Since the dawn of the space age, NASA has been relying on radio communications technology to send and receive data to and from spacecraft. Although it has developed higher data-rate radio frequency systems, data-compression, and other techniques to boost the amount of data that its current RF systems can handle, they can't keep pace with the projected data needs of advanced instruments and further human exploration. To break this bottleneck, NASA is turning to optical communications technology that would use lasers to increase data rates over existing systems by anywhere from 10 to 100 times.
NASA's current legacy radio-based network includes a fleet of tracking and data relay satellites and a network of ground stations. At the current limit of 6 Mbps for the Mars Reconnaissance Orbiter (MRO), NASA says it currently takes 90 minutes to transmit a single HiRISE high-resolution image from Mars back to Earth. However, the new optical communications system would reduce the transmission time down to just five minutes and even allow streaming of high definition video from distances beyond the Moon.
The Laser Communications Relay Demonstration (LCRD) is designed to enable NASA, other governmental agencies and the commercial space industry to undertake future, complex missions by providing significantly higher data rates for approximately the same mass, power, and volume as a comparable RF system.
NASA says laser-based space communications will enable missions to use bandwidth-hungry instruments, such as hyperspectral imagers, synthetic aperture radar (SAR), and other instruments with high definition in spectral, spatial, or temporal modes. Laser communication will also make it possible to establish a "virtual presence" at a remote planet or other solar system body.
The LCRD is expected to fly as a hosted payload on a commercial communications satellite developed by Space Systems/Loral, of Palo Alto, California. The experimental payload will include telescopes, lasers, mirrors, detectors, a pointing and tracking system, control electronics, and two different types of modems; one ideal for communicating with deep space missions or tiny, low-power smallsats operating in low-Earth orbit, and another that can handle much higher data rates, particularly from Earth-orbiting spacecraft, including the International Space Station.
A team at the NASA Goddard Space Flight Center in Greenbelt, Maryland, which is leading the development of the system, will encode digital data and transmit the information via laser light from specially equipped ground stations to the experimental payload on the commercial communications satellite. Once the payload receives the data, it would then relay it back to the ground stations that are now scheduled to operate in Hawaii and Southern California.
The multiple ground stations are important, as the optical system requires a clear line of sight between the transmitter and receiver. So if bad weather prevents a signal from being sent or received at one location, the network could hand things over to one of the other ground stations or store it for later transmission.
"Just as the home Internet user hit the wall with dial-up, NASA is approaching the limit of what its existing communications network can handle," said LCRD Principal Investigator Dave Israel. "What we're trying to do is get ahead of the curve. We want to get to the point where communications is no longer a constraint on scientists who want to gather more data, but are worried about getting their data back from space," Israel added. "With the higher-speed modem type, future systems could support data rates of tens of gigabits per second," he said.
NASA plans to demonstrate the LCRD system in 2016, with the demonstration expected to run two to three years. It is one of three proposals NASA has selected as Technology Demonstration Missions because of their potential to provide tangible, near-term products and have high-impact of NASA's future space exploration and science missions.
The other two proposals selected were a Deep Space Atomic Clock designed to provide the unprecedented stability needed for next-generation deep space navigation and radio science and an in-space demonstration of a mission-capable solar sail to enable propellantless in-space navigation for missions such as advanced geostorm warning, economic orbital debris removal, and deep space exploration. More on these later.