Signal Processing - May 2017 - 12

The system's precise measurement caThe new Space Optical Communication
pability is tied directly to high-precision
and Navigation System is a miniaturized
frequency synthesizing, low-noise implelasercom transceiver comprising commermentation of frequency and timing deteccially available components that simution. "The final instrumentation will be
late both ground and space terminals. In
carried out in a digital domain with lots of
recent laboratory tests, the system showed
digital signal processing," Yang says. "The
that it could provide micrometer-level
types of digital signal processing we used
distance and speed measurements over
include digital frequency synthesizing,
a 622 Mb/s laser communication link.
digital-phase detecting, digital-phase lock
"This technology decreases ranging and
loops, digital filters, and digital-time interrange-rate errors by orders of magnitude,"
val counters for ranging measurements," he
says Guangning Yang, an optical physicist
explains. The measurements are processed
at NASA's Goddard Space Flight Center
in a software tool that
in Greenbelt, Maryrelies on an extended
land. "We can more
Yang says that the biggest Kalman filter to proprecisely determine
challenge faced so far
duce high-precision
a spacecraft's orbit
has been conducting
orbit-state estimates.
relative to an absoYang says that the
lute location." Besides
high-precision frequency
biggest
challenge faced
transmitting data at
measurements of
so
far
has
been conductLLCD's record-breaka Doppler-shifted
ing high-precision freing rate of 622 Mb/s,
clock signal within
quency measurements
the new transceiver
a digital domain.
of a Doppler-shifted
measured speed within
clock signal within a
a precision of less than
digital domain. "The obvious advantage
10 nm/s and distances within 20 n m.
with digital implementation is size and flexThe system achieved the precise meaibility," Yang notes. "A single field-prosurements by incorporating a Doppler
grammable gate array can accomplish
frequency enabled by an FFT.
much more in a small form factor."
NASA's current space communicaYang says that the project is moving
tion and navigation systems, such as
ahead in two directions. "On one hand, we
the tracking data and relay satellite
will try to implement the current version
(TDRS), provide two major services-
of this technology on an existing hardcommunication and satellite tracking.
ware platform, such as Goddard's NavTracking keeps tabs on a satellite's locaCube (a spacecraft bus that is typically no
tion, speed, and orbit. "We do this by
larger than a shoebox), which is a powerful
continuously measuring the spacecraft's
navigational technology," he remarks. "At
distance and speed relative to a fixed
the same time, we are pushing to further
reference point with an RF communiadvance the technology into continuous
cation satellite, like TDRS," Yang says.
optical-phase measurement."
"With these measurements, we can calculate the spacecraft's speed, distance,
and orbit." Space optical communicaAtomic gyroscope
tions promise to provide similar servicU.S. National Institute of Science and
es, but with higher data bandwidth and
Technology researchers have develenhanced tracking precision.
oped a compact, low-power atomic gyro"With significantly improved satellite
scope design that promises to give precise
location and speed information, we will
navigational capabilities to spacecraft,
enable better scientific-data collection
submarines, and other vehicles hampered
and processing," Yang says. "This highby size, weight, and power restrictions.
precision instrumentation, which is low
The gyroscope can also simultaneously
mass, consumes less power, is relatively
measure acceleration, enabling navigation
small, and will enable many other scienby "dead reckoning" without reference to
tific instruments that require high-preciexternal landmarks or stars.
sion ranging, such as flying information
The gyroscope, an atom interferwith a constellation of satellites."
ometer, is based on an expanding cloud
12

IEEE Signal Processing Magazine

May 2017

of laser-cooled atoms, an approach
first demonstrated at Stanford University. Traditional optical interferometry
involves combining or "interfering" the
electromagnetic waves in light and then
extracting information about the original
light paths from the resulting wave patterns. An atom interferometer leverages
the ability of atoms to act as both particles and waves, interfering these waves
to measure the forces exerted on atoms.
When atoms speed up or rotate, their
matter waves shift and interfere in predictable ways that are visible in interference patterns.
The basic concept behind the new
gyroscope is similar to the principle
underlying optical ring-laser gyroscopes,
says Gregory Hoth, a postdoctoral research associate in NIST's Time and
Frequency Division. "We take a wave
and split it into two parts, he says. "We
arrange for the two wave packets to travel
along different paths and then recombine
them and look at the amplitude of the
wave that comes out." If the separated
paths enclose an area, the output wave
amplitude will depend on whether or
not the device is rotating. "This is often
called the Sagnac effect," Hoth says. For
optical gyroscopes, the waves are light
waves, and the amplitude corresponds to
the intensity of the light. "For our gyroscope, the waves are matter-waves and
the amplitude corresponds to the probability for an atom to occupy a specific
energy state," Hoth says.
At the gyroscope's heart is a small
glass chamber containing a sample of
about 8 million cold rubidium atoms that
are continuously trapped and released.
As the atoms fall under gravity, a laser
beam causes them to transition between
two energy states. The process gives the
atoms momentum and forces their matter waves to separate and later recombine to interfere. The cold atom cloud
expands to as much as five times its initial size during the 50 ms (thousandths
of a second) measurement sequence,
which creates a correlation between each
atom's speed and its final position. The
interference effect on an atom depends
on its speed, so rotations generate interfering bands of atoms across images of
the final cloud (Figure 3).

Table of Contents for the Digital Edition of Signal Processing - May 2017