Space communications have relied on radio since the first Sputnik in 1957. It’s a mature, reliable technology, but it’s reaching its limits. The amount of data sent has increased exponentially for decades and NASA expects the trend to continue. The current communications systems are reaching their limits, so NASA and ESA are going beyond radio as a solution. As part of this effort, ESA has finished tests of part of a new communications system, in preparations for a demonstration in October in which it will receive a laser data download from a NASA lunar orbiter.
Lasers have already proven capable of carrying enormous amounts of data in fiber-optic cables and engineers believe that if they could be used in space communications, they could carry up to 622 megabits per second (Mbps) of data. Another advantage of lasers is that they use a smaller wavelength than radio by a factor of 10,000. This means that lasers can be aimed in a narrow beam and need very small antennas compared to radio for the same signal strength. All of this means smaller sets at both ends with a subsequent cost savings. Furthermore, the narrowness of a laser beam allows for much more secure communications.
The platform for the October test is NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE). As its name implies, its purpose is to study the almost non-existent lunar atmosphere and the dust that may be lifted up from the lunar surface by static electric charges. It’s based on the Modular Common Spacecraft Bus, which is a NASA program for building spacecraft faster and cheaper by using a common design. One of its four major experiments is the Lunar Laser Communications Demonstration (LLCD).
NASA’s LLCD is similar to the space agency’s OPALS project, but instead of testing it across hundreds of miles to the International Space Station, the LLCD will test the technology over a quarter of a million miles. The test involves the transmission of hundreds of millions of laser light pulses from the Lunar Lasercomm Space Terminal (LLST), aboard the LADEE spacecraft orbiting the Moon. The 65-lb (29.4-kg) system was developed by MIT and consists of three modules: an optical module with a 4-in (10.1-cm) telescope mounted on a gimbal outside the spacecraft, a modem, and a controller electronics module. This fires a 0.5-watt infrared laser at the Earth and can also receive data from the ground stations, but only at a rate of 20 Mbps.
These transmissions will be picked up by two ground stations in New Mexico and California and a third in Spain. This will use the Lunar Lasercomm Ground Terminal (LLGT). It’s made up of an array of eight telescopes ranging from 6 to 17 inches (15.2 to 43.1 cm) housed in a fiberglass enclosure.
ESA’s contribution to the project is through the use of its Optical Ground Station on Tenerife, Spain. In preparation, the space agency is upgrading its LLGT. The technology was tested in July in Zurich using a new detector and decoding system, a ranging system and a transmitter. Meanwhile, NASA and MIT supplied a laser simulator that allows engineers to test compatibility with the US system.
“The testing went as planned, and while we identified a number of issues, we’ll be ready for LADEE’s mid-September launch,” says Zoran Sodnik, manager for ESA’s Lunar Optical Communication Link project. “Our ground station will join two NASA stations communicating with the LADEE Moon mission, and we aim to demonstrate the readiness of optical communication for future missions to Mars or anywhere else in the Solar System.”
LADEE will be launched later this year atop a NASA Minotaur V booster rocket at the Goddard Space Flight Center's Wallops Flight Facility in Virginia, with the first LADEE test expected in mid-October, four weeks after launch.
According to NASA, the use of lasers will allow for better data transmission, real-time communications and 3D high-definition video transmissions. As an example of this increased capacity, S-band radio transmission would take 639 hours to transmit an HD feature-length movie, while LLCD technology could do the same in eight minutes.
The animation below shows the LADEE spacecraft.