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.”