NASA's next era of deep space communications network (DSN) lies in the field of X-ray and laser

Original author: Bruce Dorminey
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Testing of space optical laser communications will begin next year, X-ray navigation and communications systems are in development.

70 meter antenna in Goldstone. Source: NASA / JPL

After half a century of using the radio to track and communicate with all devices, from the first lunar Rangers to Voyager probes, now crossing the boundary of the solar system and going into interstellar space, NASA is investing $ 2 billion in a long-distance space communications network (DSN) based on the optical and x-ray spectrum.

Next yearNASA plans to launch a demonstration flight to test optical laser communications for the LADEE mission on the moon (Lunar Atmosphere and Dust Environment Explorer - a program for studying the lunar atmosphere and the dust environment of its orbit). And soon optical missions will follow to test the capabilities of the laser repeater in the geostationary Earth orbit (GEO).

“The DSN works almost flawlessly, doing everything we ask,” said Leslie Deutsch, chief technology officer at the Interplanetary Networks Office of the NASA's Jet Propulsion Laboratory. “There were no cases when space missions were lost during a call through DSN, but in several cases the DSN was used to save the mission.”

Using three ground complexes: in Goldstone, California; Canberra, Australia; and Madrid, Spain - The DSN tracks about 35 spacecraft with success exceeding 98 percent.

But from time to time, NASA uses other radio telescopes. Deutsch notes that during the recent landing of the Mars Science Lab, the Parkers Australia DSN radio observatory was used as backup capabilities in order to track MSL signals during entry, descent and landing on Mars.

“We have bottlenecks in the fact that instruments on Mars could return more data if we had more powerful communication channels,” said Deutsch.

Wherever there is a lot of research activity, Deutsche says, it’s also meaningful to create GPS-like capabilities to help navigate the surface of the planet. Deutsch notes that the GPS capability for Mars is still being explored and possibly realized in the coming decades.

Meanwhile, NASA will experience laser communications. LLCD (Lunar Laser Communications Demonstration - demonstration of laser communication with the Moon) will be launched with LADEE in January next year and will demonstrate the speed of the laser transmission line from the Moon at 622 megabytes [per second].
Then, at the end of 2017, a laser relay relay project (LCRD - the Laser Communications Relay Demonstration Project) will be launched in conjunction with the commercial satellite Space Systems / Loral. From geostationary orbit, LCRD will continuously experience high-speed data transmission through optical communication for two years.

LCRD will use 0.5W lasers; which roughly corresponds to the current power when burning DVDs. But increasing this indicator to just 5 watts will allow LCRD technology to provide a communication line with an outgoing speed of 1 gigabyte per second and an incoming speed of 100 megabytes per second in terrestrial orbit. This is 10 to 100 times faster than DSNs on radio frequencies now provide.

“We need an optical repeater in geostationary orbit by 2022,” said David Israel, NASA's space communications engineer at the Goddard Space Flight Center.

Although Israel claims that NASA will use an “eye-safe” wavelength and ensure that their lasers never cross the path of an airplane or satellite, he notes that conventional clouds pose the greatest technical challenge for optical communications.

Thus, when looking for a place for ground-based optical receivers, why not just go to areas where the sky is almost constantly clear?

“Great reception at some isolated mountain peaks is perfect for astronomy,” Israel said. “But if you have a high data transfer rate from orbit there, then there may not be a [effective] way to get this data from the mountain.”

Thus, one of the problems for ground-based optical communication telescopes is to strike a balance between optimal “visibility” and the use of existing communication infrastructure lines needed to quickly redirect incoming data back to remote researchers.

NASA is also exploring the suitability of natural astrophysical x-ray sources to create a space-based navigation system that functions on a solar-system scale similar to GPS. The idea is to use [accreting millisecond] pulsars, rotating neutron stars, emitting x-rays with a frequency of milliseconds, to accurately determine the ship's course and its position.

Voyager 1 Source: NASA / JPL

“The XNAV system,” said Keith Gendreau, an astrophysicist at NASA’s Goddard Space Flight Center, “will need a tracking x-ray detector to observe multiple pulsars over time.”

“Pulsars produce regular pulses that can compete in accuracy with atomic clocks for months and years,” Gendreau said. - GPS is a grouping of satellites, each of which contains an atomic clock transmitting the exact time. GPS receivers receive time signals from several satellites, and according to these data they calculate their coordinates. For XNAV, our watches will be pulsars distributed on a galactic scale, which will allow us to provide GPS navigation both throughout the solar system and beyond. ”

Today, to navigate to outer planets, they use a long-range space communications system and spacecraft star sensors to calculate the exact position. But Deutsch says that XNAV can make autonomous spacecraft navigation even more accurate.

“XNAV would create 3-dimensional positional data from pulsars located in different directions in the sky,” says Gendreau. He also notes that in addition to the three pulsars that the spacecraft will use to determine its position, the fourth pulsar will provide independent time measurements.

Researcher of the internal structure of neutron stars - (NICER - Neutron Star Interior Composition Explorer) is an experiment proposed by NASA for pulsar timing, which could demonstrate XNAV by the end of 2016.

“By the time the miners go into space into the asteroid belt, we can say with confidence that they will use XNAV,” Israel said.

The most accurate clock in nature can make Galactic GPS possible. (Illustration from Wikipedia)
A radio search found 17 new millisecond pulsars by examining a list of unidentified sources obtained by the Fermi gamma-ray space telescope. Colored circles indicate the positions of new pulsars discovered in 2009, over the year of operation of the telescope, on a panoramic map of the entire sky.

Meanwhile, NASA researchers in Goddard are also working on X-ray communication (XCOM), which uses photovoltaic modulation of the photocathode for communication using an ultraviolet source. The advantage of x-rays over laser communication is that the wavelength of the x-ray radiation is shorter, and it can penetrate into areas inaccessible to radio and optical frequencies, for example, when entering the atmosphere.

Gendreau says that one of the main advantages of x-rays over lasers is that the short wavelength allows very thin beams to be transmitted, and therefore lose much less energy when communicating over long distances.

“Very high X-ray energy can also penetrate the plasma envelope surrounding the capsule entering the atmosphere and provide a low-speed connection for such a hypersonic apparatus,” said Jendreau. “If NICER takes off, then by 2018 we could use it as a receiver for the first XCOM demonstration in space.”

What is the future of the deep space communications network?

Deutsch says data rates are orders of magnitude higher than today; continuous DSN coverage for people in such remote areas as the far side of the moon, as well as Internet-like empowerment where NASA sends astronauts or cars.

What about the radio?

“I think that space radio communications can never be completely abandoned,” said Deutsch. “She's very simple and easy.”

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