Linking the Earth and a Satellite with a LASER - NASA's Breakthrough

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We have come a long distance in communication. A decade ago, Communication and its related gadgets were kind of luxury but now they have become a basic necessity which are affordable. With the advent of GPS in modern day mobile, we can roam with ease even in an unknown place. What exactly is that driving force behind all these services?    Obviously “SATELLITES”.                                                                                                             Satellites have become an integral part of this 21^st century’s life. You pick one service and there will be a satellite communication involved in a way or other for its proper functioning. We are very much dependent on these satellites unknowingly and it’s not so easy to comprehend the effects of satellite shutdowns on our regular activities. In short we can say that our life will be as messy as you cannot imagine without the satellites and the network they use to communicate with all of us.

Click here to take a glance at our life without satellites

So, with this article we thought of making our readers aware of some basics of satellite, their functionality, and the way they communicate with all of us and eventually we take you to a revolutionary update that is going to change the way satellites handle the information.

      Satellite engineering unlike other faucets of engineering is very complex and requires a lot of expertise in almost all branches of science. Right from the design phase to the launching phase of the satellite a lot of brilliance and foresight is required because it’s all about designing a prototype which is going to function in an outer space and thorough knowledge of the space and its characteristics must be required. As you all know transforming the satellite from a conceptual stage to its final working model is a tedious and money hungry activity and so a lot of clarity and mental acumen is a mandatory. These satellites after testing will be put in an orbit with the help of launching vehicles and once put into the orbit they start providing their services.

Why satellites need to communicate with a ground station?

       We all think that the job is done when once the satellite is placed in its intended orbit but actually it’s just half of the job that is done. Once the satellite is placed in the orbit its trajectory must be controlled and it is a continuous process. In order to control the trajectory the satellite needs to send its position information to the control stations present on the earth and the corrections will be sent in the form commands from the earth stations to the satellite with which it can rectify the offset. For this, communication is very vital and without this, satellite will just be fancy object roaming in the space. Also satellites whether it be a meteorological satellite or Positioning satellite, its main job is to send the information it gathers from the space to the earth station so that analysis will be carried out on that data to get some useful information. So, with this you can imagine the importance of the communication systems in the satellite.

How do satellites communicate?

        Satellites will be equipped with Transceivers (Device which can transmit and as well as receive data) which form the major component of the communication system. Along with the transceivers processing units and other electronic circuitry will be present to support the transceivers. Say for example there is a variation in the orientation of the satellite and this needs to be sent to the earth station. So, the data from the orientation sensors will be taken into the processor and will be properly modulated and will be fed to the transceivers which then transmit the data to the earth station. In the earth station the data will be received by antennas which are constantly oriented in the direction of the satellite and is sent to the earth station for analysis. After analyzing the data received from the satellite transceivers, the operators will give a specific command which is transmitted to space through transmitters stationed in the earth station. These signals are then received by the transceivers and are transferred to the processing unit in the satellite. The processing unit takes up necessary action for a specific command as per the software written in the processor. This is how the satellite is corrected to its intended orientation (It must be noted that all the vital parameters of the satellite are continuously monitored and are altered from earth station only). All the steps that we have discussed in the above lines can be summed up to a single term called “TELEMETRY”. 

       In telemetry, in order to establish a connection between satellite and earth station Radio Frequency (RF) signals are used. It will be so startling to know that the information related to position, health of equipment aboard a satellite will be communicated in the form  of numbers which are then decoded into useful information at the end place. These RF signals are being used for a long time in satellite communication but with the necessity to receive more information from the satellites, the data load has become humongous that it’s getting difficult for the RF to serve the purpose. If we take a look at the future, the space missions demand tonnes and tonnes of information from space in order to thoroughly understand the happenings in the dark space. RF technology is not so promising for this exponential growth in data demand and thus a quest for an alternative approach has started several years ago and finally NASA succeeded in fetching an efficient alternative for the existing approach.

NASA’s High speed data transfer – A Breakthrough:-

         The problem with RF signals is that they possess high wavelengths and also they spread to a wider range in space. If you consider an RF signal sent from a satellite to an earth station, the signal will have a foot print (range it spreads) of around 100miles and this results in several reception problems. This phenomenon increases with increase in distance from the surface of the earth. Also to accommodate bigger wavelengths, the antennae used in the satellite must be very big (sometimes bigger than the satellite itself). Apart from all these disadvantages the data transfer speed is very low in case of RF signalling. To address all these problems with a single solution, NASA along with MIT worked on LASER signalling to transfer data to and from the satellite and earth station. This approach was tested successfully with NASA’s recent LADEE (Lunar Atmosphere Dust Environment Explorer) mission.

How it works?

          LADEE was sent by NASA into space to study the properties of lunar atmosphere and the dust prevailing over there. Aboard LADEE an LLST (Lunar Laser Space Terminal) module was provided which takes care of the communicating part of the satellite. As discussed earlier, the crucial information that needs to be sent to the earth station will be encoded by LLST in the form of light and is then transmitted to the earth station. This approach is quite similar to the optical communication through which you are getting internet to your home but the difference is the medium of transfer. Internet data transfer needs optical fibre cable through which the data travels in the form of light whereas in the case of extra-terrestrial communication, the medium is space itself and this is quite challenging. LLST uses near infrared spectrum for transmitting and receiving data with its on ground counterparts (LLGT). Both LLST and LLGT put together should be termed as LLCD (Lunar Laser Communication Demonstration).  

     LLST consists of mainly 3 modules namely Optical Module (OM), MODEM module (MM) and Controller Electronics module (CE). OM consists of a telescope mounted on a gimbal which can transmit and receive encoded infrared data to and from the ground terminals. The telescope is coupled to the fibres through which it gives and receives data from a MODEM. Modem module encodes the incoming and outgoing data with the help of optical transmitters and receivers. It is also equipped with a 0.5 watt IR LASER transmitter. CE module possesses all the control algorithms related for proper functioning of LLST. A similar arrangement will also be present at the ground stations (LLGT). With this technology NASA succeeded in increasing the speed of transmission 5 folds with a maximum downlink data speed being 622 Mbps also an uplink speed of 20 Mbps. This technology can transmit data over lakhs of miles with a speed which is almost hundreds of times faster than our conventional internet speeds. As a matter of comparison between the new revolutionary approach and the conventional data transfer methods, NASA reported that:

       

     “LADEE spacecraft would take 639 hours to download an average-length HD movie. Using LLCD technology, download times will be reduced to less than eight minutes”. 

                                           (Taken from NASA official website).

Apart from robust data transfers, LASER transmission is secure and the transceiver equipment required is very handy (LLST weighs only 30 kg) thus saving a lot of payload and space on the satellite. Since IR waves are about 10,000 times smaller in wavelength, the footprint (dispersion of data signal during uplink and downlink) will be less and data can be transferred in narrow beams. This kind of narrow and tight beam gives away maximum signal power to the receivers and hence reduces power losses and improves the quality of reception. It also facilitates in effective utilization of bandwidth.        

      This breakthrough technology is a first step towards achieving a real time reception and transmission of data to and from the satellite and lets all hope that this communication strategy propels the space research to a whole new level wherein the dark universe unleash its latent secrets.

                                                        Source :  NASA official Website