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Satellite Missions Catalogue

ISS Utilization: XCOM (X-ray Communications)

Last updated:Apr 25, 2019





Mission complete

Quick facts


Mission typeNon-EO
Mission statusMission complete
Launch date04 May 2019
End of life date04 May 2020

ISS Utilization: XCOM (X-ray Communications) + NISTEx II of STP-H6 payloads

A new experimental type of deep space communications technology is scheduled to be demonstrated on the International Space Station. Currently, NASA relies on radio waves to send information between spacecraft and Earth. Emerging laser communications technology offers higher data rates that let spacecraft transmit more data at a time. This demonstration involves XCOM (X-ray Communications), which offers even more advantages. 1)

X-rays have much shorter wavelengths than both infrared and radio waves. This means that, in principle, XCOM can send more data for the same amount of transmission power. The X-rays can broadcast in tighter beams, thus using less energy when communicating over vast distances.

If successful, the experiment could increase interest in the communications technology, which could permit more efficient Gbit/s data rates for deep space missions. These extremely high-speed rates of data transfer are not currently common, but new research projects have pushed computing capability toward this range for some technologies.

"We've waited a long time to demonstrate this capability," said Jason Mitchell, an engineer at NASA's Goddard Spaceflight Center in Greenbelt, Maryland, who helped develop the technology demonstration, which relies on a device called the MXS (Modulated X-ray Source). "For some missions, XCOM may be an enabling technology due to the extreme distances where they must operate," Mitchell said.

Perhaps more dramatically, at least as far as human spaceflight is concerned, X-rays can pierce the hot plasma sheath that builds up as spacecraft hurdle through Earth's atmosphere at hypersonic speeds. The plasma acts as a shield, cutting off radio frequency communications with anything outside the vehicle for several seconds — a nail-biting period of time dramatically portrayed in the movie, Apollo 13. No one has ever used X-rays in a communications system, though, so other applications not yet conceived could emerge, Mitchell said. "Our goal for the immediate future is finding interested partners to help further develop this technology," Mitchell said.

Encoding Digital Bits

To demonstrate this new communications technology, NASA will use the MXS to generate rapid-fire X-ray pulses. Operated by another Goddard-developed computing and navigation technology called NavCube, MXS will turn on and off many times per second while encoding digital bits for transmission.

From the experimental payload, the MXS device will then send the encoded data via the modulated X-rays to detectors on the NICER (Neutron-star Interior Composition Explorer), which is located ~50 m away on the space station. In this way, NICER becomes the receiver of a one-way X-ray signal.

Although the first XCOM test will involve the transmission of GPS-like signals, Mitchell said the team may attempt to transmit something more complicated after the initial attempt. "It's important is that we transmit a known code we can identify to make sure NICER receives the signal precisely the way we sent it," Mitchell said.

Although primarily built to gather data about the densest objects in the universe — neutron stars and their pulsating next-of-kin, known as pulsars — NICER was also designed to demonstrate advanced technology. In addition to the XCOM demonstration, the mission proved the effectiveness of X-ray navigation in space, showing in 2017 that pulsars could be used as timing sources for navigational purposes.

During that two-day demonstration, which the NICER team carried out with an experiment called SEXTANT (Station Explorer for X-ray Timing and Navigation Technology), the mission gathered 78 measurements from four millisecond pulsars. The team fed that data into onboard algorithms to autonomously stitch together a navigational solution that revealed the location of NICER in its orbit around Earth as a space station payload. Within eight hours of starting the experiment, the system converged on a location within the targeted 6.2 miles and remained well below that threshold for the rest of the experiment.

NICER's ability to carry out science and demonstrate emerging, revolutionary technologies has captured the attention of those planning NASA's next era of human spaceflight. Missions that perform multiple functions are now considered a model, said Jake Bleacher, lead exploration scientist responsible for identifying areas where Goddard scientists can support human exploration of the Moon and Mars.

Technology Heritage

The idea to use X-rays to communicate and navigate originated more than a decade ago when NICER Principal Investigator Keith Gendreau began work on enabling technologies for a proposed black hole imager aimed at directly imaging the event horizon of a supermassive black hole or the point of no return where nothing — neither particles nor photons — can escape.

The idea was to establish a constellation of precisely aligned spacecraft that would in essence create an X-ray interferometer, an instrument used to measure displacements in objects. He conceived the idea of using X-ray sources as beacons to enable highly precise relative navigation. Using research and development funding, he developed the MXS.

Gendreau then reasoned that if he could modulate X-rays through a modulator, he could also communicate, thus giving birth to the NICER three-in-one mission concept.

In partnership with the U.S. NRL (Naval Research Laboratory), NASA is deploying XCOM as a payload on the U.S. Air Force's STP -H6 (Space Test Program-H6) experiment pallet to be deployed on the International Space Station. The Air Force Space Test Program is providing payload integration and test, launch services, and up to a year of operations. The XCOM demonstration was also supported by NASA's Space Communications and Navigation program within the Human Exploration and Operations Mission Directorate. NICER is an Astrophysics Mission of Opportunity within the Explorers program. The Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation.

NavCube — is the result of a union between Goddard's homegrown SpaceCube, a reconfigurable, very fast flight computing platform, and Navigator GPS, an enabling navigation technology on NASA's flagship MMS (Magnetospheric MultiScale) mission. 2)

"This new product is a poster child for our R&D efforts," said Goddard Chief Technologist Peter Hughes, who announced that the NavCube development team had been selected to receive the FY16 IRAD Innovators of the Year award. "Both SpaceCube and Navigator already proved their value to NASA. Now the combination of the two gives NASA another, more robust navigational tool. NavCube represents technology infusion at its best."

Figure 1: NavCube, the product of a merger between the Goddard-developed SpaceCube 2.0 and Navigator GPS technologies, could play a vital role helping to demonstrate X-ray communications in space — a potential NASA first (image credit: NASA)
Figure 1: NavCube, the product of a merger between the Goddard-developed SpaceCube 2.0 and Navigator GPS technologies, could play a vital role helping to demonstrate X-ray communications in space — a potential NASA first (image credit: NASA)

This more powerful and flexible technology is slated to fly on the Defense Department's STP-H6 (Space Test Program-H6), an external experiment pallet to be deployed on the International Space Station in 2019. Once deployed, NavCube will demonstrate its enhanced navigational and processing capabilities afforded by the merger of its technological parents and provide precise timing data needed to enable XCOM (X-ray Communications).

A Marriage Made in Heaven

As part of this planned demonstration, NavCube will drive the electronics for the device MXS (Modulated X-ray Source). Also advanced through Goddard R&D support, MXS is critical to demonstrating XCOM and is one of two technology demonstrations that Principal Investigators Keith Gendreau and his deputy, Zaven Arzoumanian, want to execute with the NICER (Neutron-star Interior Composition Explorer), once it's deployed as an attached payload on the International Space Station in 2017. NICER primarily will study neutron stars, the densest objects in the universe, and their rapidly spinning next-of-kin, pulsars.

Figure 2: Goddard's Steve Kenyon is the mechanical and packaging "wizard" for the MXS and STP-H6 XCOM hardware. The equipment shown are various incarnations of the hardware needed to demonstrate X-ray communications in space (image credit: NASA)
Figure 2: Goddard's Steve Kenyon is the mechanical and packaging "wizard" for the MXS and STP-H6 XCOM hardware. The equipment shown are various incarnations of the hardware needed to demonstrate X-ray communications in space (image credit: NASA)

However, Gendreau's team purposely designed the mission to demonstrate two potentially revolutionary technologies — XCOM as well as X-ray navigation, or XNAV. For the latter, the team will use NICER's 56 X-ray telescopes to detect the highly predictable X-ray pulsations emanating from the sweeping magnetic poles of rotating pulsars to provide high-precision timing data. With this information and specially developed algorithms, they want to show that they can stitch together a navigational solution to demonstrate one-way XCOM — the other advanced technology — MXS will generate rapid-fire X-ray pulses, turning on and off many times per second, encoding digital bits for transmitting data. Positioned inside another box on the STP-H6 pallet, MXS will transmit data via the modulated X-rays to NICER's receivers placed about 50 m away on the opposite side the space station truss.

NavCube's all-important job is running MXS's on-and-off switch, said Jason Mitchell, a Goddard engineer who helped advance the MXS and manages NICER's XNAV effort. Because NavCube combines SpaceCube's high-speed computing with Navigator's ability to retrieve GPS signals even in weak-signal areas, the device also will allow the team to experiment with X-ray ranging, a technique for measuring distances between two objects.

"We've known about NavCube for a long time," Mitchell said. "For us, NavCube provided the best solution for running this experiment. The combination of these powerful technologies was a marriage made in heaven."

Having flown many times before, including on previous STP experiment pallets, SpaceCube now enjoys a growing list of customers. A couple high-profile missions, NASA's RRM3 (Robotic Refueling Mission 3) launche on 5 December 2018 and Restore-L in 2020, have baselined SpaceCube 2.0 for their computing operations.

"We're actively working commercialization agreements for SpaceCube 2.0 technology," said Dave Petrick, a Goddard branch chief engineer and SpaceCube 2.0 architect, who has earned prestigious engineering awards for his work on SpaceCube 2.0. "We don't want to be in the business of building computers for the aerospace market. We'd rather spend our time building the next one."

The other technology — Navigator GPS — was purposely designed to detect, acquire, and track faint GPS signals for NASA's MMS mission. Navigator, which has since been commercialized, now is providing positioning information to the four spacecraft that must fly in a pyramid-shaped, highly elliptical orbit to gather data.

Since MMS's launch, Navigator has set records. At the highest point of the MMS orbit, Navigator has tracked as many as 12 GPS satellites. The team originally expected to detect no more than two or three GPS satellites. "This is indeed one of the greatest engineering accomplishments Goddard has done," said Dean Chai, the acting chief for technology of Goddard's Mission Engineering and Systems Analysis Division.

The Union

Even before MMS launched, however, the Navigator team had already begun improving the technology, which ultimately led to NavCube. "Navigator is adequate for MMS, but it's not very extensible," said Luke Winternitz, Navigator's chief architect. "This isn't the one we wanted to move forward with."

"At the time, we needed a more robust, re-programmable and extensible processing platform," added Monther Hasouneh, NavCube's hardware lead. "SpaceCube was already there. Furthermore, we figured that missions using SpaceCube 2.0 as a science data processor also could benefit from having a GPS receiver as a low-cost add-on," he added.

Hasouneh and his team ported the Navigator software and firmware into the SpaceCube reprogrammable platform and developed a compatible GPS radio-frequency, or RF, card — and in doing so, reduced Navigator's size. Using R&D support, the team also added new GPS signal capabilities and enhanced Navigator's sensitivity to make it appropriate for a broader range of applications, not just MMS-style flagship missions.

"The end result is NavCube, which is more flexible than previous Navigators because of its ample computational resources. Because we added the ability to process modernized GPS signals, NavCube has the potential to significantly enhance performance at low, and especially, high altitudes, potentially even to cis-lunar space and lunar orbits," Winternitz said. Yet, still, further improvements are afoot, Winternitz added.

The team is investigating adding a transponder capability to support a future, next-generation beacon service. In addition, the team has plans to further reduce NavCube's size to make it appropriate for CubeSats, particularly those that fly in constellations to gather real-time, simultaneous measurements. In addition, members of the NavCube team are working with Stanford University, under a recently awarded Smallsat Technology Partnerships initiative effort, to develop and demonstrate precision GPS-based formation-flying algorithms for small satellites.

"We're really excited about winning that. The development of these algorithms won't make NavCube smaller, but they could be a good application for NavCube or a future further-miniaturized version of this technology," Winternitz said. "One thing is certain. We will continue improving this technology."



The SpaceX CRS-17 (Commercial Resupply Service-17) with a Dragon spacecraft on a Falcon 9 Block 5 rocket was launched on 04 May 2019 (02:48 EST, or 06:48 UTC) from the Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida. Major payloads on this flight were: 3) 4)

OCO-3 (Orbiting Carbon Observatory-3) of NASA

STP-H6-XCOM (Space Test Program-Houston 6-X-ray Communication)

PBR (Photobioreactor)

Hermes Facility

Organs on Chips

Orbit: Near circular orbit, altitude of ~ 400 km, inclination = 51.6º.

The spacecraft will take two days to reach the space station before installation on May 6. When it arrives, astronaut David Saint-Jacques of the Canadian Space Agency will grapple Dragon, with NASA astronaut Nick Hague serving as backup. NASA astronaut Christina Koch will assist by monitoring telemetry during Dragon's approach. After Dragon capture, mission control in Houston will send commands to the station's arm to rotate and install the spacecraft on the bottom of the station's Harmony module.

The Dragon spacecraft will spend about four weeks attached to the space station, returning to Earth with more than 1900 kg of research, hardware and crew supplies.



STP-H6-XCOM (Space Test Program-Houston 6-X-ray Communication)

PIs (Principal Investigators) of STP-H6-XCOM are Keith C. Gendreau of NASA/GSFC and Paul Ray of NRL (Naval Research Laboratory).


• The STP-H6-XCOM investigation space-qualifies a new modulated X-ray source that produces a beam of X-ray that switches on and off at timescales of nanoseconds, which is much faster than traditional X-ray sources.

• STP-H6-XCOM demonstrates space communication and ranging using modulated X-rays for the first time.

• STP-H6-XCOM includes a SpaceCube 2.0 with a new dual-frequency Global Positioning System (GPS) receiver that demonstrates high-precision GPS in a flexible multi-purpose space computer. This computer provides the precise timing and command and control interface for STP-H6-XCOM.

• STP-H6-XCOM is the first step towards operational X-ray communications systems and other potential applications of DoD (Department of Defense) and NASA interest.

The Primary goal of STP-H6-XCOM is to perform a spaceborne demonstration of a new technology for generating beams of modulated X-rays. This MXS (Modulated X-ray Source) was invented and patented at NASA's Goddard Space Flight Center. It is capable of switching times of order 1 nanosecond, making it usable for high-rate communication and ranging applications. It can also be configured to work at a range of X-ray energies, though the MXS on STP-H6-XCOM is limited to a maximum energy of 4 keV by its high voltage supply. The source is modulated by an ultraviolet (UV) light emitting diode (LED) that is computer controlled. The UV light stimulates the emission of electrons in a photocathode and those electrons are multiplied and accelerated in the device before impacting on a target and generating X-rays characteristic of the materials in the target. These X-rays are collimated into a 20 degree cone and beamed down the ISS (International Space Station) truss towards the NICER (Neutron Star Interior Composition Explorer) investigation. 5)

Figure 3: XCOM investigation prior to integration on to STP-H6 payload (image credit: Paul Ray, NRL)
Figure 3: XCOM investigation prior to integration on to STP-H6 payload (image credit: Paul Ray, NRL)

The control of the investigation and the precise timing are provided by the NavCube – a dual-frequency, GPS L1/L2C, receiver built on the flexible and reprogrammable-FPGA based SpaceCube 2.0 digital signal processing platform. The NavCube produces and controls the modulation pattern of the MXS with times precisely synchronized to GPS time. Another goal of this investigation is to demonstrate GPS L1 and L2C signal tracking on NavCube and demonstrate the benefit of the GPS L2C signal on the position, velocity and timing solution and its utility in reducing errors due to Ionospheric propagation delay. The performance of the NavCube dual frequency GPS receiver is monitored and analyzed on the ground.

The receiver for the STP-H6-XCOM transmissions is the NICER X-ray telescope, which operates on Express Logistics Carrier 2 (ELC 2) on the ISS since June 2017. NICER records the time of each X-ray photon it detects to an accuracy of better than 100 ns and telemeters full time and energy information for each event to the ground for analysis. NICER is on a 2-axis gimbal, which allows it to track celestial X-ray sources in its normal operation mode, or to point towards ELC 3 to get the XCOM investigation in its field of view for these planned investigations.

STP-H6-XCOM is the first step towards operational X-ray communications systems of DoD (Department of Defense) and NASA interest. Potential applications are deep space probes, or communicating with hypersonic vehicles where plasma sheaths prevent traditional radio communications.

The STP-H6 payload consists of eight SERB experiments, including experiments related to star tracker and communications technology and space environment monitoring. ACES-RED (Army Cost-Efficient Spaceflight -Research Experiments and Demonstrations). ACES-RED has a primary focus on attitude determination and control components. The primary payload is a MAI-400 ADCS. 6)

Figure 4: Illustration of the STP-H6 payload (image credit: The Aerospace Corporation, USAF/SMC) 7)
Figure 4: Illustration of the STP-H6 payload (image credit: The Aerospace Corporation, USAF/SMC) 7)



NISTEx II (Navy Interferometric Star Tracker Experiment II)

NISTEx II of the STP-H6 (Space Test Program-Houston 6 payload is a star tracker capable of identifying stars to 5th magnitude and measures attitude to 62 mas (milli-arcseconds) in the cross axes and 124 mas in the roll axis (3σ). STP-H6-NISTEx II demonstrates assured PNT (Positioning, Navigation, and Timing) via CN (Celestial Navigation) and SSA (Space Situational Awareness) detection by post-processing image frames. STP-H6-NISTEx II also provides high accuracy, current epoch bright star astrometric catalog updates. 8)

Figure 5: NISTEx II as-built hardware for STP-H6 shows the interferometric star tracker (foreground) and custom designed and three-dimensional printed sun shade (image credit: NASA)
Figure 5: NISTEx II as-built hardware for STP-H6 shows the interferometric star tracker (foreground) and custom designed and three-dimensional printed sun shade (image credit: NASA)

NISTEx II demonstrates technology that increases by 100 times the accuracy of star detection and direction measurements (the equivalent of detecting and measuring the width of a human hair from 500 m away). STP-H6-NISTEx II provides high accuracy attitude measurements and stellar locations and magnitudes of stars – enabling astrophysical science measurements on CubeSat class and larger orbiting platforms. These high performance astrometry measurements will help improve the US Naval Observatory star catalogs and provide future missions an alternative to European-built star trackers.


Research Overview

• The current state of US-based star trackers pales in comparison to those built by European companies (e.g. Galileo).

• There is a dearth of US companies that can provide high accuracy (< 1 arcsec), reliable star trackers for flagship missions.

• Space Test Program-Houston 6-Navy Interferometric Star Tracker Experiment (NISTEx II) (STP-H6-NISTEx II) provides an independent measure of star directions and inertial attitude measurements for comparison.

• STP-H6-NISTEx II also more accurately determines stellar locations and magnitudes (astrometry) for inputs to the star catalogs developed by the US Naval Observatory.

• The accuracy of the star magnitudes and directions is utilized in a myriad of ways (astrometry, inertial attitude estimation, inertial rate estimation, and celestial navigation).

• STP-H6-NISTEx II provides much higher accuracy inertial attitude measurements than is available from existing star trackers – benefitting astrophysics, heliophysics, and earth observing missions.

The STP-H6-NISTEx II hardware features a sun-shade at the front of the device, an optical shutter, interferometric components, housing for the Stingray custom designed optics, an electronics processing and power interface using a radiation hardened BAE CIS1910 detector. This hardware also includes custom software developed to process interferometric measurements, to identify stars in the data and to determine an inertial attitude based on measured star positions.

The interferometer is designed using birefringent gratings, Wollaston prisms, and LSI spacers to create the grating lateral shearing interferometer. The interferometer is carefully aligned with the optics by personnel from the OPC (Optical Physics Company) until a perfect fringe pattern is obtained. Once aligned with the optics, the assembly is calibrated on a custom goniometer using a collimated light source.

The lateral shearing angular period is designed to have a 200 µrad (micro-radian) period, which gives a theoretical performance of 41 mas with near-zero inertial rates, and an accuracy of 206 mas at a slew rate of 1º/s. The hardware update rate is set to 1 Hz to ensure enough processing time is allotted to determine a solution. The optics are designed to provide a 10º FOV (Field of View) with a 20 mm aperture, which requires approximately 15 ms of integration time to collect enough stellar photons. With a slew rate on the ISS (International Space Station) platform of approximately 0.067º/s and the parameters of the BAE detector, the margin for smear is 473%, which allows for longer integration times in case the conditions in orbit necessitate it.

The software onboard is able to process the interferometric quadrature signals, identify the stars in the image down to 5th magnitude and then compute an inertial attitude solution once the star identification is complete. The US Naval Observatory provided the star catalog for this investigation and is a co-investigator. The software and electronics also handle the interfaces with the STP-H6 C&DH (Command and Data Handling) system as well as the voltage reduction necessary to operate on the ISS platform.

Space Applications

Accurate measurement of the magnitude and direction of stars directly affects space platforms and missions, including inertial attitude estimation and celestial navigation. Having a measurement tool with the accuracy of STP-H6-NISTEx II may result in eliminating the need for spacecraft to have inertial rate-sensing devices, which typically are large, heavy and use significant power. The new capability greatly simplifies mission guidance and control systems, offering significant cost savings on hardware, software development, integration and test costs.



1) "NASA set to demonstrate X-ray communications in space," NASA/GSFC, Public Release: 19February 2019, URL:

2) "A Progeny Is Born," Cutting Edge, Volume 13, Issue 1, Fall 2016, pp: 4-5, 20, URL:

3) Joshua Finch, Courtney Beasley, Sean Potter, "SpaceX Dragon Heads to Space Station with NASA Science, Cargo," NASA Release 19-035, 04 May 2019, URL:

4) Stephen Clark, "Launch Schedule," Spaceflight Now, 29 April 2019, URL:

5) "Space Test Program-Houston 6-X-Ray Communication," NASA, URL:

6) Mason Nixon, Chris Duron, Jameson Hilliard, Eric Becnel, Gauge Day, John Gould, Jessica Shrontz, Evan Swinney, Andrew Webb, Elizabeth Neilson, "ACES RED Experiment #1 Environmental Test Results for Industrial Grade, Non-traditional, and Other Components Lacking Flight Heritage," Proceedings of the 32nd Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 4-9, 2018, paper: SSC18-IV-07, URL:

7) Barbara Manganis Braun, Sam Myers Sims, James McLeroy, Ben Brinning, "Breaking (Space) Barriers for 50 Years: The Past , Present, and Future of the DoD Space Test Program," Proceedings of the 31st Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 5-10, 2017, paper: SSC17-X-02, URL:

8) "Space Test Program-Houston 6-Navy Interferometric Star Tracker Experiment (NISTEx II)," NASA,19 March 2019, URL:


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 (