Optical Communications "Ground-to-Orbit Lasercom Demonstration" (GOLD)

One of the primary goals identified within NASA's most recent strategic plan is "to establish a virtual presence throughout the solar system." The fidelity of this virtual presence will in large part be defined by the communications bandwidth that will link robotic spacecraft and the scientists, engineers, and public who interact with them from Earth.

In this picture, a bidirectional optical communications demo is sending data at rates of up to 1 Mb/second between JPL's Table Mountain Facility (TMF), located in Southern California, and the Japanese Engineering Test Satellite VI spacecraft. This study was known as the Ground-to-Orbit Lasercom Demonstration (GOLD).

New radio frequency and optical technologies are poised to provide breakthrough increases in deep-space telecommunications capacity, allowing NASA to meet the needs of a growing and increasingly challenging mission set while offering increased data return to individual missions.

NASA is developing laser communications technology for sending data back from future space missions. Because of its narrow beamwidths, laser communications can enable significant (10-100x) increases in data return rates, while simultaneously decreasing the mass, power consumption, and size impact of the communications system on the space mission vehicle (e.g. 1/2 mass, 1/2 power, 1/10 volume).

To accurately point such narrow beams back to the Earth receiver, a beacon laser signal must be transmitted from the ground station up to the spacecraft. However, turbulence in the atmosphere can cause the beacon laser signal to break up and fluctuate in power (scintillate) as it passes through the atmosphere. (This is the same phenomenon that causes stars to twinkle).

JPL has demonstrated a way to reduce the effects of scintillation on an uplink beacon laser signal. The characteristic size of the atmospheric refractive index variations (the changes that cause scintillation) is usually below 10 cm (4 inches).

By separating the beacon laser signal into several (four in this case) independent and coaligned beams, and then transmitting each beam up from different parts of the ground telescope primary mirror (which are separated by more than the characteristic atmospheric fluctuation size), the amount of atmospherically-induced intensity fluctuation at the spacecraft can be significantly reduced. This results in a much more constant beacon signal power level at the distant spacecraft, and hence a more stable pointing of the return beam back to the ground site.

The picture shows the transmission of a four-beam beacon signal through the ground telescope toward a distant spacecraft. Experimental measurements of the received signal at the spacecraft confirmed the reduced sensitivity to the effects of atmospheric turbulence.

Read More About It:

http://tmot.jpl.nasa.gov/Program_Overview_Information/ Technology_Newsletter/Issue5b.pdf

http://www.hq.nasa.gov/office/oss/osstech/sec112.htm