Farahmand, Mitra

Getchius, Joel

Hassouneh, Munther A.

Long, Anne

Mitchell, Jason W.

Winternitz, Luke B.

NASA Technical Reports Server

Predicted Performance of an X-ray 
Navigation System For Future Deep Space 
and Lunar Missions 
Joel Getchius#, Anne Long‡, Mitra Farahmand‡,
Luke B. Winternitz†, Munther A. Hassouneh†, Jason W. Mitchell†
† NASA Goddard Space Flight Center
‡a.i. solutions, Inc. 
#Omitron, Inc.
American Astronautical Society
42nd Annual Guidance and Control 
Conference
Beaver Run Resort, Breckenridge, CO
February 4, 2019
https://ntrs.nasa.gov/search.jsp?R=20190001154 2019-08-30T10:23:44+00:00Z
2X-ray Pulsar Navigation (XNAV)
• Millisecond pulsars (MSPs): rapidly rotating 
neutron stars that pulsate across 
electromagnetic spectrum
• Some MSPs rival atomic clock stability at 
long time-scales
– Predict pulse arrival phase with great 
accuracy at any reference point in the Solar 
System via pulsar timing model on a 
spacecraft
– Compare observed phase to prediction for 
navigation information
• Why X-rays?
– Many stable MSPs conveniently detectable 
in (soft) X-ray band 
– X-rays immune to interstellar dispersion 
thought to limit radio pulsar timing models
– Highly directional compact detectors possible
• Main Challenge: MSPs are very faint!
Crab Pulsar (1/3 speed), Cambridge
University, Lucky Image Group
3Pioneer plaque (Pioneer 10,11 1972-73) 
with pulsar periods and relative 
distances to our Sun
X-ray Pulsar Navigation (XNAV)
Applications
• XNAV can provide autonomous navigation and timing that is of 
uniform quality throughout the solar system
– Is enabling technology for very deep space missions
– Provides backup autonomous navigation for crewed 
missions
– Augments Deep Space Network (DSN) or op-nav
techniques
– Allows autonomous navigation while occulted, e.g., 
behind Sun
History
• Pulsars were discovered in 1967 and immediately recognized 
as a potential tool for Galactic navigation
• US Naval Research Laboratory (NRL) (1999-2000)
– Unconventional Stellar Aspect (USA) Experiment
• DARPA XNAV, XTIM Projects (2005-2006, 2009-2012) 
• Significant body of research (international interest, academic 
research, several Ph.D. dissertations, etc.)
• NICER/SEXTANT successfully demonstrates real-time, 
onboard, autonomous XNAV (Nov 2017)
4• Launched on June 3, 2017 on 
Space-X CRS-11 to ISS 
• Neutron-star Interior Composition 
Explorer (NICER) 
– Fundamental investigation of ultra-
dense matter: structure, dynamics, 
& energetics
– Nearly ideal XNAV detector 
combination: low-background, large 
effective collecting area, precise 
timing, scalability, and low-cost
– Assembly of 56 X-ray concentrators 
and detectors, ~1800 cm2 effective 
collecting area in soft X-ray band
– Scalable design, e.g., reduce to 
1,4,10, etc. concentrators
• SEXTANT – Successful 
demonstration results reported in 
Mitchell (2018) and Winternitz 
(2018)
NICER/SEXTANT Overview
5• NICER/SEXTANT focused primarily on LEO/ISS orbit and required ground 
support systems
• NICER/SEXTANT XNAV Flight Software (XFSW) consists of two main 
components
– Event/measurement processing
– Goddard Enhanced Onboard Navigation System (GEONS) navigation filter (EKF)
• GEONS Ground MATLAB Simulation (GGMS)
– General tool for running GEONS simulations from convenient MATLAB wrapper
– Includes NICER/SEXTANT flight software XNAV measurement models 
• This work examines performance of XNAV vs. 2-way ground tracking from 
Deep Space Network (DSN) for 3 scenarios beyond LEO
– Measurements are simulated and processed by GEONS/GGMS
– Focus on top 5 XNAV pulsar configurations
that provides good geometry 
– Assume perfect clock 
– Conduct single run(s), not Monte
Carlo
Simulation Setup
External 
Input Data
Truth Trajectory
Truth Attitude
Maneuver 
Accelerations
Earth 
Orientation 
Parameters
MATLAB Simulation 
Script
Simulation Driver
Measurement Simulation
Data Analysis Functions
GEONS MATLAB API
I/O Functions for Data and 
Commanding
GEONS header files
GEONS 
Shared 
Dynamic 
Library
GEONS 
Functionality 
Compiled From 
C Code
6Notes:
• Two classes of operations: crewed 
vs. un-crewed
• Un-crewed operations are quiescent 
and similar to a robotic spacecraft
• Crewed operations involve 
significant increase in perturbations 
due to more out-gassing (waste, 
CO2, etc.)
Gateway Simulation
Candidate orbit for NASA’s 
proposed Gateway is a Near-
Rectilinear Halo Orbit (NRHO)
NRHO:
• 1800 km x 68,000 km 
• Period of 6.5 days
Ground navigation:
• 2-way range and Doppler 
alternating from Goldstone, 
Madrid, and Canberra
• Limit to 8 hrs of tracking per day
• Use DSN level of accuracy
Simulation details:
• Run for 45+ days
• Trade number of XNAV 
concentrators (56, 10, 4, and 1)
7Gateway Results (Uncrewed)
• Performance promising for backup applications
• Large integration times to formulate measurements (> 13 min)
• Velocity spikes at periapsis due to combination of rapidly changing 
dynamics and large integration times
Steady State Statistics
RMS 
Position 
Error (km)
RMS 
Velocity Error 
(m/s)
DSN 0.157 0.0035
XNAV 56 
Concentrators 3.5 0.1331
XNAV 10 
Concentrators 5.3 0.1631
XNAV 4 
Concentrators 9.1 0.4101
XNAV 1 
Concentrators 9.2 0.5814
10 Concentrators
8Gateway Results (Crewed)
• Performance degraded as compared to un-crewed
• Large velocity spikes at periapsis still present
• At XNAV level of performance additional disturbances have only 
minor effect 
Steady State Statistics
RMS 
Position 
Error (km)
RMS 
Velocity Error 
(m/s)
DSN 2.73 0.052
XNAV 56 
Concentrators 6.32 0.177
XNAV 10 
Concentrators 7.89 0.275
XNAV 4 
Concentrators 11.91 0.465
XNAV 1 
Concentrators 16.45 0.977
10 Concentrators
9Notes:
• Demanding bandwidth 
requirements limit the amount of 
available ranging in favor of 
download of scientific data
• Station keeping maneuvers 
required every 4 weeks
• Momentum unloads required 
weekly
WFIRST Simulation
Proposed mission in halo orbit 
about Sun-Earth L2 common 
for telescope missions
Sun-Earth L2:
• 1.6 million km y-axis in Rotating 
Libration Point (RLP) frame
• Period of 6 months
Ground navigation:
• 2-way range and Doppler from 
White Sands and Canberra
• 1 hr of range per station contact
• Use DSN level of accuracy
Simulation details:
• Run for 1 year
• Trade number of XNAV 
concentrators (56,10, 4, and 1)
10
WFIRST Results
• No velocity spikes as dynamics through perigee are more benign 
than for Gateway
• Possible semi-annual variation likely due to pulsar geometry 
changes relative to orbit
• The 56 or 10 concentrator configuration exhibits performance 
acceptable for primary navigation
Steady State Statistics
RMS 
Position 
Error (km)
RMS 
Velocity Error 
(m/s)
DSN 1.5 0.0005
XNAV 56 
Concentrators 1.7 0.0016
XNAV 10 
Concentrators 3.4 0.0024
XNAV 4 
Concentrators 4.5 0.0034
XNAV 1 
Concentrators 7.2 0.0046
10 Concentrators
11
Notes:
• Although New Horizon’s 
navigation plan includes 
combination of 3-way, 2-way, 
ΔDOR, and optical we only use 2-
way
• Overlapping 2-way is equivalent to 
3-way but NOT ΔDOR and optical
New Horizons Simulation
Robotic probe on a Solar 
System escape trajectory
Escape Trajectory:
• Interested in swath near Saturn 
orbit crossing
• Spacecraft in hibernation mode
Ground navigation:
• 2-way range and Doppler from 
Goldstone, Madrid, and Canberra
• Use all available contacts
• Use as reported transponder 
accuracies
Simulation details:
• Run for 30 days
• Trade number of XNAV 
concentrators (56,10, 4, and 1)
12
New Horizons Results
• Lack of ΔDOR skews the reported DSN results
• XNAV exhibits excellent performance for this profile
• The linear trajectory is insensitive to long integration times to 
generate measurements
Steady State Statistics
RMS 
Position 
Error (km)
RMS 
Velocity Error 
(m/s)
DSN 66.76 0.0508
XNAV 56 
Concentrators 2.67 0.0038
XNAV 10 
Concentrators 6.63 0.0090
XNAV 4 
Concentrators 5.72 0.0111
XNAV 1 
Concentrators 18.98 0.0125
10 Concentrators
13
• Demonstrated the potential performance of XNAV for three 
mission profiles
– Gateway: suitable for backup navigation capability
• Matures support for Deep Space Transport backup navigation
– WFIRST: potentially suitable for primary navigation capability in 
Sun-Earth L2 
– New Horizons: potentially suitable for primary navigation capability 
in deep space
• Illustrated sensitivities in XNAV performance
– Geometric dependence vs. integration time
– Number of concentrators traded vs. performance
• Future work includes:
– Further refinement of simulation models based on NICER/SEXTANT 
results
– Inclusion of limitations such as solar / planetary occultations
– Analysis of XNAV performance against other navigation techniques 
such as ΔDOR
– Monte Carlo or linear covariance analysis to produce statistically 
robust performance predictions
Conclusions & Future Work


Predicted Performance of an X-Ray Navigation System for Future Deep Space and Lunar Missions

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