What do you think about when you hear the word, Laser? Besides the funny sounds they use in cartoons and laser beam swords hot enough to cut through anything, the idea of a laser seems like something that comes to mind when we think about the future! Lasers play a big part in a lot of the technology we use today. From simple laser pointers we use in the classroom and barcode scanners in grocery store, to LASIK lasers, that are powerful enough to cut through tissue, which is why we use them for surgery, you will find that lasers are actually more common in our lives than you'd think. So what do you think is possible if we combine the idea of lasers with communication technology? Something futuristic, right?Satellite Communications
When it comes to satellite communication, there are two types we use: satellite to satellite and satellite to ground. The entire process of how we receive data from various space missions often involves something called data relay. Much like a relay race, a spacecraft that's studying, say, Mars, sends the data it's collected to another satellite, the data relay satellite, whose primary job is to pass on information, sending the information onto another satellite that shoots the information back to Earth. In the beginning, NASA used to have many ground stations to keep up with getting information from space, but nowadays NASA only keeps a few ground stations open, and uses a number of data relay satellites instead. Like all digital devices we use daily, such as computers and smart phones, the satellites use a number system called binary code, which is made up of only 0's and 1's. Satellite messages in binary code or in bits (number of 0's and 1's) are formed, and sent through the antenna of a transmitting system, like when you talk to someone through a phone, and are captured by another antenna of a receiving system, like when you hear someone talk to you through the speakers on the phone.
What did light and the ocean do when they passed each other? They waved! Light and the ocean are similar in the fact that they both have waves. How can this be? They look nothing like each other! So, if you are standing on a beach watching the ocean, you can count the number of times per minute the waves are crashing onto the beach. This is period of watching the waves hit the shore is called frequency, and we measure it in hertz, which sorts everything to the number of full cycles per second. If we look at how a wave looks, notice how there is a distance between the peaks of one wave to the next; this distance is called the wavelength of the ocean wave. So let's apply these new terms, frequency and wavelength to light and what do we get? Light, including lasers, acts like a wave with different frequencies to form different colors of a rainbow. But not only that, light is part of range of different frequencies called the electromagnetic spectrum as humans can only see a tiny part of this spectrum! One very interesting thing to remember about light is that while it moves in waves, it also moves like particles. If you have seen a bingo ball machine, looking at how the little bingo balls bounce around in their containers is how particles move on a subatomic level. Because light can move as waves and particles, we can make technology that focuses on one or both of these features. Take a digital camera for example. The sensors in the camera pick up the light particles called photons, which are little packets of energy, to form the image of a scene that you are trying to capture with your camera.
As for radio waves, scientists have discovered that electricity (the thing that powers our homes and schools) and magnetism (like the magnets on your fridge) are linked. When put together, they form waves that again look like the waves of an ocean. These waves are not visible like visible light, the light we can see, and have much lower frequencies. Examples would be frequencies used AM and FM radio stations, as well as in satellite and other wireless communications systems. Waves with higher frequencies than those used for AM and FM broadcasting, and often called microwave or Radio Frequency (RF). These are very important as NASA currently uses them operationally.
The biggest difference between laser and RF communications methods is the difference in the frequencies. Lasers have frequencies at 100,000 times higher than that of radio frequencies, which means using lasers we can send and receive more bits of data per second than using RF. For example, in 2013 NASA completed a lunar mission called Lunar Atmosphere and Dust Environment Explorer (or the LADEE). If we wanted to download our favorite movie in HD onto the LADEE spacecraft using RF, it would have taken 639 hours to download. That means it would have taken little over 26 days to download a 2-hour movie! Can you imagine waiting almost a month to watch your favorite movie? In comparison, using laser communications, the download time would have taken only 8 minutes. 8 minutes versus 26 days: that's a huge difference! It's important to remember that laser and RF both travel at the speed of light, which is the fastest speed we know of in the universe. So the difference between these two communications methods is not in the speed of the signals, but is in the number of bits we can transfer in the same amount of time. In addition, there are so many improvements laser communication adds to the data relay system that it's something NASA can't help but be really excited about!
A laser communications system, complete with transmitters and receivers on laser terminals and other equipment, is lighter in weight and smaller in size than an RF communications system because sending and receiving a higher frequency of a laser requires less equipment compared to RF. Lasers have a narrower beam and have special requirements to support the laser equipment so that it doesn't wobble when sending/receiving data. This advantage of lasers over RF not only makes the communications equipment easier to send into space, but also saves costs both in space and on the ground.
One important fact about the laser communications method is that it is still a new technology. The laser communications system on the LADEE mission was the first full demonstration of this technology from Moon to Earth, which will be followed by a more complete and longer demonstration based on laser communication on following missions. If this is successful, NASA plans to use laser systems alongside existing RF devices to send and receive data to and from future science satellites.
One of the other challenges of laser communications is that clouds can block lasers, interrupting the signal between transmitters and receivers. Why does this happen? If you imagine pointing a flashlight at fog, you will notice that all the light does not go through the fog, and some bounces back or scatters. For this reason NASA has to be careful about picking locations where it would set up grounds stations that are able to receive laser signals for satellite to ground communications, however with satellite to satellite communications, there is no problem. RF signals, on the other hand, can go through clouds more easily than lasers to reach their receivers. However, RF signals at certain frequencies can also get lost in rain or fog due to water droplets absorbing the RF energy. You can observe this if you try to heat up a cup of water in the microwave; it'll boil because of this reason! This is why neither technology can be a replacement for the other. Rather NASA imagines using both technologies working together as a team to bring the stuff we learn about in space back to Earth.
In 2013, NASA completed its first laser communications demonstration installed aboard the Lunar Atmosphere and Dust Environment Explorer (or the LADEE) spacecraft. On October 18, LADEE successfully completed testing the laser system onboard, sending its message back to Earth at record speed of 622 megabytes per second. This data was downloaded across quarter million miles to a ground station in New Mexico! Shortly after completing its mission to observe the environment around the moon, LADEE purposefully crashed into the surface of the moon as planned because flying it back to Earth was never in the plan. It gave NASA a great chance to observe in a controlled experiment of how an object would impact the moon.
Another NASA mission LCRD or Laser Communications Relay Demonstration is an example of how NASA could use laser communications together with radio frequency communications. The ultimate plan is to have satellites to use both radio frequency and laser communications systems. Because neither method works perfectly on their own, put together they can cover each other's shortcomings. LCRD is promised to achieve much higher data rates than Radio Frequency communication. This is much faster compared to the average Internet download speed in the US. In comparison, LCRD's lowest data rates are still many times faster than the average Internet download speed in the US. Think about downloading a movie from the Internet, LCRD can achieve this more quickly. LCRD aims to further our knowledge on laser communications. NASA has specifically chosen ground stations located in both California and Hawaii to send and receive laser signals. Having both types of communications available on future spacecraft allows us to receive more data from lunar satellites and planetary missions operating in our solar system! There's so much to learn about our world, space and earth, that we need to upgrade how we get information to quench our quest for knowledge.
1 Woodford, C. (2008, January 1). Barcodes and barcode scanners. Retrieved March 11, 2015, from http://www.explainthatstuff.com/barcodescanners.html
2 U.S. Food and Drug Administration. (n.d.). Retrieved March 11, 2015, from http://www.fda.gov/medicaldevices/productsandmedicalprocedures/surgeryandlifesupport/lasik/default.htm
3 Download Speed in the United States. (n.d.). Retrieved March 11, 2015, from http://www.netindex.com/download/2,1/United-States/