Scientists and engineers at the
Applied Physics
Laboratory will be applying their expertise from more
than 40 years of space research to help determine whether
X-ray signals from celestial sources, such as stars and
pulsars, can be used for satellite navigation in deep
space.
Pulsars, a type of extremely dense collapsed star, may
provide a means to determine a precise location in space
and time and could be used as navigation beacons for
satellites and deep space probes.
"We've begun working on algorithms and electronics
that spacecraft equipped with X-ray sensors will be able to
use to track timing signals from X-ray pulsars," said
project leader Daniel G. Jablonski, referring to those that
radiate in the X-ray portion of a spectrum. "In a manner
somewhat similar to the Global Positioning System, the
ability to track the highly stable signals from pulsars
will allow spacecraft to navigate alone, anywhere in the
universe, without depending on Earth-based signals or
systems," said Jablonski, a navigation systems and
technologies expert in the Lab's Space Department.
The team's research is directed by the Defense
Advanced Research Projects Agency, or DARPA, which recently
awarded the Lab a contract to design critical portions of a
spacecraft X-ray navigation system. APL will be working
with Ball Aerospace, the Los Alamos National Laboratory and
the National Institute of Standards and Technology on the
project, known as the X-ray Source-based Navigation for
Autonomous Position Determination program, known as
XNAV.
"With XNAV, we hope to demonstrate that we can
determine time, position and attitude of an object in deep
space using X-ray sources," said Darryll Pines of DARPA,
XNAV's program manager. "This will allow us to 'see'
spacecraft at much farther distances from Earth than we can
today."
XNAV would complement existing low-Earth-orbit
navigation systems, like GPS. However, this technology
would enable satellites to use X-rays from pulsars and
other celestial sources to navigate in deep space, where
Earth-based signals cannot be seen.
Pulsars are neutron stars that emit brief repetitive
pulses of energy instead of the steady-state radiation
associated with other natural sources. For the purposes of
navigation, APL engineers are focusing on
"rotation-powered" pulsars, which are rapidly rotating
neutron stars whose axes of rotation are not aligned with
their magnetic poles. This causes the pulsars' signal
intensity to vary as they rotate, thus providing highly
stable, time-dependent signals that can be used for precise
timing and navigation.
Several universities, particularly the University of
Maryland, have laid the technical foundation for successful
spacecraft navigation using signals from pulsars and other
celestial X-ray sources. APL engineers are extending this
preliminary work, using the navigation techniques developed
by the Lab as far back as 1959, when it unveiled TRANSIT,
the first navigation by satellite system and the precursor
to GPS and other space-based navigation systems. "X-ray
navigation is noteworthy because the timing signals to be
used are neither man-made nor limited to near-Earth use,"
Jablonski said. "X-ray signals from pulsars can be observed
from anywhere outside the Earth's atmosphere."
APL's John Goldsten, who is spearheading the design of
some of the electronics that would go on board the
satellites, said, "The beauty of the pulsar signals lies in
their remarkable stability, which can rival our best atomic
clocks. However, the signals are very weak and will require
sensitive detectors and electronics that can distinguish
these signals from other interfering sources of background
radiation."
Using X-ray pulsar timing data obtained from the
Chandra X-ray Observatory and other space-borne X-ray
telescopes, APL is evaluating various traditional and
nontraditional tracking loop technologies, including novel
variations on the traditional Kalman Filter, a numerical
method used to track a time-varying signal in the presence
of noise.
In addition to their other roles on the XNAV project,
NIST and Los Alamos will provide critical support for
processing the raw X-ray data prior to its insertion in the
APL-developed algorithms and tracking loops. Ball Aerospace
will perform the system integration tasks needed to fly an
X-ray experiment aboard a spacecraft or the International
Space Station.
Once the XNAV team can demonstrate the ability to
recover and track X-ray timing signals, APL engineers will
develop the navigation algorithms necessary to convert the
timing data from pulsars into a three-dimensional,
real-time navigation fix referenced to the center of our
solar system.
At the 24th DARPA Systems and Technology Symposium,
held this summer, Steven H. Walker, the program manager for
DARPA's Space Activities Tactical Technology Office, said
that XNAV may present some of the agency's most difficult
challenges.
"We need your help developing supersensitive X-ray
detectors, navigational algorithms to infer time and
position, timing models for pulsars, the supernova stars
that emit electromagnetic energy and new methods to fix the
precise inertial position of those pulsars," Walker said.
However, if successful, "XNAV would take us beyond the
star-tracker cameras and sensors now in use and free a
satellite completely from the need for navigational
assistance on Earth."
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