According to the NASA news article NASA’s NavCube Could Support an X-ray Communications Demonstration in Space — A NASA First:

The new technology, called NavCube, combines NASA’s SpaceCube, a reconfigurable and fast flight computing platform, with the Navigator Global Positioning System (GPS) flight receiver. Navigator


“A Match Made in Heaven”

As part of the potential XCOM demonstration, NavCube will drive the electronics for a device called the Modulated X-ray Source, or MXS, which generates rapid-fire X-ray pulses, turning on and off many times per second.

From this article I understand that NavCube is a unique, fast computer for space applications, combined with a unique, sensitive GPS receiver also optimized for space applications.

The X-ray communications investigation/demonstration will put an X-ray transmitter at one end of the ISS and an X-ray receiver at the other. In this case the receiver will be NICER, an array of X-ray telescopes that will primarily be used to study astronomical sources such as neutron stars, and to look into the possible use of timing from pulsars as a deep space positioning system for navigation.

Would NavCube then actually just serve as a signal generator for some high-speed pulsed X-ray generator?

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above: "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. Credits: NASA/W. Hrybyk" from here

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above: NICER, from Astrophysics on the International Space Station - Understanding ultra-dense matter through soft X-ray timing.

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above: NICER X-ray timing telescope/concentrator array, from About NICER.


Yes, it appears that NavCube will primarily serve as a source of signal for the X-ray generator. In fact, one article explicitly confirms this:

NavCube’s job is to run MXS’s on-and-off switch, said Jason Mitchell, an engineer at Goddard who helped advance the MXS.

I agree that this initially doesn't sound like much of a job. However, the article also says this:

It also may help demonstrate — for the first time — X-ray communications in space, a capability that would allow the transmission of gigabits per second throughout the solar system.

If we are to take this literally, a multiple gigabit throughput would be an enormous upgrade. The ISS's current data downlink appears to be on the order of 300 megabits. Except for possible internal optical fiber links, there's simply no reason network hardware capable of supporting gigabit rates would be on the Station.

Given this, I think it's likely that NavCube is flying a clock and FPGAs that will enable proof of concept for the entire link.

While the X-ray source could be driven at a lower rate, that would only test portions of the technology. NavCube may enable emulation of a much more flight-like system, hence the comments about a ""match made in heaven".

  • $\begingroup$ I assume NICER can measure timing to ns precision, but I am not sure it is built to handle (capture and buffer/save) GHz data rate. Pulsar rates are only roughly Hz to kHz, and since that's sufficient for both science/astronomy and pulsar-based position measurements, it may not have been given a high data stream rate capability. Still it could be the only fully controllable/configurable signal generator available. $\endgroup$ – uhoh Jan 23 '17 at 14:58
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    $\begingroup$ I'm sure they won't want to fly more capability than they need, true. However, one of NICER's stated science goals is measuring "stability of pulsars as clocks" as well as "properties of [pulsar] outbursts, oscillations, and precession". Measuring small fluctuations in a kHz rate could potentially require GHz sampling speeds. Combine this with the fact that ISS has plenty (by aerospace standards) of power and storage, and I wouldn't be surprised if NICER could handle the rate. $\endgroup$ – Bear Jan 23 '17 at 16:29
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    $\begingroup$ I'll dig in and see what I can find, but I think that each individual concentrator on the NICER array has its own detector, and the count rate in each individual detector is very low. In other words, it each detector might actually detect a single X-ray photon from the target source (e.g. neutron star) once per second or once per minute. for example, and for each event the pulse time and height will be logged, and binned. After some time, the average pulse shape will be shown in the histogram. I'll see what more I can find, or ask a new question here in a few days. $\endgroup$ – uhoh Jan 23 '17 at 21:55
  • $\begingroup$ Like this for example: i.stack.imgur.com/XjXhL.png from here - radio pulse timing shown on top, X-ray in the middle, or this i.stack.imgur.com/vp72m.gif from here. $\endgroup$ – uhoh Jan 23 '17 at 22:05

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