MIT tackled Earth's atmosphere to give the moon broadband
- 24 May, 2014 08:29
Four transmitting telescopes in the New Mexico desert, each just 6 inches in diameter, can give a satellite orbiting the moon faster Internet access than many U.S. homes get.
The telescopes form the earthbound end of an experimental laser link to demonstrate faster communication with spacecraft and possible future bases on the moon and Mars. Researchers from the Massachusetts Institute of Technology will give details about the system and its performance next month at a conference of The Optical Society.
The Lunar Laser Communication Demonstration (LLCD) kicked off last September with the launch of NASA's LADEE (Lunar Atmosphere and Dust Environment Explorer), a research satellite now orbiting the moon. NASA built a laser communications module into LADEE for use in the high-speed wireless experiment.
LLCD has already proved itself, transmitting data from LADEE to Earth at 622Mbps (bits per second) and in the other direction at 19.44Mbps, according to MIT. It beat the fastest-ever radio communication to the moon by a factor of 4,800.
NASA hopes lasers can speed up communication with missions in space, which use radio to talk to Earth now, and let them send back more data. Laser equipment also weighs less than radio gear, a critical factor given the high cost of lifting any object into space.
The project uses transmitting telescopes at White Sands, New Mexico, to send data as pulses of invisible infrared light. The hard part of reaching the moon by laser is getting through Earth's atmosphere, which can bend light and cause it to fade or drop out on the way to the receiver.
One way the researchers got around that was by using the four separate telescopes. Each sends its beam through a different column of air, where the light-bending effects of the atmosphere are slightly different. That increases the chance that at least one of the beams will reach the receiver on the LADEE.
Test results have been promising, according to MIT, with the 384,633-kilometer optical link providing error-free performance in both darkness and bright sunlight, through partly transparent thin clouds, and through atmospheric turbulence that affected signal power.
One reason it works is that there's plenty of signal power to spare. The transmission power from the Earth antennas totals 40 watts, and less than a billionth of a watt is received on the LADEE. But that's still 10 times the signal needed to communicate without errors, according to MIT. On the craft, a smaller telescope collects the light and focuses it into an optical fiber. After the signal is amplified, it's converted to electrical pulses and into data.