthetic (a moving antenna) – this allows SAR to create high-resolution images with comparatively small physical antennas. For a fixed antenna size and orientation, objects which are further away remain illuminated longer – therefore SAR has the property of creating larger synthetic apertures for more distant objects, which results in a consistent spatial resolution over

4644-642: The artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunication Union (ITU), which defines radio waves as " electromagnetic waves of frequencies arbitrarily lower than 3000  GHz , propagated in space without artificial guide". The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses. Higher-frequency, shorter-wavelength radio waves are called microwaves . Radio waves were first predicted by

4730-617: The azimuth-range coordinates provided by 2-D SAR focusing, gives the third dimension, which is the elevation. The first step requires only standard processing algorithms, for the second step, additional pre-processing such as image co-registration and phase calibration is used. In addition, multiple baselines can be used to extend 3D imaging to the time dimension . 4D and multi-D SAR imaging allows imaging of complex scenarios, such as urban areas, and has improved performance with respect to classical interferometric techniques such as persistent scatterer interferometry (PSI). SAR algorithms model

4816-761: The beginning of Sentinel-1A operational life. Copernicus data and services as provided by the Sentinel-satellites and Copernicus services are available on a full, open and free-of-charge basis. The satellite was used as part of a in-orbit verification of the ESA European Data Relay System . In 2014, data from the Sentinel-1A satellite in LEO was transmitted via an optical link to the Alphasat in GEO and then relayed to

4902-410: The carrier, altering some aspect of it, encoding the information on the carrier. The modulated carrier is amplified and applied to an antenna . The oscillating current pushes the electrons in the antenna back and forth, creating oscillating electric and magnetic fields , which radiate the energy away from the antenna as radio waves. The radio waves carry the information to the receiver location. At

4988-461: The collision. Preliminary data, taken during commissioning, demonstrated the versatility and capability of the imagery produced by Sentinel-1A even before the radar system is fully calibrated. Mapping sea ice to ensure the safe passage of marine vessels, providing data to validate ice loss models and determining the difference between land used for forest, agriculture and urban areas. Data became available to all system users on 6 October 2014, marking

5074-413: The covariance matrix, the forward-only Capon uses only the forward data vectors to estimate the covariance matrix. The APES (amplitude and phase estimation) method is also a matched-filter-bank method, which assumes that the phase history data is a sum of 2D sinusoids in noise. APES spectral estimator has 2-step filtering interpretation: Empirically, the APES method results in wider spectral peaks than

5160-421: The electrons in the antenna emit the energy in discrete packets called radio photons, while in a receiving antenna the electrons absorb the energy as radio photons. An antenna is a coherent emitter of photons, like a laser , so the radio photons are all in phase . However, from Planck's relation E = h ν {\displaystyle E=h\nu } , the energy of individual radio photons

5246-438: The energy is deposited. For example, the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm . Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating. A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts . Since

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5332-459: The fact that "radar reflections from discrete objects in a passing radar beam field each [have] a minute Doppler, or speed, shift relative to the antenna". Carl Wiley, working at Goodyear, Arizona, (which later became Goodyear Aerospace, and eventually Lockheed Martin Corporation) in 1951, suggested the principle that — because each object in the radar beam has a slightly different speed relative to

5418-518: The heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Radiofrequency electromagnetic fields have been classified by the International Agency for Research on Cancer (IARC) as having "limited evidence" for its effects on humans and animals. There

5504-464: The levels of electric and magnetic field strength at a measurement location. Another commonly used unit for characterizing an RF electromagnetic field is power density . Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the far field zone of the radiation pattern. In closer proximity to the transmitter, i.e., in

5590-434: The metal antenna elements. For example, a dipole antenna consists of two collinear metal rods. If the rods are horizontal, it radiates horizontally polarized radio waves, while if the rods are vertical, it radiates vertically polarized waves. An antenna receiving the radio waves must have the same polarization as the transmitting antenna, or it will suffer a severe loss of reception. Many natural sources of radio waves, such as

5676-452: The more reliable the target characterization. Multiple captures can be obtained by moving a single antenna to different locations, by placing multiple stationary antennas at different locations, or combinations thereof. The advantage of a single moving antenna is that it can be easily placed in any number of positions to provide any number of monostatic waveforms. For example, an antenna mounted on an airplane takes many captures per second as

5762-430: The other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in a demodulator . The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices. A digital data signal is applied to a computer or microprocessor , which interacts with

5848-543: The outputs of Sentinel 2 missions, can be downloaded from the Sentinel Data Hub. Sentinel-1A arrived at its launch site in Kourou , French Guiana on 25 February 2014, ahead of its launch which was at the time planned for 28 March. Sentinel-1A was launched on 3 April 2014 by a Soyuz rocket at 21:02:31 GMT (23:02:31 CEST). The first stage separated 118 seconds later, followed by the fairing (209 s), second stage (287 s) and

5934-558: The plane travels. The principal advantages of multiple static antennas are that a moving target can be characterized (assuming the capture electronics are fast enough), that no vehicle or motion machinery is necessary, and that antenna positions need not be derived from other, sometimes unreliable, information. (One problem with SAR aboard an airplane is knowing precise antenna positions as the plane travels). For multiple static antennas, all combinations of monostatic and multistatic radar waveform captures are possible. Note, however, that it

6020-494: The ramp-up of the 6 Copernicus Services in the fields of atmosphere-, marine- and land-monitoring, climate change, emergency management and security. Copernicus data and services are available on a full, open and free-of charge basis to the public sector, the private sector, scientists, and citizens. The programme provides multi temporal Synthetic Aperture Radar images with a temporal gap of 12 days. This data can, for example, be used for SAR Interferometry . Such data, as well as

6106-401: The range of practical radio communication systems decreases with increasing frequency. Below about 20 GHz atmospheric attenuation is mainly due to water vapor. Above 20 GHz, in the millimeter wave band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Above 300 GHz, in the terahertz band , virtually all the power

6192-429: The raw data was recorded on film and the postprocessing by matched filter was implemented optically using lenses of conical, cylindrical and spherical shape. The Range-Doppler algorithm is an example of a more recent approach. Synthetic-aperture radar determines the 3D reflectivity from measured SAR data. It is basically a spectrum estimation, because for a specific cell of an image, the complex-value SAR measurements of

6278-421: The reality of Maxwell's electromagnetic waves by experimentally generating electromagnetic waves lower in frequency than light, radio waves, in his laboratory, showing that they exhibited the same wave properties as light: standing waves , refraction , diffraction , and polarization . Italian inventor Guglielmo Marconi developed the first practical radio transmitters and receivers around 1894–1895. He received

6364-410: The received signals builds a virtual aperture that is much longer than the physical antenna width. That is the source of the term "synthetic aperture," giving it the property of an imaging radar. The range direction is perpendicular to the flight track and perpendicular to the azimuth direction, which is also known as the along-track direction because it is in line with the position of the object within

6450-446: The receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna. This voltage is applied to the radio receiver , which extracts the information signal. The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all

6536-433: The relative moving track, the backprojection algorithm works very well. It uses the concept of Azimuth Processing in the time domain. For the satellite-ground geometry, GEO-SAR plays a significant role. The procedure of this concept is elaborated as follows. Capon and APES can yield more accurate spectral estimates with much lower sidelobes and more narrow spectral peaks than the fast Fourier transform (FFT) method, which

6622-452: The scene as a set of point targets that do not interact with each other (the Born approximation ). While the details of various SAR algorithms differ, SAR processing in each case is the application of a matched filter to the raw data, for each pixel in the output image, where the matched filter coefficients are the response from a single isolated point target. In the early days of SAR processing,

6708-501: The solar arrays delivering more power than the satellite needs (so mild damage to them would not lead to power shortages). Synthetic-aperture radar Synthetic-aperture radar ( SAR ) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide finer spatial resolution than conventional stationary beam-scanning radars. SAR

6794-416: The sun, stars and blackbody radiation from warm objects, emit unpolarized waves, consisting of incoherent short wave trains in an equal mixture of polarization states. The polarization of radio waves is determined by a quantum mechanical property of the photons called their spin . A photon can have one of two possible values of spin; it can spin in a right-hand sense about its direction of motion, or in

6880-412: The surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues. The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity ; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of

6966-588: The surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection , refraction , polarization , diffraction , and absorption . Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate: At microwave frequencies, atmospheric gases begin absorbing radio waves, so

7052-437: The theory of electromagnetism that was proposed in 1867 by Scottish mathematical physicist James Clerk Maxwell . His mathematical theory, now called Maxwell's equations , predicted that a coupled electric and magnetic field could travel through space as an " electromagnetic wave ". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist Heinrich Hertz demonstrated

7138-411: The tuned circuit and not passed on. Radio waves are non-ionizing radiation , which means they do not have enough energy to separate electrons from atoms or molecules , ionizing them, or break chemical bonds , causing chemical reactions or DNA damage . The main effect of absorption of radio waves by materials is to heat them, similarly to the infrared waves radiated by sources of heat such as

7224-480: The upper assembly (526 s). After a 617-second burn, the Fregat upper stage delivered Sentinel into a Sun-synchronous orbit at 693 km altitude. The satellite separated from the upper stage 23 min 29 s after liftoff. At 09:43 and at 11:21 on 5 April 2014 ACRIMSAT was rated as having a high risk of collision (<20m) with the newly launched Sentinel-1A. A 39-second burn of Sentinel-1A during LEOP successfully avoided

7310-420: The wave's electric field to the next, and is inversely proportional to the frequency f {\displaystyle f} of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is where Equivalently, c {\displaystyle c} , the distance that a radio wave travels in vacuum in one second, is 299,792,458 meters (983,571,056 ft), which

7396-442: The waves. Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit volt per meter (V/m) is used for the electric component, and the unit ampere per meter (A/m) is used for the magnetic component. One can speak of an electromagnetic field , and these units are used to provide information about

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