November 26, 2012
Credit: Credit: NASA/JPL
Deep-space exploration depends on more than big rockets and sophisticated spacecraft. Without some way to receive information from the instruments and astronauts we send beyond the Moon, there isn't much point in going in the first place. Today, deep-space data travels in the form of radio waves that need a global network of huge tracking antennas to catch their digital whispering. There is another way—embedding the data in high-bandwidth laser light that can be picked up with small optical telescopes.
The higher the frequency, the more information that can be packed into a signal, and light beats radio hands-down in that department. Two space experiments in the works at NASA's Goddard Space Flight Center and a third at the Jet Propulsion Laboratory (JPL) are set to send laser test signals from space beginning next summer.
“For the same power you get more bandwidth with optical comm,” says Donald Cornwell, mission manager for the Lunar Laser Communications Demonstration (LLCD) scheduled for launch next August as a testbed payload on NASA's Lunar Atmosphere and Dust Environment Explorer (Ladee). “We hope to demonstrate 622 mbps, which is excellent from lunar distance. “[The Lunar Reconnaissance Orbiter] had Ka-band [radio frequency] that did about 100 mbps.”
While laser data transmission is much more efficient, the aiming problem is more difficult. Put an antenna in the footprint at Earth's surface of a Ka-band signal arriving from the Moon (see image), and you've got the signal. The blue arrow at left, and the red circle in the righthand image show how small the LLCD laser footprint will be when it reaches its receiver at White Sands Missile Range, N.M.
The LLCD will send its 1,150-nanometer laser through a gimbaled space terminal built by the Massachusetts Institute of Technology's Lincoln Laboratory. To hit the target—an array of four 17-in. telescopes at White Sands, and a duplicate backup on Table Mountain in Southern California—the space terminal will aim at a beacon laser beam sent up from the ground terminal, give it enough of a “lead” to account for the distance the spacecraft will move before the signal reaches the ground, and stay on target with software designed to account for the jitter of the spacecraft as it station-keeps in lunar orbit.
The actual laser tests will come during Ladee's first 30 days, while the spacecraft's scientific instruments are being commissioned. The laser data rate will be higher than needed for Ladee science, so most of the tests will be conducted with simulated data instead of real science.
Science and a lot more will be routinely transmitted by laser if the follow-on to Cornwell's experiment works out. The Laser Communications Relay Demonstration (LCRD) will test essentially the same laser gear as Ladee in a more operational environment at geostationary Earth orbit (GEO). The two-year trial from a commercial communications satellite positioned in view of White Sands and Table Mountain will explore how laser communications can be scheduled among the multiple ground stations that would be needed in an operational system. Unlike radio waves, laser comm can be blocked by clouds, so a laser data relay will need protocols for handing off the signal from point to point on the ground, says David Israel of Goddard, the principal investigator on LCRD. Ultimately, those protocols could be applied to a high-performance version of the Tracking and Data Relay Satellite System (TDRSS) that now serves the International Space Station (ISS) and a variety of Earth-observing spacecraft.
“TDRSS is the target infusion point for this technology,” Israel says.