ISS Utilization: NICER/SEXTANT
ISS Utilization: NICER/SEXTANT (Neutron-star Interior Composition ExploreR / Station Explorer for X-ray Timing and Navigation Technology)
NICER is an ISS instrument of NASA, devoted to the study of neutron stars through soft X-ray timing. Neutron stars are unique environments in which all four fundamental forces of nature are simultaneously important. They squeeze more than 1.4 solar masses into a city-size volume, giving rise to the highest stable densities known anywhere. The nature of matter under these conditions is a decades-old unsolved problem, one most directly addressed with measurements of the masses and, especially, radii of neutron stars to high precision (i.e., better than 10% uncertainty). With few such constraints forthcoming from observations, theory has advanced a host of models to describe the physics governing neutron star interiors; these models can be tested with astrophysical observations. 1)
NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft (0.2-12 keV) X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena, and the mechanisms that underlie the most powerful cosmic particle accelerators known. The NICER mission achieves these goals by deploying an X-ray timing and spectroscopy instrument on the ISS (International Space Station).
By answering a long-standing astrophysics question — How big is a neutron star? — NICER will confront nuclear physics theory with unique measurements, exploring the exotic states of matter within neutron stars through rotation-resolved X-ray spectroscopy. The capabilities that NICER brings to this investigation are unique: simultaneous fast timing and spectroscopy, with low background and high throughput. NICER will also provide continuity in X-ray-timing astrophysics more broadly, post-RXTE (Rossi X-ray Timing Explorer), through a Guest Observer program. — Finally, in addition to its science goals, NICER will enable the first space demonstration of pulsar-based navigation of spacecraft, through the SEXTANT (Station Explorer for X-ray Timing and Navigation Technology ) enhancement to the mission, funded by the NASA Space Technology Mission Directorate's Game-Changing Development program.
NICER/SEXTANT, which NASA's Science Mission Directorate selected in 2013 as its next Explorer Mission of Opportunity, is a one-of-a-kind investigation that not only will gather important scientific data, but also demonstrate advanced navigation technologies — all from a relatively low-cost instrument that takes advantage of an already-existing platform, the ISS. The ISS orbit, that ranges between ±51.6 º latitudes, will give the instrument a good view of the cosmos to accomplish both its scientific and technology objectives. 2) 3) 4)
Once the NICER/SEXTANT instrument deploys as an external attached payload on one of the ISS ECL (ExPRESS Logistics Carriers), its 56 X-ray optics and silicon detectors will observe and gather data about the interior composition of neutron stars and their pulsating cohort, pulsars. In pulsars, the magnetic poles are especially luminous, affording an opportunity to also demonstrate celestial-based XNAV ( X-ray Navigation), a capability that could enhance NASA's ability to pilot to the far reaches of the solar system and beyond.
Due to the pulsars rapid pulsations, with repetition periods that range from seconds to milliseconds, the powerful beams of radiation emanating from their magnetic poles sweep around much like a lighthouse, and are as seen as flashes of light at Earth. Because of their predictable pulsations, pulsars can provide high-precision timing just like atomic-clock signals, supplied through the GPS (Global Positioning System ), which weaken the farther one travels out beyond Earth and the GPS constellation.
Two-in-One Mission: To demonstrate XNAV, the payload will detect X-ray photons within the pulsars' sweeping beams of light to estimate the arrival times of the pulses. With these measurements, the system will use specially developed algorithms to stitch together an onboard navigational solution.
The NASA NICER/SEXTANT team with Keith Gendreau as PI, includes partners/collaborators from MIT (Massachusetts Institute of Technology), Moog Inc., headquartered in East Aurora, New York, NRL (Naval Research Laboratory), and universities across the U.S., Canada, and Mexico. The experiment/investigation may also demonstrate a second technology — X-ray communication — that, if advanced, potentially could allow space travelers in the future to transmit gigabytes of data per second over interplanetary distances.
At the heart of this potential demonstration is the MXS (Modulated X-ray Source), which the team developed to test and validate the XNAV concept. The technology generates rapid-fire X-ray pulses, turning on and off many times per second, encoding digital bits for transmitting data. If the team attracts additional support, perhaps from other government agencies interested in advancing X-ray communication, the team plans to develop an MXS-based transmitter that would fly on a future ISS supply spacecraft. As the craft approached the space station, MXS would transmit data via the modulated X-rays, which the NICER/SEXTANT hardware would then receive.
Background: Neutron stars squeeze more than 1.4 Solar masses into a city-size volume, giving rise to the highest stable densities and pressures known anywhere (Figure 1). The nature of matter under these conditions, in which all four fundamental forces of Nature are simultaneously important, is a decades-old unsolved problem, one most directly addressed with measurements of the masses and, especially, radii of neutron stars to high precision (i.e., to better than 10% uncertainty). Existing instrumentation does not provide the critical measurement capability — combined time and spectral resolutions in X-rays — needed to probe the physics of neutron star interiors. With order-of-magnitude advances in time-coherent sensitivity and timing resolution, NICER/SEXTANT will carry out rotation-resolved spectroscopy of rapidly rotating neutron stars, enabling lightcurve analyses with unique power to constrain models of neutron star structure, dynamics, and energetics. 5) 6)
Figure 1: NICER will infer the masses and radii of neutron stars to reveal the composition of matter in their interiors, at the highest stable densities allowed in nature (image credit: NASA, NICER Team)
Science of neutron star investigations:
Constraining the presence or absence of exotic phases within neutron star cores (the question mark in Figure 1) is key to understanding the basic physics of dense matter as well as the astrophysics of late stellar and binary evolution, which govern the relative abundances of neutron stars and black holes. Whatever the nature of matter beyond nuclear density, it exists in abundance: millions of neutron stars inhabit the Galaxy. Usually observed as pulsars, the number of known neutron stars has expanded dramatically in recent years, especially as the Fermi γ-ray space telescope has proven to be a remarkable engine of pulsar discovery.
A compact star's interior structure is captured, in a global sense, by the EOS (Equation of State), a distinct hypothesis of the nature of dense matter. The EOS relates density to pressure within a star; equivalently, for a given theory of gravity, it relates the star's mass M to its radius R. Most EOSs predict that R will shrink as M grows and the self-gravitational force increases, but different assumptions about interior composition produce differences in the detailed mass-radius relation. Thus, measurements of M and R probe dense matter.
Nuclear theory predicts distinct mass-radius relationships — the S-shaped curves in Figure 2 — for many proposed models of particle composition and interaction in cold, dense matter. NICER will confront these predictions with measurements, isolating those models that are consistent with observed neutron stars and ruling out large numbers of alternatives. Graphically, NICER's mass and radius measurements will define "allowed" regions in the M-R plane, as shown for PSR J0437–4715 in Figure 2. EOS curves that do not pass through every allowed region must be ruled out, while the remaining models are viable.
Figure 2: Mass and radius measurements probe the nature of dense matter by testing proposed equations of state (EOSs; representative labeled curves). Current weak radius bounds allow virtually all EOSs. NICER will improve radius measurements by an order of magnitude, isolating viable models of dense matter (image credit: NASA, NICER Team)
In pursuit of its key science objective, NICER exploits a well-founded and accessible approach to mass and radius constraints: analysis of X-ray flux modulations due to rotating hot spots on the surface of a neutron star. NICER enables such lightcurve analysis at a level that achieves the standard set by nuclear theory for distinguishing between proposed EOS models: ±5% uncertainty on radius measurements. To realize such precision, large numbers of photons, typically 105–6, must be collected.
Rotating hot spots are seen from both rotation- and accretion-powered pulsars. Lightcurve analysis to constrain neutron star properties has been demonstrated in both cases — for steady thermal X-ray pulsations from nearby MSPs (Millisecond Pulsars), and for transient accretion-powered pulsations and burst oscillations from low-mass X-ray binaries — but with broad statistical and, in the case of the accreting systems, systematic uncertainties. NICER delivers data suited to both investigations; its prime focus, however, is on the non-accreting MSPs, which are unaffected by the complexities of accretion flows. Rotation-powered MSPs are ideal for this approach: they appear frequently in binary systems, offering independent mass measurements, their radiative properties are well described by hydrogen atmosphere models, and they are always "on" with lightcurves stable in time, producing steady and predictable gains in SNR with increasing exposure.
Figure 3: Lightcurve modeling constrains the compactness (M/R) and viewing geometry of a non-accreting millisecond pulsar through the depth of modulation and harmonic content of emission from rotating hot-spots, thanks to gravitational light-bending (image credit: NASA, NICER Team)
Legend to Figure 3: Science measurements reveal the stellar structure through lightcurve modeling, long-term timing, and pulsation searches. A distant observer sees X-ray intensity grow and fall as hot-spots on a neutron star surface spin through the line of sight. The far-side spot becomes more visible for smaller stars through gravitational light-bending, which depends on M/R; thus, depth of modulation constrains compactness.
Figure 4: Right: Two sets of simulated NICER lightcurves, for stellar radii differing by ±5%, show measurable differences in several energy bands for a 1 Msec exposure: 4–6σ differences per phase bin pinpoint the star's radius (image credit: NASA, NICER Team)
Using XMM-Newton data for PSR J0437–4715, Bogdanov et al. (2007) established the viability of constraining the neutron star EOS through lightcurve analysis. Their model successfully reproduces both the pulsar's lightcurve and its phase-averaged spectrum. Their result (Figure 2), however, encompasses virtually all EOS families, including SQM (Strange Quark Matter), condensate (the region between the PAL6 and FSU curves), and soft and stiff nucleonic models (black curves). Because of limitations in XMM's onboard timekeeping, these radius constraints cannot be improved upon with additional XMM observations by more than a factor of two, and there is no possibility of attaining ±5% (3σ) uncertainty in R, which is NICER's goal and an improvement by a factor of more than 12. Simulations show that NICER achieves this goal for PSR J0437–4715 with an accumulated 1 Msec exposure.
To infer neutron star masses, NICER offers collecting area and photon time-tagging precision sufficient for measurement in X-rays (for the first time) of the general-relativistic Shapiro delay in binary systems. For orbits viewed edge-on, this retardation of a pulsar signal traversing the gravitational well of its companion is the most accessible of the well-known "post-Keplerian" timing effects traditionally accessible only to radio observations. Delays of tens of µs are measurable whenever timing precisions of a few µs are achieved on timescales small compared to the orbital period. For orbital periods longer than a few days, in systems containing either rotation- or accretion-powered MSPs, NICER's pulse TOA (Time-Of-Arrival) measurements will yield masses through the Shapiro delay.
Additional core NICER science investigations will include:
• Searches for coherent pulsations and QPOs (Quasi-Periodic Oscillations) in steady and transient systems. Discovery of a neutron star's rotation rate, its most consequential observable property, is important for eventual lightcurve analysis, characterization of orbits and component masses in binaries, and for understanding the origins of kHz QPOs, which probe the extreme neutron star environment. NICER also offers the prospect of a new capability to perform asteroseismology measurements of neutron stars.
• Definitive determination of the intrinsic clock stabilities of MSPs (Millisecond Pulsars), free of the interstellar propagation effects that plague radio timing observations.
• Cooling histories of young neutron stars. Existing temperature measurements suffer from poor statistics, blending together cooling-surface emission, heated polar caps, and nonthermal magnetospheric emission. NICER will disentangle the various components through phase-resolved spectroscopy, including constraining the origins of absorption features near 1 keV seen in some objects.
• Characterization of spin variations and outbursts associated with "glitches" in normal pulsars and magnetars.
• Determining radiation patterns, spectra, and relative phases of nonthermal emissions across wavelength bands.
Figure 5: NICER capabilities and measurement products are highly complementary to those of other X-ray, radio, and γ-ray facilities for neutron star studies. Interplay between them amplifies the scientific returns from all (image credit: NASA, NICER Team)
The NICER Guest Investigator/Observer program:
The NICER mission includes a proposed GI/O (Guest Investigator/Observer) program. Modeled after the Swift mission's GO program, the first NICER AO (Announcement of Opportunity) will solicit GI proposals for concurrent observations of NICER's neutron star targets in the radio, optical, and γ-ray bands (Figure 6), and also to support theoretical work relevant to NICER investigations. The first GI period will overlap with the first year of the mission, during which the NICER observing program will be dedicated to fulfilling its core neutron-star science investigation. To facilitate planning and coordination of complementary observing efforts, NICER target schedules will be published well in advance. GI/O proposals received within this and subsequent AOs will be selected via a peer-review process.
The second AO will solicit GO proposals, for observing time using NICER to target objects not part of the core NICER science agenda, concurrently with baseline science observations during a 12-month period. The RXTE (Rossi X-ray Timing Explorer), recently decommissioned after 16 years of service, revolutionized X-ray timing astrophysics, discovering many new phenomena and resulting in more than 2,500 publications; NICER will provide a natural extension of much RXTE science, while similarly opening a new discovery window in soft X-rays. With the highest time resolution of any astronomy instrument flown, 30 times the sensitivity of RXTE to background-dominated sources, and a factor of 8 improvement in energy resolution, NICER will enable new observing strategies and science for a wide variety of sources, from active stars to clusters of galaxies. Unique science outcomes could include establishing the existence of intermediate mass black holes through characterization of QPOs (Quasi-Periodic Oscillations) in ULXs (Ultraluminous X-ray sources); unification of stellar-mass and supermassive black holes via measurement of break timescales in AGN power-spectra; testing models of polar accretion onto magnetic white dwarfs in binary systems; detecting photon bubble oscillations driven by high-rate accretion; probing galaxy evolution in clusters with Fe line measurements in the intracluster medium out to redshifts z ~ 1 or more; and others.
Figure 6: The NICER payload in its stowed, deploying, and fully deployed states (image credit: NASA)
• June 8, 2016: The NICER instrument arrived at NASA/KSC, Cape Canaveral, FL on June 8, 2016. The forthcoming ISS (International Space Station) payload was transported from NASA's Goddard Space Flight Center in Greenbelt, Maryland, aboard a climate-controlled, air-suspension truck. 7)
- "Thanks to a terrific development team, we're pleased to have delivered NICER two weeks ahead of our original schedule crafted almost four years ago," said Keith Gendreau, NICER's principal investigator at Goddard. "We're looking forward to launching on a SpaceX rocket and integrating with the ISS. From this platform, NICER will provide both unique insights into neutron star physics and validation of a technology that may one day lead humanity into deep space."
Figure 7: A view of the NICER XTI (X-ray Timing Instrument) without its protective blanketing shows a collection of 56 close-packed sunshades-the white and black cylinders in the foreground-that protect the X-ray optics (not visible here), as well as some of the 56 X-ray detector enclosures, on the gold-colored plate, onto which X-rays from the sky are focused (image credit: NASA, Keith Gendreau)
• Dec. 14, 2015: NICER has cleared another major milestone review, and the nearly-complete flight payload is poised to begin environmental (electromagnetic, vibration, thermal, etc.) testing. Delivery to NASA's Kennedy Space Center is anticipated in June 2016. 8)
• May 12, 2015: NASA mission NICER/SEXTANT, that embodies the virtues of faster, less expensive access to space, has sailed past all major development milestones and is scheduled to be delivered to Cape Canaveral on time for its launch in late 2016. 9)
Figure 8: Technicians assemble a new 7 m high test facility, equipped with a 1 m parabolic optical mirror, which will be used to align NICER/SEXTANT's 56 optics and detectors (image credit: NASA)
• In September 2014, NICER/SEXTANT successfully passed the CDR (Critical Design Review), after completing the design. While completing the XTI design, a number of ETUs (Engineering Test Units) were developed and subjected to preliminary environmental testing, e.g., XRC (X-ray Concentrator), detector packaging.
• NICER's KDP-C (Key Decision Point-C) review was held in late February 2014, and shortly there after, NICER was confirmed to proceed to Phase C, Final Design and Fabrication.
• The PDR (Preliminary Design Review) of NICER/SEXTANT was held in December 2013 at the end of Phase B, Preliminary Design and Technology Completion.
• The NICER mission was formally selected for Formulation in April 2013 after completing Phase A, Concept and Technology Development and holding a very successful Phase A site visit in January 2013 conducted by a standing review board.
Launch: The NICER/SEXTANT instrument assembly will be flown to the ISS in 2017 on SpaceX CRS-11 (Commercial Resupply Services-11). NICER/SEXTANT will be hosted as an externally attached payload on the ISS via the ELC (ExPRESS Logistics Carrier).
Orbit of ISS: Near-circular orbit, altitude of ~ 400 km, inclination = 51.6º, period = 92.6 minutes.
NICER/SEXTANT installation: Once berthed, NICER is installed robotically via the SSRMS (Space Station Remote Manipulator System) and the SPDM (Special Purpose Dexterous Manipulator), referred to as Dextre. During this process, NICER must be able to survive for at least 6 hours without power, when provided sufficient notice to allow for pre-heating of the payload. Dextre will transfer NICER to its operational location, Site 7 on ELC-2 (EXPRESS Logistics Carrier-2), which is zenith, outboard, and ram on ISS. Once the payload FRAM (Flight Releasable Attachment Mechanism ) is mated to the ELC 2's FRAM and powered on, engineering assessment will begin. Assessment will continue through deployment and science calibration of the instrument. Once commissioned, primary mission operation will commence.
Figure 9: Artist's rendition of the NICER/SEXTANT payload mounted on the ELC-2 of the ISS with active pointing over 2π steradians (image credit: NASA)
NICER / SEXTANT (Station Explorer for X-ray Timing and Navigation Technology) instrumentation
The NICER/SEXTANT instrument consists of 56 X-ray telescopes in a compact bundle, their associated silicon detectors, and a number of other advanced technologies. NICER's XTI (X-ray Timing Instrument) represents an innovative configuration of high-heritage components. The heart of the instrument is an aligned collection of 56 X-ray "concentrator" optics (XRC) and SDD (Silicon Drift Detector) pairs. Each XRC collects X-rays over a large geometric area from a roughly 30 arcmin2 region of the sky and focuses them onto a small SDD. The SDD detects individual photons, recording their energies with good (few percent) spectral resolution and their detection times to an unprecedented 100 nanoseconds RMS relative to Universal Time. Together, this assemblage provides a high SNR (Signal-to-Noise Ratio) photon-counting capability within the 0.2-12 keV X-ray band, perfectly matched to the typical spectra of neutron stars as well as a broad collection of other astrophysical sources.
SEXTANT is a NASA funded technology demonstration. SEXTANT will, for the first time, demonstrate real-time, on-board XNAV (X-ray Pulsar-based Navigation), a significant milestone in the quest to establish a GPS-like navigation capability available throughout our Solar System and beyond (Ref. 3). 10) 11)
SEXTANT will, for the first time, demonstrate realtime, on-board XNAV (X-ray Pulsar-based Navigation), a significant milestone in the quest to establish a GPS-like navigation capability available throughout our Solar System and beyond. The SEXTANT demonstration will exploit the large collecting area (> 1800 cm2), low background (< 0.2 counts/s), and precise timing (< 100 ns, 1σ) provided by NICER's X-ray timing instrument.
SEXTANT will demonstrate the use of MPSs (Millisecond Pulsars), rapidly spinning neutron stars, as deep-space navigation beacons which could someday guide humankind out of Earth orbit, to destinations throughout the Solar System and beyond. SEXTANT's primary objective is to demonstrate realtime orbit determination with an uncertainty better than 10 km with 2 weeks of measurements in the highly dynamic LEO (Low Earth Orbit) of the ISS. SEXTANT also includes the development of a ground testbed, called the GXLT, (Goddard XNAV Lab Testbed) that enables realtime and faster than realtime simulation of navigation scenarios. With the addition of a unique MXS (Modulated X-Ray Source), flight-like X-ray detector, and time-tagging electronics, this testbed provides a test-as-you-fly HWIL (Hardware-In-The-Loop) simulation capability.
The SEXTANT demonstration is a technology enhancement to the NICER mission, which is an X-ray Astrophysics Mission of Opportunity to the ISS that is currently in Phase C and plans to launch in 2017. NICER will undertake a fundamental investigation of extremes in gravity, material density, and electromagnetic fields of rapidly spinning neutron stars via time-resolved X-ray spectroscopy . NICER achieves this objective by deploying an X-ray telescope instrument as an attached payload on a zenith-side ELC-2 (ExPRESS Logistics Carrier-2) aboard the ISS, Figure 10. NICER offers over an order-of-magnitude improvement in time-coherent sensitivity and timing resolution beyond the capabilities of any X-ray observatory flown to date. As a flight software augmentation to NICER, SEXTANT will use a subset of the data collected for the NICER science program, on-board and in real-time, to accomplish its objective.
The ubiquity, reliability, and accuracy of the GPS (Global Positioning System) has revolutionized terrestrial navigation over the past two decades. Space users, within the GPS Space Service Volume (SSV), have also benefited from the abundance of GPS radiometric measurements to autonomously obtain position, velocity and time on-board and in real-time. The use of GPS for space navigation is well established in LEO (Low Earth Orbit). More recently, its use has been extended to GEO (Geosynchronous Earth Orbit) and HEOs (Highly Eccentric Orbits). It has also been explored for lunar libration point and lunar transfer orbits.
Unfortunately, GPS is available only to space users within Earth's vicinity. To enable autonomous navigation for future missions operating far from Earth-based navigation beacons, the SEXTANT (Station Explorer for X-ray Timing and Navigation Technology) will use rapidly spinning neutron stars to demonstrate a GPS-like navigation capability available throughout the Solar System. This technology also holds the ultimate promise of enabling travel beyond our Solar System, to other stars.
X-ray pulsar navigation history:
Use of radio pulsars as navigation beacons was first considered shortly after their discovery. 13) The idea was later extended to X-rays using the earliest established X-ray pulsars, but the achievable accuracy was severely limited by the noise characteristics of the X-ray pulsars known at the time. X-rays do not penetrate the Earth's atmosphere to sea level. Thus, the X-ray form of the concept is inherently limited to operations above atmospheres and perhaps planetary surfaces with thin atmospheres or none, but has the advantage that it permits comparatively small sensors to be used. The first X-ray satellite instrument developed with a specific goal of exploring the feasibility of X-ray satellite navigation techniques was the USA (Unconventional Stellar Aspect) Experiment flown in 1999 on the DoD ARGOS Satellite, under the Space Test Program. 14) This experiment explored a broader vision of X-ray navigation, not limited to use of pulsars for position determination, but also studying use of occultations, which require no pulses and may use bright sources, a technique suitable for satellites in orbits near planets. It furthermore was not limited to position determination but also evaluated X-ray sensors for attitude determination and time transfer. For these applications it primarily conducted feasibility assessment exercises rather than full operational demonstrations.
During the late 1990s X-rays began to be detected from MSPs (Millisecond Pulsars), which had previously been known only as radio pulsars. 15) This removed the earlier limitation associated with the intrinsic pulsar clock noise but entailed observing a considerably fainter class of X-ray sources. The USA Experiment lacked sufficient sensitivity and on-board timekeeping accuracy to study this source class effectively, but it was recognized that X-ray MSPs would greatly improve the accuracy of an X-ray pulsar-based navigation system, hence the conceptual development path was laid out in paper studies and a patent (US Patent 7,197,381). A DARPA program emphasizing MSP navigation represented the next phase of DoD development and introduced the term XNAV for the specific pulse timing based methodology. The deep space navigation application of XNAV described above was also analyzed . During this program the first laboratory facility to simulate X-ray pulsars was built at NASA/GSFC and used to test sensor prototypes. As of 2014, the concept of X-ray navigation is being widely discussed, in several other countries as well as the US. 16) 17)
As the DARPA program advanced, it became increasingly clear that the detector concept most appropriate for the X-ray MSP population would need to include optics to enhance SNR (Signal-to- Noise Ratio). Optical systems that focus X-rays have been developed for many different astronomical purposes. High performance systems provide full imagery for extended sources or groups of point sources. A by-product of the focusing is that SNR is greatly improved because photons are collected over the full aperture of the focusing system while background is collected only over the much smaller active area of the detector; SNR is enhanced, roughly, by the ratio of those areas. When the goal is to obtain the spectrum or time history of a single source, it is possible to obtain the latter benefit without the cost of the former by using optics of lower performance that can be made low weight as well as low cost. NASA/GSFC has broad experience in such designs, dating back to the Broad Band X-ray Telescope, flown on space shuttle Columbia in 1990.
A modern, low-cost and low-mass optic combined with a SDD (Silicon Drift Detector) emerged as an excellent design for observing X-ray MSPs for navigational purposes. This same design was also ideal for the science that forms the NICER mission goals in astrophysics. In this way, a scientific mission (NICER) and an engineering demonstration (SEXTANT) became realizable in a single package. Moreover, the driving technical requirements flowing down from the scientific objectives were able to cover the requirements for SEXTANT, i.e., the latter did not levy additional requirements beyond those imposed by the purely scientific mission goals.
The SEXTANT technology demonstration objective is to perform real-time, on-board XNAV-only orbit determination, via sequential observation of multiple MSPs (Table 1). In the highly dynamic ISS orbit, SEXTANT will use an intentionally degraded initial orbital position, provided by NICER's GPS receiver, then maintain its orbital position knowledge by processing only XNAV measurements. The demonstration will be considered successful if the on-board position knowledge error reaches ≤ 10 km, worst direction, with two weeks of valid measurements derived from a navigation focused observation schedule. The baseline experiment includes two attempts to achieve this objective: one early in mission operations using ground-based radio observatory derived pulsar timing models, and one later in mission operations using NICER augmented timing models. The performance of the XNAV system will be determined by comparison with the available on-board GPS solution.
If time permits during NICER mission operations, and with complementary observation schedules, a number of additional objectives will be pursued. In line with the primary objective previously stated, a stretch primary objective is to attempt to reach ≤ 1 km on-board position knowledge error, worst direction, with up to 4-weeks of valid measurements from a navigation focused observation schedule. This presents a supreme challenge in the highly dynamic ISS orbit.
The study of long-term pulsar clock stability is a secondary objective that is shared with NICER's fundamental science. Since MSPs rival terrestrial atomic clocks, XNAV observations may be used to support spacecraft time and frequency maintenance, or spacecraft clock synchronization for coordinated measurements over long distances.
Since all NICER photon data will be telemetered to the ground and archived, it will be available for use in ground experiments in which SEXTANT will explore variations and enhancements to its on-board algorithms. Planned ground investigations include exploring the effect on navigation performance of intentionally degrading photon event timestamps resulting from an imperfect spacecraft clock, and attempting to eliminate the need for an initial seed state by batch processing an extended observation sequence—Event times are referenced to GPS time.
Requirements: Demonstrating XNAV-only navigation in LEO on ISS is a considerable challenge. Several factors work to limit the available time to observe MSPs: 1) the highly dynamic perturbation-rich ISS orbit, 2) payload mechanical pointing limitations, 3) ISS structural interference and source occultations, and 4) Sun, Earth, and Moon exclusion zones. This is exacerbated by the faintness of the most desirable MSPs for navigation. Consequently, this objective is less than the ultimate potential accuracy of XNAV-only position determination, which is expected to be on the order of hundreds of meters with a NICER-like instrument in lower dynamic environments, e.g., an interplanetary cruise phase.
To achieve SEXTANT's technology objective, a set of basic technical requirements were developed. The requirements, enumerated below, assume a detector capable of providing source and background count rates as specified in Table 1.
Table 1: SEXTANT pulsar catalog with source and background count rates for a NICER-like detector
* Source and background count rates for a NICER-like detector configuration include consideration of effective detector area, field of view, energy band, operational regime background radiation, etc.
Table 2: SEXTANT mission requirements
While these requirements satisfy SEXTANT, they are not specifically levied as elements of the NICER mission requirements because the requirements that drive the NICER science objectives meet or exceed the SEXTANT requirements specified above. Further, a dedicated period of observing time will be made available wherein SEXTANT objectives receive consideration in optimizing the target observation schedule. The interval will be long enough to realize the requirements given in Table 2.
XNAV (X-ray Pulsar Navigation):
X-ray observations of celestial sources can provide useful navigation information to spacecraft in a range of applications from LEO to interplanetary, and even interstellar, space. One source of such information are X-ray emitting pulsars, which are neutron stars whose X-ray emission is modulated at the rotational period of the star. In the XNAV concept, X-ray observations of such pulsars are being used for spacecraft navigation. A subset of pulsars, the millisecond pulsars, are highly stable clocks, with long term stability comparable to laboratory atomic clocks. For these pulsars, a simple physical model with a small number of parameters can predict the arrival time of pulses to microsecond accuracy over months or years. A measurement of the difference between the arrival time of a pulse at a spacecraft and the predicted arrival time according to an onboard navigation solution can provide an error signal that can be used to measure the location of the spacecraft in a manner similar to GPS (Figure 11).
Figure 11: Geometry of an XNAV observation (image credit: NASA, NICER/SEXTANT Team)
The use of radio pulsars as navigation beacons was first considered shortly after their discovery . 18) The idea was later extended to X-rays using the earliest established X-ray pulsars, 19) but the achievable accuracy was severely limited by the noise characteristics of the X-ray pulsars known at the time. The first X-ray instrument with a specific goal of exploring X-ray navigation techniques was the USA (Unconventional Stellar Aspect) experiment, flown in 1999 on the DoD ARGOS satellite, under the Space Test Program . This experiment explored a broader concept of X-ray navigation, not limited to pulsars and also not limited to position determination but considering also time transfer and aspect determination.
During the 1990s X-rays began to be detected from millisecond pulsars previously known only as radio pulsars. 20) This development greatly improved the expected accuracy of an XNAV system and spurred detailed studies, resulting in a patent on the idea (US Patent 7,197,381). A DARPA program emphasizing millisecond pulsar methodologies represented the next stage of DoD development, and it was during this program that the first laboratory facility to simulate X-ray pulsars was developed at NASA/GSFC. By now the concept of X-ray navigation is being pursued in several other countries as well as the U.S. 21) 22)
XNAV has the potential to become an enabling technology for very deep space exploration missions and an important augmentation to NASA's DSN (Deep Space Network), the current standard for interplanetary navigation and communication. 23)
The NICER XTI (X-ray Timing Instrument):
The key measurable for an XNAV instrument are pulse arrival times determined from a set of detected X-ray photons. The instrument must be designed to be able to produce a high signal-to-noise pulse profile in an integration time that is short compared to the timescale for a propagated orbit to deviate from the true trajectory by more than the required navigation accuracy. In turn, this requires a detector with the following characteristics: large effective area in the region of the X-ray spectrum where MSP pulsations can be observed (roughly 0.2–8 keV), high precision time tagging of each X-ray photon, and low background rates. To be useful in future spacecraft navigation applications, this should be achieved with the lowest possible mass, volume, and power requirements.
The NICER XTI is extremely well suited to this task (Figure 12). It is a modular array of 56 identical telescopes, making it easily scalable to a range of potential applications. Each telescope consists of a lightweight grazing-incidence optic made up of concentric foil mirrors. The mirrors concentrate X-rays onto a small (1 mm radius unobstructed circular aperture) detector area using a single bounce, in contrast to typical (e.g. Wolter I) 24) imaging X-ray optics that require two bounces and thus incur a significant efficiency penalty to achieve quality imaging. The XTI provides over 1800 cm2 of area in a package with frontal area of 6400 cm2, an areal efficiency of 28%.
Figure 12: Block diagram of NICER XTI showing the main components: concentrators, focal plane modules, detectors and measurement and power unit (image credit: NASA, NICER/SEXTANT Team)
The X-ray detectors are commercial (Amptek) silicon drift detectors that have very high quantum efficiency over the photon energy range of interest. The detectors are read out by dual-channel electronics chains that provide both high time resolution (100 ns) and excellent spectral resolution (120 eV) with very low dead time. Particle backgrounds are low because of the small detector volume made possible by the concentrating optics, while particles that do interact in the detector can be rejected at high efficiency by their energy deposition and by filtering events that occurred outside of the illuminated area of the detector by a comparing the pulse heights determined by the slow and fast electronics channels. In addition to reducing particle backgrounds, the focusing of the mirrors reduces the backgrounds from the cosmic diffuse X-ray background and neighboring sources that are outside the ~6 arcmin (FWHM) field of view. The expected background rate in the critical 0.4–2 keV band is< 0.2 counts/s.
SEXTANT system architecture overview:
The SEXTANT system architecture is comprised of four main components: 1) the NICER XTI, 2) the flight software and algorithms, 3) the ground testbed, and 4) the ground system. The main components of the SEXTANT system and the data flow relationship among these components are shown in Figure 13 (Ref. 10).
1) XTI (X-ray Timing Instrument): NICER's XTI is an array of 56 identical X-ray telescopes optimized for observations of neutron stars with high time resolution, good throughput, and low background (description above). Since the instrument does not require good imaging performance, the additional size, mass, and complexity and poorer efficiency of true imaging optics are not needed.
Event times recorded by the XTI are referenced to GPS time provided by NICER's GPS receiver system, which gives absolute timing accuracy referenced to UTC to an accuracy of 100 ns RMS. In a future XNAV application outside of Earth orbit, this simplification would not be available. Such an XNAV system would require a stable clock that is either synchronized with UTC time via two-way time transfer, or steered based on XNAV pulsar observations to a pulsar-based time standard. This additional degree of freedom can be accommodated by observing more than the minimum 3 pulsars required to determine position alone.
Figure 13: SEXTANT system architecture showing the four main components (image credit: NASA, NICER/SEXTANT Team)
2) Flight Software and Algorithms: Photon events, timestamped with GPS time, with associated pulse heights, which are proportional to photon energy, are the fundamental data provided by the NICER XTI to the SEXTANT XFSW (X-ray Pulsar Navigation Flight Software) application. The observed photon events are modeled as the arrival times of a NHPP (Non-Homogeneous Poisson process) with time varying mean cumulative count function. The photon arrival process at the detector is modeled as a delayed version of that at a hypothetical reference observatory, e.g., located at the Geocenter or at the SSB (Solar System Barycenter), with a delay given by the light propagation time of the pulse wavefront moving from the detector to the reference observatory. The relationship between the rate of the arrival process at the spacecraft and that at the reference observatory furnishes a measurement equation that connects the statistical model for the fundamental photon arrival process to the desired spacecraft state parameters. The phase evolution at the reference observatory is provided by the pulsar timing software Tempo2.
The XFSW implements the XNAV algorithms in C and runs as a single application hosted by the NICER IFSW (Instrument Flight Software) , which is based on the NASA/GSFC CFS (Core Flight System). As an application within the IFSW, it receives commands and sends telemetry via the CFS provided publish-and-subscribe software message bus. These messages include the photon events generated by NICER's XTI, the ground commands to configure and manage the on-board pulsar almanac, and the GPS receiver position, which is used to initialize the orbit propagator. The XFSW algorithms are included in the application as a shared library containing two core components: photon processing algorithms, specially developed for SEXTANT, and navigation filter software based on an XNAV-enhanced version of the GEONS (Goddard Enhanced Onboard Navigation System) flight software package. The shared library allows the flight source code to be tested from within MATLAB via the C shared library interface.
SEXTANT XFSW operates as follows. Several MSPs from the SEXTANT catalog are observed in a sequence taking into account observation schedule and visibility constraints. After accumulating a sufficient number of photon events from a given MSP, the collected events are batch processed to extract pulse phase and Doppler measurements. These measurements are then passed to the GEONS navigation filter, where they are blended with models of the spacecraft dynamics to update an estimate of the spacecraft state.
Figure 14: Schematic of the SEXTANT ground testbed architecture showing the three simulation flows from orbit simulation through truth measurement simulation, photon processing and navigation filtering algorithms. The three levels of simulation split at measurement truth and primarily differ in the way estimates are produced. In Level 0, red (upper path) arrows, the measurements are simulated using the measurement model. In Level 1, green (lower path) arrows, the measurements are produced from software simulated photon events. In Level 2, blue (central path) arrows, the measurements are produced from HWIL simulated photon events (image credit: NASA, NICER/SEXTANT Team)
3) Ground Testbed: The GXLT (GSFC X-ray Navigation Laboratory Testbed) is a unique hardware and software test environment developed in support of the SEXTANT mission. The GXLT leverages several GSFC GN&C (Guidance, Navigation, and Control) software tools and X-ray source and detector technologies, and allows for rapid, high-fidelity, end-to-end simulation and performance evaluation of various spacecraft XNAV scenarios.
The overall end-to-end simulation architecture of the SEXTANT ground testbed, depicted in Figure 14, provides three simulation process flows which are indicated by the colored arrow paths. A simulation scenario definition specifies the simulation level and length, X-ray detector parameters, observation schedule that takes into account visibility constraints and observation times, pulsar target list and their models, a truth ephemeris file, event simulation options, photon processing algorithms, and orbit propagator parameters and navigation filter options.
The three simulation process flows, or levels, differ primarily in the fidelity and way XNAV measurements are estimated. The simulation levels are described below.
Level 0: Pulsar pulse phase and Doppler measurements are generated by the XNAV measurement model used by the navigation filter. Noise is intentionally added to these measurements based on expected uncertainty in generating the phase and Doppler estimates from the photon data. Simulation of measurements is a standard operating procedure for fast simulations and for navigation performance evaluation, i.e., no hardware-in-the-loop. This flow is indicated by the red (upper path) arrows in Figure 14.
Level 1: Measurement generation fidelity is increased by simulating the photon arrival process in software, then extracting the realized pulse phase and Doppler estimates. This flow is indicated by the green (lower path) arrows in Figure 14.
Level 2: Measurement fidelity is further increased by replacing the software simulated photon process with a HWIL (Hardware-in-the-Loop) process obtained from the GXLT X-ray pulsar simulator hardware, which is driven by various XNAV pulsar observation scenarios. This flow is indicated by the blue (central path) arrows in Figure 14.
4) Ground System: The NICER ground system will reside in a SMOC ( Science Mission Operations Center) at NASA/GSFC, and will have responsibility for the health and safety of the NICER payload, as well as the scheduling of observations in a manner designed to meet the primary mission science objectives. The SEXTANT ground system is comprised of two components: a monitoring and trending element, which resides within the NICER SMOC, and an external component that maintains the pulsar almanac and provides scheduling recommendations to NICER. Since the primary pulsar targets are common between NICER and SEXTANT, scheduling needs can be met within the constraints of NICER's nominal operations concept.
The primary function of SEXTANT's ground system is to generate and maintain the pulsar information needed to support the XNAV demonstration objectives. This includes parameterized pulse timing models, polynomial-based pulse phase predictions used by the flight software, X-ray profile templates, and count rate estimates. The ground system receives pulsar timing data from ground-based radio telescopes, X-ray telescopes, and NICER itself. The SEXTANT ground system also collects telemetry data and analyzes it for navigation performance monitoring purposes (Figure 15).
The SEXTANT pulsar catalog, consists of pulsars that have been identified as suitable for navigation. The pulsar selection criteria are based on predictive accuracy of the timing model and the precision of TOAs (Time-of-Arrivals) measured with XTI in a 30 minute observation, which is determined from the X-ray brightness, pulse period, lightcurve shape, and unpulsed background rate. The current SEXTANT pulsar catalog contains 11 pulsars, shown in Table 3 and Figure 16, and will be supplemented by new pulsars discovered and characterized before launch and by NICER once operational.
The SEXTANT ground system uses the Tempo2 pulsar timing software to generate timing models by fitting parameterized models to measured radio and X-ray pulse TOAs. During SEXTANT operations, the ground system will measure the phase relationship between the radio and X-ray templates and track variations in the pulsar dispersion measure to maintain alignment.
The Tempo2 timing model parameters are not suitable for direct use by the flight software due to computational complexity. Instead, we use Tempo2 in its predictive mode to generate piecewise polynomial approximations to the full timing model that can be rapidly evaluated. These polynomials, together with astrometric parameters estimated as part of the timing model update, comprise the pulsar almanac that will be uploaded to the XFSW at regular intervals.
Legend to Table 3 symbols:
a D is the distance to the pulsar in kpc (1 pc = 3.26 light-years)
b The measurement accuracy for this observation length is 1 µs, but the intrinsic rotational instability of the Crab Pulsar imposes a limit on the model prediction accuracy of about 10 µs over a few days.
c The phase error is given by σ = 2P / √⟨IpT) with T = 1800 s.
Figure 15: SEXTANT Ground System: Pulsar catalog and almanac maintenance process (image credit: NASA, NICER/SEXTANT Team)
SEXTANT will have two modes of operation during the NICER mission. Whenever NICER is observing MSPs, SEXTANT will attempt to make X-ray pulsar measurements and produce a navigation solution in an opportunistic mode. There will also be a dedicated two week window in which SEXTANT will directly influence the observation schedule in order to achieve the previously stated technology demonstration objective. The schedule must provide adequate observation time for each pulsar while obeying the visibility constraints of the pointing system. A navigation focused observation schedule will be developed to maximize navigation performance during this two week window.
Typical observation times will range from 10 min to several hours to produce navigation measurements. Ideal observation windows are often broken up by visibility restrictions either due to physical blockage of the pulsars or hardware constraints. For example, in a 90 minute ISS orbit, the Earth can block the line of sight to a pulsar for up to 45 minutes. Annually, the portion of the celestial sphere in the neighborhood of the Sun is also blocked from view of NICER (Figure 16). Pointing restrictions from the hardware include gimbal actuator limits and ISS structure exclusion zones.
Even with these visibility restrictions, generally, at least one pulsar is available for observation. Careful selection of pulsar sequencing in this challenging environment will produce a good navigation solution. The final observation schedule is created by weighing the impact of pulsar availability against the navigation quality of the measurements produced. Verification of the schedule's impact on navigation performance is done in the high fidelity end-to-end simulation environment. Initial results have shown navigation performance meeting or exceeding SEXTANT's goals.
Figure 16: The sky locations, in ecliptic coordinates, of SEXTANT's top 10 navigation targets, and the Crab pulsar, are labeled around the figure and indicated as blue circles. The enclosed purple region represents the effect of a 45º Sun avoidance angle on a 3-month period, centered on December 21, with the exclusion zones shown at the beginning (yellow), middle (orange), and end (red). Targets that are unavailable due to Sun avoidance constraints appear as unfilled blue circles, while targets that remain visible during the period are filled (image credit: NASA, NICER/SEXTANT Team)
Ground system operations: The SEXTANT ground system will receive telemetry which will be input into the monitoring and trending functions located in the NICER SMOC. On a monthly cadence, the SEXTANT ground system will retrieve new radio and NICER pulse TOAs for all catalog pulsars, then update the pulsar almanac. More frequently, a new pulsar upload ephemeris will be generated and sent to the XFSW. The upload period is expected to be weekly, except for the Crab pulsar which requires more frequent updates.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).