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Scientists and engineers at The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., 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," those that radiate in the X-ray portion of a spectrum, explains Daniel G. Jablonski, a navigation systems and technologies expert in the Lab's Space Department, and the project leader for this effort. "In a manner somewhat similar to the Global Positioning System (GPS) 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."

Their research is directed by the Defense Advanced Research Projects Agency (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 Institutes of Standards and Technology (NIST) on the project, known as the X-ray Source-based Navigation for Autonomous Position Determination (XNAV) program.

"With XNAV, we hope to demonstrate that we can determine time, position and attitude of an object in deep space using X-ray sources," notes DARPA's Darryll Pines, 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 Power the Navigation

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, rapidly rotating neutron stars whose axes of rotation are not aligned with their magnetic poles. This causes their signal intensity to vary as the pulsar rotates, 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 the TRANSIT navigation system, 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 explains. "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, says "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 non-traditional 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 XNAV, 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.

More than a Star-Tracker

At the 24th DARPA Systems and Technology Symposium 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."