Late last year, a team of researchers from the Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory working with NASA demonstrated a laser-based communication uplink between the moon and Earth. At CLEO 2014, the team will present new details and the first comprehensive overview of the on-orbit performance of the uplink, which beat the previous record transmission speed by a factor of 4,800. Earlier reports have stated what the team accomplished, but have not provided the details of the implementation.
‘This will be the first time that we present both the implementation overview and how well it actually worked,’ said Mark Stevens of MIT Lincoln Laboratory. ‘The on-orbit performance was excellent and close to what we’d predicted, giving us confidence that we have a good understanding of the underlying physics.’
The team made history last year when their Lunar Laser Communication Demonstration (LLCD) transmitted data over the 384,633km between the moon and Earth at a download rate of 622 megabits per second, faster than any radio frequency (RF) system. They also transmitted data from the Earth to the moon at 19.44 megabits per second, a factor of 4,800 times faster than the best RF uplink ever used.
‘Communicating at high data rates from Earth to the moon with laser beams is challenging because of the 400,000km distance spreading out the light beam,’ Stevens said. ‘It’s doubly difficult going through the atmosphere, because turbulence can bend light, causing rapid fading or dropouts of the signal at the receiver.’
A ground terminal at White Sands, New Mexico, uses four separate telescopes to send the uplink signal to the moon. Each telescope is about 6 inches in diameter and fed by a laser transmitter that sends information coded as pulses of infrared light. The total transmitter power is the sum of the four separate transmitters, which results in 40W of power.
The reason for the four telescopes is that each one transmits light through a different column of air that experiences different bending effects from the atmosphere, Stevens said. This increases the chance that at least one of the laser beams will interact with the receiver, which is mounted on a satellite orbiting the moon. This receiver uses a slightly narrower telescope to collect the light, which is then focused into an optical fibre.
From there, the signal in the fibre is amplified about 30,000 times. A photodetector converts the pulses of light into electrical pulses that are in turn converted into data bit patterns that carry the transmitted message.
Of the 40W signals sent by the transmitter, less than a billionth of a watt is received at the satellite – but that’s still about 10 times the signal necessary to achieve error-free communication, Stevens said.
The CLEO presentation, which will take place on 9 June at 4pm, will also describe how the large margins in received signal level can allow the system to operate through partly transparent thin clouds in the Earth’s atmosphere, which the team views as a big bonus.
While the LLCD design is directly relevant for near-Earth missions, the team predicts that it’s also extendable to deep-space missions to Mars and the outer planets.