At the beginning of this month, the IEEE/ION Position, Location and Navigation Symposium (PLANS) 2008 conference in Monterey, California featured two interesting concepts for the use of these highly accurate X-ray sources. The first proposal called "Noise Analysis for X-ray Navigation Systems" headed by John Hanson of CrossTrac Engineering, introduces a scaled-up version of terrestrial GPS, using pulsars rather than man-made satellites. The system is called X-ray navigation, or "XNAV" for short. Primarily focusing on space missions beyond Jupiter, XNAV would use the Solar System as the base co-ordinate and then measure the phase of the incoming X-ray emission from the mapped pulsars. As the X-ray pulses are so accurate, onboard systems could measure and compare the signal from multiple pulsar sources and automatically deduce the position of the spacecraft to a high degree of certainty. I suppose it would be an advanced 3D version of the traditional sextant as used by ships to measure the elevation of stars above the Earth's horizon.
The second concept entitled "Online Time Delay Estimation of Pulsar Signals for Relative Navigation using Adaptive Filters", is headed by Amir Emadzadeh at the UCLA Electrical Engineering Department. Emadzadeh suggests that the location of two spacecraft can be worked out if both ships are looking at the same, known pulsar. The periodic emission measured by both ships will have a differential time delay proportional to the distance between the ships. In addition, the UCLA group suggest a method to derive their relative inertial position by observing a distribution of X-ray sources throughout the cosmos.
These are very interesting concepts, but until we begin routinely venturing beyond the orbit of Jupiter I doubt we'll see these ideas come to fruition any time soon…
