Lasercom System Also Measures Distance, Speed
GREENBELT, Md., Oct. 30, 2015 — Lasers used in space communications could also be used to make precise distance and speed measurements, benefiting spacecraft navigation.
Called the Space Optical Communication and Navigation System (OCNS), the breadboard technology is made up of commercially available components simulating both ground and space terminals. It recently demonstrated in laboratory testing that it could provide micron-level distance and speed measurements over a 622-Mb/s laser communications link.
"Combined with the large communication bandwidth, high-precision ranging over an optical communication network will bring about significant advances in navigation and communications, to say nothing of science gathering, notably in the area of geodesy," said technology developer Guan Yang, an optical physicist at NASA's Goddard Space Flight Center.
Guan Yang, front, and research associate Wei Lu in front of the laser communications breadboard they created to demonstrate high data-rate download and uplink speeds, as well as precise distance and speed measurements. Courtesy of NASA/W. Hrybyk.
Geodesy is the science of measuring variations in Earth's gravitational field caused by changing land mass.
Because of its diminutive size, the OCNS could be used on CubeSats, an increasingly popular spacecraft that typically are no larger than a shoebox.
The ground-based test was similar to one carried out in late 2013 using NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE).
The Lunar Laser Communication Demonstration (LLCD) experiment downloaded and uploaded test data to and from lunar orbit at 622 Mb/s and 20 Mb/s, respectively, proving that it could operate as well as any NASA radio system. Mission operators also used LLCD's lasercom system to download LADEE's stored science and spacecraft data. It took just four minutes to download the data, a feat that would have taken several days if using only LADEE's onboard radio system at 50 kb/s.
But before the LLCD mission could break the data-transmission records, it needed to know the speed of the LADEE spacecraft as it orbited the moon, as well as its distance from the LLCD ground terminals. This required the high-frequency laser beam also to obtain precise distance and speed measurements.
It succeeded. LLCD's instrument gathered velocity measurements with a precision better than 10 mm/s; its position calculations were precise to within 12 mm, the best measurements ever recorded over lunar distances.
Yang's miniaturized lasercom transceiver improved upon LLCD's precision by several orders of magnitude in laboratory testing. It measured speed within a precision of less than 10 μm/s and distances within 20 μm.
The system achieved these unprecedented measurements by incorporating a Doppler frequency enabled by fast Fourier transform.
"When you're trying to predict where something is, one of the issues is eliminating measurement errors," said Dennis Woodfork, Goddard's assistant chief for technology specializing in navigation and communication technologies. "If errors build too much, you will lose position, and therefore, you won't know where the spacecraft will be in the future. Guan's measurements are at least an order-of-magnitude better. He got great ranging."
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