Background

The Neutron star Interior Composition Explorer (NICER) has produced a rich archive of precision X-ray timing observations since its installation on the International Space Station in 2017. In addition to its fundamental astrophysics objectives, NICER demonstrated for the first time autonomous X-ray pulsar-based navigation in space through the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) program.

The archival NICER dataset represents a unique opportunity to investigate new concepts in applied astrophysics, particularly in areas relevant to NASA's Moon-to-Mars initiative and future deep-space exploration.

The NICER Guest Investigator Cycle 9 call for proposals encourages investigations that leverage archival NICER observations to advance technologies involving autonomous navigation, precision timing, spacecraft timekeeping, and future X-ray instrumentation.

Motivation

The SEXTANT demonstration established the feasibility of spacecraft navigation using stable, X-ray-bright millisecond pulsars. Since that demonstration, significant advances have occurred in pulse timing techniques, orbit-propagation estimation, machine learning, and other computational approaches. These advances may motivate re-analysis of NICER data to determine what additional performance could be achieved using modern techniques and to inform the definition of requirements for future operational systems.

Areas of Interest

Improved Pulsar Navigation Algorithms

Investigations to develop and evaluate state-of-the-art navigation algorithms using archival NICER photon event data could include:

• Bayesian estimators
• Machine-learning-assisted estimators
• Background mitigation and robust estimators during poor observing conditions
• Joint navigation and clock estimation.

Relevant questions include: How much better can navigation accuracy become? Can acquisition times be shortened? How much computation is required?

Reduced Instrument Concepts

NICER data are acquired with 52 X-ray concentrator optics and their associated detectors. Future spacecraft may only accommodate a much smaller instrument. Investigations could assess the impact on navigation accuracy and/or converge time of:

• Subsets of NICER detectors for various optimization criteria
• Minimum effective collecting area
• Detector redundancy
• Graceful degradation following detector failures or in changing observing conditions.

Timing Standards and Clock Requirements

Determine timing requirements for operational X-ray navigation systems. Questions include:

• Required clock stability
• Onboard atomic clock integration
• Autonomous clock calibration using pulsars.

Navigation Using Alternative X-ray Sources

SEXTANT made use of highly stable millisecond pulsars, with observations that spanned a few days and an onboard library of pulsar spin ephemerides. The NICER archive enables investigation of alternative navigation beacons, such as young or accreting pulsars, or other variable X-ray sources. Such substitute celestial references may be especially interesting in particular scenarios — e.g., if navigation information is needed only during finite operational intervals — or when pulsar spin ephemerides are unreliable or unavailable.

Potential questions that may be addressed include: How much ephemeris accuracy is required? Can the Crab outperform MSPs over several hours?

NICER observations obtained before, during, and after ISS reboost maneuvers may provide useful test cases for evaluating algorithm robustness under realistic operational disturbances.

Candidate pulsars observed by NICER:

Last modified: Wednesday, 15-Jul-2026 14:13:18 EDT