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Collaboration Key to Successful Technology “Push”

Bill Farr

Above, former DSOC Project Manager Bill Farr in his lab at NASA’s JPL. Credit: NASA

The Lunar Laser Communications Demonstration (LLCD) mission made history in October 2013 when it succeeded in transferring data at 622 Megabits per second, a rate six times that of comparable radio frequency systems, like going from dial up to a high-speed Internet connection. But this technological achievement in laser communications was at risk had it not been for the “push” researchers experienced when an important component, a photodiode detector, failed to perform as necessary during testing.

In the world of emerging technologies, a “push” is any activity attempting to expand on advancements to current challenges or limitations. Within NASA’s Space Technology Mission Directorate (STMD), projects like Deep Space Optical Communications (DSOC) seek to do just that. When LLCD was faced with the detector failure, a potential replacement was identified—one with a challenge: it was still under development with DSOC.

The LLCD experiment, now well known for its achievement, launched onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) from NASA’s Wallops Flight Facility in Virginia on September 6, 2013. A series of LLCD experiments began in late September with the first successful downlink from LADEE on September 28, just before LADEE reached lunar orbit. LLCD mission operations began in mid-October, and by October 21 six links were successfully completed.

Getting to that successful point, however, was not a straightforward path and required numerous collaborative efforts among individuals and organizations across NASA and industry.

The Lunar Lasercom Ground Terminal at White Sands, New Mexico. Credit: MIT

The Lunar Lasercom Ground Terminal at White Sands, New Mexico. Credit: MIT

Early in the mission life cycle, it became evident that there was a high probability of limited or no communications link opportunities for the LADEE launch due to clouds or inclement weather during the monsoon season at the optical ground station at White Sands Center in New Mexico. NASA’s Space Communications and Navigation (SCaN) Office stepped in by funding a back-up ground station at the NASA/Jet Propulsion Laboratory (JPL) Optical Communications Telescope Laboratory. The JPL back-up ground station project is referred to as LLOT, or the Lunar Lasercom OCTL Terminal. The JPL ground station has a telescope specifically designed for space optical communications experiments. The back-up station project required a demonstration only at the lowest downlink rate of 39 Mb/s. During early testing of that capability, the baselined commercial intensified photodiode detector failed to adequately detect data at 39 Mb/s.

The need to overcome this limitation was clear; fortunately the answer was already in the works.


The optical module of the Lunar Laser Communication Demo’s Space Terminal aboard LADEE during environmental testing. Credit: NASA

Back in the summer of 2011, under SCaN funding, Bill Farr and Jeff Stern of JPL had begun WSi detector development in collaboration with the National Institute of Standards and Technology, building on what Farr described as NIST’s “ground-breaking achievements.”

“This naturally flowed into STMD’s Game Changing Development DSOC project starting in the fall of 2011,” said Farr. “Our DSOC project goal has been to make large arrays of WSi detectors to go behind 5- to 12-m diameter telescopes. We are presently fabricating 64-pixel arrays. At an interim step we fabricated the 8- and 12-pixel devices, which were suitable for use behind a 1-m telescope, such as at the JPL ground station.”

Farr and Stern fabricated and began testing their first WSi devices at the start of March 2012.

“In collaboration with NIST, by the end of April 2012 we had a record setting 93-percent system detection efficiency with single-pixel devices, and under the DARPA-funded InPho program performed a record setting 13-bits per photon demonstration using pulse-position-modulation (the preferred deep-space optical communications modulation format) with one of these devices,” Farr said of the testing results.

In September 2012, after the critical nature of issues with the commercial photodiode detector was deemed insurmountable, the challenge was firmly set. The LLOT project found that to succeed, it would be necessary to switch to the WSi detector and moving forward was review-board approved.

With that approval, the push was now truly on.

Farr’s own words best describe the dynamic collaborative efforts:

“I knew a local vendor, Photon Spot, Inc., (Monrovia, Ca.) starting a business in superconducting nanowire detectors. The LLOT project worked with Photon Spot to quickly assemble and lease a cryostat that would achieve the required 1-K operating temperature for the WSi detectors.

“The cryostat was delivered to JPL in April 2013. Matt Shaw and Kevin Birnbaum at JPL then led the effort under the LLOT project to get the detector array installed into this cryostat and then interfaced to the data acquisition system, which was originally selected to operate with the photodiode detector. Kevin came up with a novel interface using only off-the-shelf electronic modules in order to meet the tight project schedule and budget.”

By June, the LLOT project demonstrated error-free communications and successfully completed compatibility testing of the WSi-based LLOT receiver with the Lunar Lasercomm Space Terminal engineering unit.

“An amazing 2-month integration effort by Matt and Kevin and the rest of the LLOT team,” said Farr.

John Rush, director for the Technology and Standards Division of NASA’s Space Communications Office, visited the JPL ground station for a final check before the LLCD experiment started. Discussions included the list of challenges the team faced in getting ready on time. “The biggest challenge was the detectors where everyone agreed that the original detectors would not have worked. But the tungsten silicide detectors that STMD invested in saved the day,” Rush said.

“The new detectors now hold the world record for efficiency at 93 percent and for a mind-boggling 13 bits per photon,” Rush added. “This is an excellent example of how working together we can achieve things that we can’t achieve by ourselves.”

Denise M. Stefula
NASA’s Langley Research Center

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Future Of NASA’s Aging Deep Space Network Lies In X-Rays And Lasers


After a half century of using radio to track and communicate with everything from the first lunar Rangers to the Voyager probes now crossing into interstellar space, NASA is moving its $2 billion Deep Space Network (DSN) firmly into the optical and x-ray spectrums.

Next year, NASA is to launch a demonstration mission to test optical laser communications in conjunction with the LADEE (Lunar Atmosphere and Dust Environment Explorer) mission to the moon. And an optical mission to test laser relay capabilities from earth geosynchronous (GEO) orbit will soon follow.

“The DSN is operating almost flawlessly, doing everything we ask,” said Leslie Deutsch, chief technologist of NASA Jet Propulsion Lab’s Interplanetary Network Directorate. “There have been no instances of the DSN causing a space mission to be lost, but there have been several instances of DSN being used to save missions.”

Using three ground complexes at Goldstone, California; Canberra, Australia and Madrid, Spain, DSN is tracking some 35 spacecraft with a success rate of better than 98 percent.

But from time to time NASA does use other radio telescopes. Deutsch notes that when the Mars Science Lab recently landed, as a backup capability the DSN used the Parkes Radio Observatory in Australia to look at its signal during entry, descent and landing.

“We do have bottlenecks where instruments at Mars could bring back more data if we had a larger communications pipeline,” said Deutsch.

Wherever there’s a lot of exploration activity, Deutsch says, it also may make sense to create a GPS-like capability to help surface navigation. Deutsch notes that a Mars GPS capability is still being studied and is a possibility within a couple of decades.

Meanwhile, NASA is proceeding with laser communications tests. The LLCD (Lunar Laser Communications Demonstration) launches on LADEE in January of next year and will demonstrate a laser downlink rate of 622 megabytes from the moon.

The Laser Communications Relay Demonstration Project (LCRD) will follow with launch in late 2017 on a commercial Space Systems Loral spacecraft. From GEO, LCRD will enable two years of continuous high data rate optical communications tests.

LCRD will use half watt lasers; about the power of a current DVD burner. But pushing that figure up to a mere 5 watts would allow LCRD technology to have downlink speeds of 1 gigabyte per second and uplink speeds of 100 megabytes per second out to near earth distances. That’s some 10 to 100 times faster than current DSN radio frequency rates.

“We should have a GEO relay with optical capability by 2022,” said says David Israel, the space communications manager at NASA Goddard Space Flight Center.

Although Israel says that NASA will use an “eye-safe” wavelength and ensure that their lasers never cross paths of an aircraft or satellite, he notes that optical communications’ biggest technical challenge are mere clouds.

So, when looking to locate ground-based optical receivers, why not just go to areas already proven to provide clear skies?

“Great viewing on top of some isolated mountain is perfect for astronomy,” said Israel. “But if you had a high data rate coming down to that location then there might not be an [efficient] way to get that data off the mountain.”

Thus, one challenge for ground-based optical communications telescopes, would be to strike a balance between optimal “seeing” and use of an existing data communications infrastructure needed to quickly ferry incoming data back to far-flung researchers.

NASA is also developing a natural astrophysical x-ray source as a jumping off point for a space-based navigation system that would function as a solar system-wide GPS. The idea is to use pulsars, rapidly spinning neutron stars that often emit x-rays on millisecond timescales to precisely determine a spacecraft’s course and position.

An XNAV system, says Keith Gendreau, an astrophysicist at NASA Goddard Spaceflight Center, would need an x-ray detector with a pointing capability in order to observe several pulsars over time.

“Pulsars produce regular pulses that rival atomic clocks on timescales of months to years,” said Gendreau. “In the GPS constellation, there are a number of atomic clocks that broadcast time. GPS receivers receive these transmissions from multiple satellites, which then work out your position. For XNAV, our clocks will be pulsars distributed on a galactic scale; enabling GPS-like navigation throughout the solar system and beyond.”

To date, outer planet navigation has used the DSN and onboard stellar background spacecraft sensors to get precise ranges. But Deutsch says XNAV could make the job of autonomous spacecraft navigation even more accurate.

XNAV would build up 3-dimensional positional data from pulsars located at different directions on the sky, says Gendreau, who notes that in addition to three pulsars that the spacecraft would use to determine its position; a fourth pulsar would provide independent time measurements.

The Neutron Star Interior Composition Explorer (NICER) is a proposed NASA pulsar timing experiment that could demonstrate XNAV by late 2016.

“By the time there are space miners heading to the asteroid belt, it’s safe to say they would be using XNAV,” said Israel.

Meanwhile, researchers at NASA Goddard are also working on x-ray communication (XCOM) using a photo-electrically driven source modulated for communication. The advantage of x-rays over laser communications is that x-ray wavelengths are even shorter and can penetrate areas blocked in the radio and optical frequencies.

Gendreau says one major advantage of X-rays over lasers is that the short wavelength allows for very tight beams and thus much less wasted energy in long distance communication.

“Very high energy x-rays could [also] penetrate the plasma shroud surrounding a re-entering capsule and provide a low data rate link to such a hypersonic vehicle,” said Gendreau. “If NICER flies, then by 2018, we could also use it as the receiver for a first XCOM space demonstration.”

What’s the Deep Space Network’s ultimate future?

Deutsch says orders of magnitude higher data rates than today; continuous DSN coverage for humans at remote locations such as the far side of the moon; and an internet-like capability extending wherever NASA sends astronauts or machines.

As for radio?

“I don’t think space radio will ever completely go away,” said Deutsch. “It’s very simple and easy.”

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