If humans want to travel about the solar system, they’ll need to be able to communicate. We communicate with robotic missions through radio signals. It requires a network of large radio antennas to do this. Spacecraft have relatively weak receivers, so you need to beam a strong radio signal to them. They also transmit relatively weak signals back. You need a large sensitive radio dish to capture the reply. For spacecraft beyond the orbit of Earth, this is done through the Deep Space Network (DSN), which is a collection of radio telescopes custom designed for the job.
When we return to the Moon and place our first footsteps on Mars, we will want not only scientific data but live video feeds, high-resolution images, and even tweets from the astronauts. Increased space exploration and the growing capability and thus data output of satellite-borne sensors operated by agencies such as NASA, ESA and JAXA impose greater demands on communication systems to operate at higher data rates and to reach across farther distances into space. Even the most sophisticated radio network isn’t capable of that level of bandwidth.
Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. “Free space” means air, outer space, vacuum, or something similar. FSO operates on the Line-of-Sight phenomenon, consisting of a LASER at source and detector at the destination which provides optical wireless communication between them. This contrasts with using solids such as optical fiber cable.
Free Space Optical or Laser communications is creating a new communications revolution, that by using visible and infrared light instead of radio waves for data transmission is providing large bandwidth, high data rate, license free spectrum, easy and quick deployability, low mass and less power requirement. It also offers low cost transmission as against radio frequency (RF) communication technology and fiber optics communication.
Commercial telecommunications is a significant driver for LAN-to-LAN, 5G, mobile backhaul, and “last mile” applications. According to Malcolm Watson, a principal research engineer at AVoptics Ltd. (an optical technology SME based in Somerset, England, and Cwmbran, Wales), the major factors driving growth are faster and more secure data transfer, radio frequency (RF) spectrum crunch, and reduced energy consumption.
“Other applications, such as building-to-building high-speed links, commercial aerospace, and within-room communications, are already using or starting to use FSO. FSO can also be used for disaster recovery or temporary building installations, as it can be quick and easy to set up a point-to-point communication link in an environment that is not suitable for laying fiber or using radio [communications],” Watson said.
Both military and civilian users have started planning Laser communication systems from terrestrial short-range systems, to high data rate Aircraft and Satellite communications, unmanned aerial vehicles (UAVs) to high altitude platforms (HAPs), near-space communications for relaying high data rates from moon, and deep space communications from mars.
A growing area for innovation in FSOC is unmanned aircraft systems (UASs) or UAVs operating at very low altitude, typically less than 200 m. Advanced UASs with high-bandwidth data transfer over long ranges are highly desirable. “Breakthrough low size, weight, and power (SWaP) FSO technology can open up innovation in precision agriculture, Earth observation, satellite uplink/downlink, infrastructure monitoring (rail, shipping, and energy), and a range of other terrestrial and maritime applications,” Watson said.
AVoptics, a major long-term supplier to the Airbus FSOC group and others in the aerospace and defense sectors, is currently collaborating with scientists at Cardiff University in Wales to develop new receivers for potential use on a UAV. “We think this is an emerging field in FSOC and have submitted a research proposal together to UK EPSRC [the Engineering and Physical Sciences Research Council] for funding support,” said Shiyu Xie, Sêr Cymru Research Fellow of Advanced Materials and Devices in professor Diana Huffaker’s group at Cardiff. “The prime advantages of FSO are license-free bands, high datarate transmission, and negligible signal interference. FSO is also compatible with quantum-based optical techniques (using single-photon sources and detectors) that can be used to encrypt the data transfer securely. Its ability to transmit and receive multiple signals at once is relatively simple thanks to wavelength division multiplexing techniques.”
However, FSOC technology has matured with many proven demonstrations : SILEX [the Semiconductor laser Intersatellite Link Experiment], GOLD [the Ground/Orbiter Lasercom Demonstration], and LADEE [NASA’s Lunar Atmosphere and Dust Environment Explorer], and the first operational laser communication service [EDRS, the European Data Relay System, dubbed the SpaceData-Highway].
Challenges of Free Space Optical Communications
The drawback of the FSO link is that its performance is strongly dependent on atmospheric attenuations. Different atmospheric conditions like snow, fog and rain scatter and absorb the transmitted signal, which leads to attenuation of information signal before receiving at receiver end. As a result of attenuation caused by atmospheric conditions the range and the capacity of wireless channel are degraded thereby restricting the potential of the FSO link by limiting the regions and times.
While fixed FSO links between buildings have long been established and today form a separate commercial product segment in local and metropolitan area networks, the mobile and long-range applications of this technology are aggravated by extreme requirements for pointing and tracking accuracy because of the small optical beam divergences involved. This challenge has to be addressed to fully exploit the benefits of optical links.
In satellite laser communication systems, accurate positioning of the beacon is essential for establishing a steady laser communication link. For satellite-to-ground optical communication, the main influencing factors on the acquisition of the beacon are background noise and atmospheric turbulence.
Furthermore, long-haul optical links through the atmosphere suffer from attenuation due to fog, Haze, Rain, Clouds, scintillation, strong fading as a result of index-of-refraction turbulence (IRT) and link blockage by obscuration. Hamid Hemmati, director of engineering for telecom infrastructure at Facebook, said at the Milcom conference that there are questions about linking satellites to ground terminals as the signal travels through weather disturbances in the atmosphere.
The advantages of free space optics are easy to come. But as the medium of the transmission is air for FSO and the light passes through it, some environmental challenges are unavoidable. Troposphere regions are the region where most of the atmospheric phenomenon occurred.
(a) Physical obstructions: flying birds, trees, and tall buildings can temporarily block a single beam, when it appears in line of sight (LOS) of transmission of FSO system.
(b) Scintillation: there would be temperature variations among different air packets due to the heat rising from the earth and the man-made drives like heating ducts. These temperature variations can cause fluctuations in amplitude of the signal which causes “image dancing” at the FSO receiving end. The effect of scintillation is addressed by Light Pointe’s unique multibeam system.
(c) Geometric losses: geometric losses which can be called optical beam attenuation are induced due to the spreading of beam and reduced the power level of signal as it travelled from transmitted end to receiver end.
(d) Absorption: absorption is caused by the water molecules which are suspended in the terrestrial atmosphere. The photons power would be absorbed by these particles. The power density of the optical beam is decreased and the availability of the transmission in a FSO system is directly affected by absorption. Carbon dioxide can also cause the absorption of signal.
(e) Atmospheric turbulence: the atmospheric disturbance happens due to weather and environment structure. It is caused by wind and convection which mixed the air parcels at different temperatures. This causes fluctuations in the density of air and it leads to the change in the air refractive index. The scale size of turbulence cell can create different type of effects given below and which would be dominant:
(i) If size of turbulence cell is of larger diameter than optical beam then beam wander would be the dominant effect. Beam wander is explained as the displacement of the optical beam spot rapidly.
(ii) If size of turbulence cell is of smaller diameter than optical beam then the intensity fluctuation or scintillation of the optical beam is a dominant one. Turbulence can lead to degradation of the optical beam of transmission. Change in the refractive index causes refraction of beam at different angle and spreading of optical beam takes place.
(f) Atmospheric attenuation: atmospheric attenuation is the resultant of fog and haze normally. It also depends upon dust and rain. It is supposed that atmospheric attenuation is wavelength dependent but this is not true. Haze is wavelength dependent. Attenuation at 1550nm is less than other wavelengths in haze weather condition . Attenuation in fog weather condition is wavelength independent.
(g) Scattering: scattering phenomena happen when the optical beam and scatterer collide. It is wavelength dependent phenomenon where energy of optical beam is not changed. But only directional redistribution of optical energy happens which leads to the reduction in the intensity of beam for longer distance. Atmospheric attenuation is divided into three types :
(1) Rayleigh scattering which is known as molecule scattering.
(2) Mie scattering which is known as aerosol scattering.
(3) Nonselective scattering which is known as geometric scattering.
The major factor concerning the free space communication market is the instability of the network system. The laser power attenuation through the atmosphere is variable and difficult to predict where changing atmospheric conditions can affect the transmission of information. Absorption of optical beam photons by water molecules in the terrestrial atmosphere reduces the power density of optical beams directly affecting the information transmission. Dynamic wind loads, thermal expansions, and weak earthquakes result in vibrations of the transmitter beam leading to a misalignment between transmitters and receivers. In addition, the fundamental limitation of the free space optics communication market arises from the atmosphere it propagates and can be severely affected by atmospheric turbulence and fog.
“It is a big problem, there is no question,” he said. That can be solved by spacing three ground stations in a cluster about every 400 kilometers apart, he said. That is normally enough space that there will be different weather patterns. There is a 95 percent probability that one ground station will be clear at any given time. John P. Leuer, executive director of next generation communications at Boeing’s space and intelligence systems division, said at the recent Milcom conference that “the commercial sector is leading the world and the nation in this technology. It used to be the other way around. It used to be the DoD.”
He further said, “There is still work to be done on networking, or the ability to switch traffic between ground segments as needed and anticipate which node transmissions have to go to, Boeing is researching these problems”. The company is partnering with the Jet Propulsion Laboratory to work on the atmospheric problems, and has an ongoing test aboard the international space station.”That is the one problem we need to retire yet before we see really significant adoption,” he said.
While there are commercial providers, the military, as usual, has its unique requirements. Systems must be rugged enough to survive the wear and tear of the battlefield. If the system is mounted on a ground vehicle or aircraft, vibration must also be taken into account. Nigara said the TALON program has worked on synchronizing transmitters and terminals on the move, whether from ship to ship, or ship to shore. They must be able to find each other and link automatically.
Free Space Optical Technology
Atmospheric factors cause an attenuated receiver signal and lead to higher bit error ratio (BER). To overcome these issues, vendors found some solutions, like multi-beam or multi-path architectures, which use more than one sender and more than one receiver. Some state-of-the-art devices also have larger fade margin (extra power, reserved for rain, smog, fog). To keep an eye-safe environment, good FSO systems have a limited laser power density and support laser classes 1 or 1M.
Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres. However the free space optics, based on 1550 nm wavelength, have considerably lower optical loss than free space optics, using 830 nm wavelength, in dense fog conditions. FSO using wavelength 1550 nm system are capable of transmitting several times higher power than systems with 850 nm and are safe to the human eye (1M class).
Additionally, some free space optics, such as EC SYSTEM, ensure higher connection reliability in bad weather conditions by constantly monitoring link quality to regulate laser diode transmission power with built-in automatic gain control. Developers around the world have now demonstrated all the most challenging parts of the technology, and free-space optical communications is finally set to transform the global and extra-global telecommunications landscape.
Free Space Optical System
Free space communications links require components such as optical transmitters and optical receivers; space-worthy, high-performance steerable telescopes; and the means for finding and stabilizing the narrow beams.
Commonly used optical channels are: Transmit; Receive; Acquisition and Tracking (e.g. beacon); and Reference (or align). The optical system transmits optical data and receives beacon signals. The transmit and receive channels may consist of separate or common apertures. The beacon signal is used for acquisition and tracking and the uplink command data from Earth or from another spacecraft. The beacon signal might be narrow band, such as a laser signal from a cooperative target, or wider spectrum sources such as celestial reference signals from stars, or a Sunilluminated Earth or moon.
The Transmit Channel consists of an optical path extending from the output of the laser transmitter to the exit aperture of the optics. The Receive Channel’s function is to accept light emerging from the fore-optics, and direct it to a photo-detector. The Acquisition and Tracking Channel images the incoming beacon signal onto the acquisition and tracking detector. Reference (or align) Channel forms an image of a portion of the transmit light at the array detector without requiring any high degree of image quality.
Laser-communications to and from airborne and space-borne assets typically involves there differing optical systems with widely different requirements. These include the flight terminal with typical telescope apertures on the order of 5 to 50 cm in diameter, the ground receiver terminal with apertures on the order of 0.5to 10 m, and the uplink command or optical beam pointer (e.g. beacon) with aperture diameters on the order of 0.5to 1 m.
Transmit [Receive Isolation: Since the transmit power is typically ten orders of magnitude larger than the receiver sensitivity levels, as much as 1 50 dB isolation of the receive channel from the transmit channel may be required for a given transceiver that must point near the Sun. Scattering from optical surfaces have to be kept to a minimum and adequate optical isolation must be built in at each stage of the design. Different isolation schemes can be implemented and include: spatial isolation; spectral isolation (e.g. filtration); temporal isolation (e.g. transmitting and receiving at different times); polarization isolation; aperture sharing; coding that utilizes codes with extreme depth of interleave; and combined Isolation.
Dedicated Star-Trackers: One or more dedicated star-trackers may be added to the optical system as the acquisition and tracking beacon. Since knowledge of alignment between these sensors and the optical channels within terminal is extremely critical (for sub-micro-radian pointing) the star tracker(s) need to be an integral part of the optical system. A dedicated star tracker will have a small aperture on the order of 6 to 8 cm, but with much wider field-of-view (a few degrees).
Mechanical, Thermal, and Temporal Stability: Structural integrity and thermal stability of the terminal are critical parameters of the design. At the same time, the telescope mass should be kept as low as possible. Thermal gradients may alter the surface figure of the optical system, as can mismatch between the thermal expansion of optics and the structure of the optics.
Reduction in SWap of FSOC system
The trend toward reductions in the SWaP of optical communications systems has opened up the skies. Compact, lightweight designs can now be fitted into microsatellites, also known as CubeSats, to deliver internet connectivity to remote and rural regions without the need for expensive and difficult-to-lay cabling. Previous hardware relied on large, complex steering mechanisms on heavy stable optical benches, in addition to power-inefficient multistage optical amplifiers. But as BridgeComm’s Sanford explains, current designs use fast-steering mirrors (FSMs) and optical closed-loop beacon tracking to provide a tighter optical communications beam in a much smaller package.
“In turn, this reduces the required power output for the optical amplifiers, resulting in a smaller electronics package,” Sanford said. “These advancements have significantly reduced SWaP, with commercially available 10- to 100-plus-Gb/s designs that are less than 5 kg with very low power consumption in form factors that can even fit into CubeSats.”
These compact commercial designs also meet the requirements of mobile commercial and military platforms, as well as terrestrial point-to-point and airborne systems. Many companies are undertaking networks of satellites in the sky — called constellations — and most look to use laser communications for intersatellite linking to create global canopies of internet coverage that can then be directed down to Earth where needed.
Transmitters and Receivers
FSO communication has used 1- to 1.5-µm bands, especially 1.55-µm fiber telecom band or the related 1.064-µm band, allowing incorporation of components developed by the huge fiber telecommunications and subsidiary markets. However in future 0.8 to 0.9-µm systems as well as visible-band and even UV are expected to be developed.
For the highest bandwidth formats, detectors need to be physically small or even single-spatial-mode, which puts special constraints on receivers in the atmosphere because of turbulence, both naturally occurring and exacerbated by aircraft moving through the air. Adaptive optics, using either deformable mirrors or coherent array concepts, can flatten out the received wavefronts and enable coupling of the light into the single-mode detection systems. Such single-mode receivers include coherent or optically preamplified architectures, both being quite mature. Also, similar to some radio frequency systems, non-demodulating transponders are feasible, based on single-mode reception and amplification.
Some receivers, such as avalanche photodiode detectors, are of particularly low complexity and do not require single-mode reception. They are usually 10 to 20 dB less efficient, but such performance may be adequate for some system designs. Finally, photon-counting detectors — semiconductor, photo-multiplier tubes or superconducting — have been shown to provide up to 5- to 15-dB better photon efficiency than coherent receivers. As an added benefit, they do not necessarily require single spatial mode reception. At present, though, most of these require more complex support systems such as cryo-refrigerators, and so are relegated to ground uses. There is no doubt, though, that because of their superlative efficiency, future developments will lead to such receivers being flyable in small aircraft or even in space.
One photon-per-bit receiver
Improving the receiver sensitivity is considered the most important method to improve the data throughput with as few photons received as possible. A better receiver sensitivity translates to a longer reach, higher data throughput and the ability to use more compact optics or a combination of the above. Although free space possesses an unrestricted optical bandwidth, the limited detection bandwidth of photon counting receivers restricts the achievable data rates, especially when applying higher-order PPM, which is spectrally very inefficient.
The demonstrated record sensitivities of photon counting receivers with PPM modulation include a sensitivity of ~1 photon-per-information-bit (PPB) at 14 Mbps8 and 1.2 PPB at 38 Mbps. The successful application of PPM and photon counting technologies at rates above 100 Mbps has been demonstrated by NASA at 622 Mbps with a 3.8-PPB sensitivity in the Lunar Laser Communication Demonstration (LLCD) and at 781 Mbps with a sensitivity of 0.5 detected PPB. However, when accounting for the insertion loss and non-ideal quantum efficiency, the latter result corresponds to a black-box sensitivity of ~8 incident PPBs
Future space communication systems such as inter-satellite and satellite-to-ground links are expected to operate at speeds of several tens of Gb/s and beyond and will thus require a major improvement over existing receiver technology both in terms of data rate and sensitivity
Superconducting nanowire-based versions of these receivers have recently been shown to provide excellent performance, including a quantum efficiency of 90% at data rates up to a few 100 Mb/s. A key drawback is, however, the need to be cooled to 2–4 K and their inability to detect photons at rates of multiple Gb/s
The sensitivity of such systems is largely determined by the noise figure (NF) of the pre-amplifier, which is theoretically 3 dB for almost all amplifiers. Phase-sensitive optical amplifiers (PSAs) with their uniquely low NF of 0 dB promise to provide the best possible sensitivity for Gb/s-rate long-haul free-space links. Researchers have demonstrated a novel approach using a PSA-based receiver in a free-space transmission experiment with an unprecedented bit-error-free, black-box sensitivity of 1 photon-per-information-bit (PPB) at an information rate of 10.5 Gb/s. The system adopts a simple modulation format (quadrature-phase-shift keying, QPSK), standard digital signal processing for signal recovery and forward-error correction and is straightforwardly scalable to higher data rates.
Flight optics and pointing, acquisition and tracking systems
Large steering motions of the beam can be implemented by gimbaling the telescope or by steering the beam at the telescope’s entrance using periscopic mirrors or Risley prisms. Small beam motions can be achieved either with these same approaches or with beam steerers — mechanical steering mirrors or solid-state devices — in the small-beam-space behind an afocal telescope.
Beam pointing is initially based on direction estimates provided by combinations of star trackers, Earth sensors, GPS and gyros. Even the best of these, though, usually have errors much larger than the beamwidth provided by the telescope (usually only tens of microradians). Therefore, a link spatial acquisition phase is almost always required, where the two terminals find each other. Here, too, there are multiple possibilities: transmitter scanning or broadening and/or special receiver sensors for detecting signals from a wide field of view, all as prescribed by an agreed-to protocol.
Because the beams are so narrow, the designer must also take into account the relative cross-velocity of the two platforms, which results in needing to “point-ahead” the transmitter in front of the receiver. Angles for this range from 17 urad for geostationary orbit (GEO) to ground, to approximately 60 urad for GEO to LEO (low Earth orbit), and up to as much as 500 urad for inter-planetary systems. Once the beams are correctly pointed, they then need to be kept stable to a small fraction of their width. Active tracking designs based on the incoming beam require a means of detecting the motion using a special detector or a conically scanned receiver, as well as a mechanism such as a small, fast mirror, behind the telescope. Also possible are non-received-beam techniques such as inertial references or actively stabilized platforms.
NASA developed integrated photonics modem for International Space Station
A NASA team is scheduled to test the space agency’s first integrated photonics modem aboard the International Space Station in 2020. The device, about the size of a mobile phone, incorporates lasers and switches onto a microchip, will use lasers to encode and transmit data at rates 10 to 100 times faster than today’s communications equipment.
-LGS Innovations, a technology company providing specialized mission-critical communications research and solutions, has been selected to support the NASA Integrated Laser Communication Relay Demonstration (LCRD) Low-Earth Orbit (LEO) User Modem and Amplifier (ILLUMA) project. For this pathfinder program, LGS will develop a free space optical modem that will fly aboard the International Space Station as the first demonstration of a fully operational, end-to-end optical communications system. The ILLUMA modem will leverage LGS Innovations’ rich heritage in free space laser communications and fiber laser technolog
“Integrated photonics are like an integrated circuit, except they use light rather than electrons to perform a wide variety of optical functions, “said Don Cornwell, director of NASA’s Advanced Communications and Navigation Division, which is funding the modem’s development. Recent developments in nanostructures, meta-materials, and silicon technologies have expanded the range of applications for these highly integrated optical chips. Furthermore, they could be lithographically printed in mass — just like electronic circuitry today — further driving down the costs of photonic devices.
“What we want to do is provide a faster exchange of data to the scientific community. Modems have to be inexpensive. They have to be small. We also have to keep their weight down,” Krainak said. The goal is to develop and demonstrate the technology and then make it available to industry and other government agencies, creating an economy of scale that will further drive down costs. “This is the pay off,” he said.
The so-called Integrated LCRD LEO (Low-Earth Orbit) User Modem and Amplifier (ILLUMA) will be installed on the International Space Station in 2020 where it will serve as a low-Earth terminal for NASA’s multi-year Laser Communications Relay Demonstration (LCRD) mission, demonstrating yet another capability for high-speed, laser-based communications. ILLUMA not only will flight qualify the technology, but also demonstrate a key capability for future spacecraft. In addition to communicating to ground stations, future satellites will require the ability to communicate with one another, he said.
LGS Innovations, a technology company providing specialized mission-critical communications research and solutions, has been selected to support the NASA Integrated Laser Communication Relay Demonstration (LCRD) Low-Earth Orbit (LEO) User Modem and Amplifier (ILLUMA) project. For this pathfinder program, LGS will develop a free space optical modem that will fly aboard the International Space Station as the first demonstration of a fully operational, end-to-end optical communications system. The ILLUMA modem will leverage LGS Innovations’ rich heritage in free space laser communications and fiber laser technology.
The ILLUMA program will use lasers to encode and transmit data at rates 10 to 100 times faster than today’s communications equipment, requiring significantly less mass and power than equivalent RF communications systems. The LGS Innovations optical modem will communicate data from the ISS to ground and back via the NASA LCRD satellite, which will fly in a geosynchronous orbit. This new capability will greatly increase the amount of scientific data transferred from the ISS, while supporting multiple channels of ultra-high-definition video to and from space.
Space Photonics Received Patent for Free Space Laser Communications in 2017
Space-qualified optics developer Space Photonics Inc. has received its seventh patent in the technology sector that includes free space laser communications. The new patent, titled “Simultaneous Multi-Channel Optical Communications System with Beam Pointing, Switching and Tracking Using Moving,” and the company’s other patents, use specific inventions for very rapid and extremely accurate angular pointing and tracking of free space laser communications beams used for ultrahigh-capacity digital signals commonly used in ground-based internet networks. The technology will provide a much needed boost for emerging global networks using satellites, aircraft and high-altitude balloons.
“This patent is by far the most valuable we’ve received. It provides innovative but simple techniques for pointing and tracking laser communications beams sent and received from moving airborne and space borne vehicles. It does this without using heavy gimbals or moving prisms and mirrors commonly used by others with techniques that are much less accurate for laser beam angular tracking and pointing. Using lasers also can replace certain standard radio frequency transceivers used on orbiting satellites,” said Chuck Chalfant, president and CEO. “The patent title can put you to sleep but is descriptive.”
Laser beam technology breakthrough could revolutionise optical communications
Scientists have created a new class of laser beam that appears to violate long-held laws of light physics. These new beams, which the team calls “spacetime wave packets,” follow different rules of refraction, which could lead to new communication technologies. Light travels at different speeds through different media, slowing down in denser materials. It’s a phenomenon that’s best summed up in a basic, middle-school science experiment: if you place a spoon in a glass of water, the spoon will appear to be broken at the water’s surface. That’s because the light is travelling slower through the water than the air, and the light rays bend as they enter the water – a phenomenon known as Snell’s Law.
This has some major implications for optical communications technologies. The team uses the example of a plane sending messages encoded in light to two submarines, at the same depth but different distances away. Normally, the message would arrive at the closer sub first, but with spacetime wave packets the pulses could be propagated to reach both at the exact same time.
While it may sound like this technology is contradicting some key laws of physics, the team stresses that it’s actually still in line with special relativity. That’s because they’re not messing with the oscillations of the light waves themselves – instead, they’re controlling the speeds at which the peaks of the light pulses travel. This is done using a device called a spatial light modulator, which reorganizes the energy of each pulse of light to intertwine its properties in space and time. “Space-time refraction defies our expectations derived from Fermat’s principle and offers new opportunities for molding the flow of light and other wave phenomena,” says Basanta Bhaduri, co-author of the study. The research was published in the journal Nature Photonics.
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