The new platform laser guidance developed by the Massachusetts Institute of technology, could help small satellites to play high speed data transmission. In 1998 was launched nearly 2,000 satellites the size of a shoebox, known as CubeSat. Due to its miniature form and the fact that they can be assembled from finished parts, These are much cheaper to build and run than the traditional monsters that cost hundreds of millions of dollars.
These changed principles for the establishment of satellites, because you can run them in packs for cheap monitoring of large areas of the Earth’s surface. But since CubeSat equipped with more and more advanced tools, a tiny spacecraft do not have time to efficiently transfer large amounts of data to Earth due to restrictions in power and size.
CubeSat: tiny messenger Earth
A new laser-guided platform for CubeSat, is described in detail in the journal Optical Engineering, “cubitum” pass the data down, using fewer on-Board resources at much higher speeds than possible at present. Instead of having to send several images, every time “cubsat” passes through the ground station, the satellites will have the opportunity to transfer thousands of high resolution images of every flight.
“To get valuable information from Earth observation, we can use a hyperspectral image that take pictures at multiple wavelengths of light and generate terabytes of data, “cubitum” it is very difficult to pass,” says Kerry Kakha, associate Professor of Aeronautics and Astronautics at MIT. “But with high-speed system lasercom we will be able to send these detailed images fast enough. And I think this ability will make the whole approach CubeSat, using a multitude of satellites in orbit, more realistic, so that we can get a global and immediate coverage.”
Outside of radio range
Satellites typically transmit data to earth using radio waves; higher-speed lines associated with large ground-based antennas. Every major satellite in outer space communicates in high-frequency radio that allows it to quickly transfer large amounts of data. But large satellites can adapt to large radiotracer and arrays, which support high-speed transmission. “They were launched” too small and have limited access to frequency bands that support high-speed channels.
“Small satellites can not use these bands, because I need to solve a lot of regulatory issues to resolution, this usually involved the big players like large geostationary satellites,” says Kakha.
Moreover, the transmitters needed for the high-speed data transmission, can use more energy than you can afford to release small satellites that support the work of the filling. For this reason, engineers have turned to lasers as an alternative form of communication for “cube-Sats”, because lasers are much more compact and more efficient in energy expenditure – they compress more data in a carefully focused beams.
However, laser communication, there are also problems: since the beams are much narrower than the beams of radio waves required much more accuracy to direct the beams to a receiver on earth.
“Imagine yourself standing at the end of a long corridor and establish thick beam like from a flashlight to a target with the bullseye at the other end,” says Kakha. “I can move my arm and the beam will still hit the jugular. But if I take a laser pointer, the beam can easily come up with the bullseye if I make a move. The challenge is how to keep the laser in the bullseye even if the satellite will wiggle”.
Demonstration of optical communications and sensors, NASA uses a laser communications system for CubeSat, which inherently tilts and pushes the entire satellite to align its laser beam with a ground station. But this steering system requires time and resources, and to achieve higher data transmission speed, a more powerful laser, which can, if necessary, use a large portion of the satellite’s power and generate a significant amount of heat on Board.
Kakha and her team decided to develop an accurate laser guidance, which minimise the amount of energy and time required for data transmission to earth, and would allow the use of less powerful lasers narrow, but to achieve higher transmission speeds.
The team has developed a platform for laser-guided, slightly larger than the “Rubik’s cube”, which includes small and ready driven MEMS mirror. A mirror which size is less than key on the keyboard that converts to a small laser situated at an angle so that the laser can bounce off mirrors in space and to go down to the ground receiver.
“Even if the whole satellite is slightly shifted, it can be corrected with the help of this mirror,” says one of the team members. “But MEMS mirrors do not give you feedback on where indicated. For example, the mirror offset in your system, this can occur due to some vibrations during launch. How do we fix this, how to find out exactly where we specify?”.
As decision scientists have developed a method of calibration, which determines how the laser is offset relative to the purpose of its ground station, and automatically corrects the angle of the mirrors to accurately direct the laser to his receiver.
This method includes an additional laser color, or wavelength, in an optical system. Thus, instead of just pass a bundle of data sent and the second calibration beam of a different color. Both the beam bounce off the mirror and calibration light passes through the “dichroic beam splitter”, an optical element that deflects a certain wavelength of light — in this case, the complementary color is from the main beam. When the rest of the laser radiation goes to a ground station, designated beam is directed back to the on-Board camera. This camera also can take upward of a laser beam, or beacon, directly from ground stations; it will help the satellite to tune to the correct ground target.
If the lighthouse beam and the calibration beam fall exactly in the same place on the detector side of the camera, the system is aligned, and the researchers can be sure that the laser is correctly positioned for communication with the ground station. However, if the rays fall in a different part of the detector of the camera, a special algorithm directs the integrated MEMS mirror so that it tilts and the calibration laser beam is aligned with the point of the beacon ground station.
“It’s like a cat-and-mouse two points coming into the camera, you need to tilt the mirror so that one dot appeared over the other.”
To check the accuracy of the method, scientists have developed a laboratory stand with a laser-pointing platform and a laser signal according to the type of beacon. The installation was to simulate a scenario in which the satellite flies at an altitude of 400 kilometers above the ground station and transmits data over a 10-minute session.
Scientists have established the minimum required pointing accuracy in the milliradian of 0.65 — the measure of angular error is acceptable for their design. In the end, the calibration method allowed us to obtain an accuracy of 0.05 milliradians, which is much more precise than required by the mission.
“This shows that on such a tiny platform, you can install the system with low power consumption and narrow beams, and it will be 10-100 times smaller than anything that’s ever been assembled like this before,” says Kakha. “The only thing that would be more interesting lab results — to see it happening from orbit. That’s what motivates the creation of such systems and output them in there.”
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