‘Line of sight’ communication is set to take back its place from radio waves, says S.Ananthanarayanan.
Early communication, over a distance, was essentially through visible signals. For example the lighthouse to alert sailing ships, the Morse flasher, the semaphore, from hilltops or even bonfires and smoke signals. The entry of the telephone, 1876, made it possible to ‘talk around corners’ and also over larger distances. Telephony had a long reign and dominated personal communication, but wireless communication also had great range and access. While optical fibre replaced the copper wire carrier in telephony and data transfer, radio waves were the ruling medium for broadcast and communication using satellites, in mobile telephony and for the space programme.
All space exploration, by NASA since 1958, or by other agencies, has used radio communication to stay in touch with satellites or spacecraft or modules that have landed on bodies in space. There have been great advances in communication equipment and increase in the extent of data that needs to be sent up to the spacecraft as well as sent by the spacecraft to the command station. The need for higher data transfer, in fact, has exploded, with better instrumentation, robotics and ambitious exploration plans. Actual data transfer, however, is still based on waves at radio frequency, and this places a limit on the amount of data that can be handled and hence on the benefits of the advances in technology.
NASA is hence in the process of putting together a system of data transfer that has substantially higher capacity, using light waves as the information carrier, in place of radio waves. This change would be like what happened with telephony when optical fibres entered the field. However, as we cannot lay an optical fibre cable out the spacecraft, the communication is by means of laser beams, from the base station to the spacecraft and from the craft to the base station. We can see that this would call for uninterrupted path from sender to receiver, which means we are back to communication along the ‘line of sight’ and another application of light, the new field of ‘photonics’ (see box).
Benefits of light
The two main limitations of radio waves are that they are low frequency waves and then that they spread out when the move over long distances. The low frequency of the waves implies that there is a limit to the quantity of information that can be conveyed with the help of the wave. The second limitation, that the waves spread out, means that the transmission has to be strong, as only a small part would be there at the place where the waves are detected. Optical signals, or light waves, on the other hand, are high frequency waves. Typically radio waves may have wavelengths in metres, or frequency in millions of cycles per second. In comparison, the wavelength of light waves is in hundreds of a billionth of a metre, or frequency in thousands of billions of cycles a second. A large number of cycles in every second means more opportunity to load data signals on the wave and hence the greater data capacity of light waves. And then, as the optical signal carrier is a laser, the beam has high directionality and there is much lesser spread of the energy before the beam reaches the receiver. This allows lower power of transmission and hence lesser loads on spacecraft that need to be moved over very large distances.
Another problem with using radio signals is that the frequencies used should not clash with the frequencies used for other purposes, like ground or airline communications or broadcast. This constraint disappears when light waves are used in place of radio waves.
The feasibility of using lasers for communication in space applications was first tried out in NASA’s Lunar Laser Communication Demonstration (LLCD), where high throughput, two-way communication with the help of an infra red laser was established between the earth and the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, a robotic spacecraft that orbited the moon and sent back text and image data for thirty days, in 2013-14. As the path of the laser beam carrier can sometimes be blocked by cloud formations and so on, the earth link is at three locations, the White Sands facility in New Mexico, the Table Mountain facility in California and European Space Agency’s observatory at Tenerife, Spain. Even orbiting stations to relay data could be considered.
The follow up to LLCD is the Laser Communications Relay Demonstration (LCRD), a two-year-long trial of technology that could enable high speed video transmission, via laser, from spacecraft deep within the solar system. The transmission process, encoding techniques and the tracking process are being refined and would be ground tested in 2017 and tested in space in an orbiting satellite in 2019. The change would be akin to the leap in bandwidth that Internet surfers experienced when we moved from dial-up links to broadband and HDMI cable and researchers would receive streaming visuals from far reaches of the solar system , where they could establish a virtual presence.
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