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Pulsar-based navigation & solar-system timekeeping

NICER's SEXTANT software payload enhancement demonstrated that X-ray pulsars can act as GPS-like beacons for spacecraft and as a basis for precision timekeeping far beyond Earth's orbit.

Why it matters

  • Autonomous navigation with reduced dependence on Earth-based measurements will be necessary for far-side lunar, Mars, and deep-space missions.
  • Reduces demand on ground-based (e.g., DSN) range, Doppler, and differential ranging support.
  • Resilient timing, positioning, and clock synchronization when Earth contact is intermittent or unavailable.
  • A practical pathway from astrophysics measurements to new operational flight capabilities.

SEXTANT demonstration

The ISS-hosted NICER X-ray telescope carried software for photon-based signal acquisition and state estimation. The performance demostration for this software surpassed its objectives for autonomous real-time orbit determination in the dynamic ISS environment using observations of four pulsars.

Future concepts include short-burn trajectory verification (e.g., ISS reboosts), relative navigation, and communication links.

XNAV utilizes pulsar signals to autonomously determine spacecraft position SEXTANT on-board performance during on-orbit demonstration determined ISS position to within 10 km
Left: XNAV utilizes pulsar signals to autonomously determine spacecraft position.
Right: SEXTANT on-board performance during on-orbit demonstration determined ISS position to within 10 km

Practical-sensor simulations show that X-ray navigation results in significantly improved on-board positioning with no need for frequent deep-space network (DSN) contact. A simulation utilizing a single-sensor detector, 1/56th of the full NICER collecting area, for the cruise phase of a future Cassini-like mission shows significant improvements over DSN-focused positioning.

Cassini cruise practical sensor simulation
End-to-end practical-sensor simulation for Cassini cruise: realistic XNAV performance compared with an optimistic DSN schedule.

Galactic Time - A Pulsar Time Standard

Long-term pulsar stability rivals laboratory atomic clocks. And pulsars are a proven time reference, enabling locally generated time that is synchronous with other sites viewing the same pulsars. In addition, X-rays are not affected by interstellar signal delays, thus minimizing instrument resource needs relative to radio-band measurement systems.

Pulsar Time concept using an ensemble of pulsars in common view of Earth and Moon
Galactic Time concept using an ensemble of pulsars in common view of Earth and Moon.

This makes pulsar timing an excellent candidate for a solar-system-wide time standard, responsive to lunar timing and interoperability needs.

Long-baseline NICER timing of six selected X-ray millisecond pulsars has pushed the boundaries of typical atomic clocks, with no evidence of low-frequency noise after 8 years of timing data.

Plot of eight years of NICER timing data showing no low-frequency noise
Plot of eight years of NICER timing data showing no low-frequency noise in PSR B1937+21, a millisecond-period X-ray pulsar suitable for X-ray navigation and timekeeping.

The use of pulsar timing as a standard is responsive to HR 2313, "Celestial Time Standardization Act," to establish time at the Moon, and beyond, that is traceable to UTC and resilient to loss of Earth contact.