The First Interplanetary Laser Communication Link

By Roland Piquepaille

NASA and MIT are teaming together to build the first interplanetary laser communication link between Mars and Earth. "In 2010, the Mars Laser Communication Demonstration (MLCD) will test the first deep-space laser communication link, which promises to transmit data at a rate nearly ten times higher than any existing interplanetary radio communication link." In fact, the speed will largely vary depending on the respective positions of March and Earth. The minimum expected speed will be one megabit per second during daytime and when Mars is at its farthest point from Earth. But when Mars is at its closest approach and reception is at night, the rate could be thirty times higher. Before 2010, a number of challenges need to be solved, including the fact that the optical frequencies of the laser can be partially be blocked by clouds.

A NASA MIT Lincoln Laboratory team will forge the first laser communication link between Mars and Earth. This unique experiment, part of NASA's Vision for Space Exploration, will greatly benefit the transmission of data from robotic spacecraft.

In 2010, the Mars Laser Communication Demonstration (MLCD) will test the first deep-space laser communication link, which promises to transmit data at a rate nearly ten times higher than any existing interplanetary radio communication link. MLCD will fly on the Mars Telecommunications Orbiter spacecraft, which is planned for launch in 2009. The experiment is a partnership among NASA's Goddard Space Flight Center, NASA's Jet Propulsion Laboratory (JPL), and MIT s Lincoln Laboratory (MIT/LL).

Below is an illustration showing the design of the Mars Telecom Orbiter to be launched in 2009, including the Mars Lasercom equipment. This diagram appears on page 7 of a 137-page presentation given at the Eighth Mission Services Customer Forum on March 18, 2004 at NASA/Goddard Space Flight Center (PDF format, 3.89 MB) (Credit: NASA).

So what kind of data transmission speed can we expect?

"If we are planning to put people on Mars, we'll need highly reliable communication links with high data rates, and our team wants to show how this can be done with lasers," said Rick Fitzgerald, Project Manager at NASA Goddard.

The expected data rate varies depending on Mars's position in its orbit, the weather and atmospheric conditions on Earth, and whether reception is occurring in daytime or nighttime. When Mars is at its farthest point from Earth and the reception is occurring during daytime, the team expects to receive data at a rate of a million bits per second, but when Mars is at its closest approach and reception is at night, the rate could be thirty times higher. Today, the maximum data rate transmitted to Earth by spacecraft at Mars is about 128,000 bits per second (for NASA's Mars Odyssey spacecraft).

Below is an illustration showing Deep Space Lasercom Compared to Geosynchronous Earth Orbit (GEO) Systems. As you can see, Mars incurs almost 80 dB additional space loss when compared to GEO links. Thus, assuming that an organization was successful in building a 10 Gbits/second class terminal that could operate from GEO to the ground, simply transporting such a terminal from Earth to Mars would result in a data rate of only 100 bits/second! An improvement of 50 dB is required to provide 10 Mbits/second from Mars. You'll find more information in "Mars Lasercom Demonstration Underway,"(Credit: NASA, The Integrator, Volume 12, No. 3, November 2003).

As you can see from the legend above, lots of progress need to be done. Here is another example.

Lasers have not been used for deep-space communications until now because they first had to be made reliable and efficient enough for use in spacecraft millions of miles from Earth. Additionally, the radio frequencies traditionally used for deep space can pass through clouds, while laser (optical frequencies) can be partially to completely blocked by them. The project hopes to overcome this limitation by employing two separate ground terminals, on the chance that if one terminal is clouded over, the other might be clear.

Sources: Massachusetts Institute of Technology News Office, August 6, 2004; NASA websites