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My understanding is that probes within the Solar system primarily rely on Earth-based tracking networks and ephemeris data to navigate. Once they get really close to their destinations, they can optionally also use visual tracking to position themselves. But how would spacecraft, e.g., those on extrasolar voyages, navigate without Earth being in range to spoonfeed them positional information? "Old school" star navigation?

coleopterist
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2 Answers2

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There are two problems here:

  • Where am I (and what is my velocity vector in some inertial frame of interest)? (Navigation) (that's what I suppose you are asking)
  • Am I flying head over heels or vice versa (and how fast am I rotating)? (Attitude determination)

For navigation:

The prevailing mode of thinking considers X-ray pulsars to be the principal avenue of advance. This is called XNAV (x-ray pulsar-based navigation and time determination).

The underlying principle here is the stability of millisecond pulsars emitting in the X-ray spectrum. A combination of pulse counts from known MSPs is transformed into pseudo-ranges and solved in a filter like any other TDOA (multilateration) problem. Please note that it is more difficult and time-consuming than navigation using GPS, where pseudo-random sequences transmitted by satellites make it easy to disambiguate the position (pulsars emit the same pulses all over).

NICER: A battery of X-ray detectors planned for installation on the ISS

An artist's rendition of NICER

Credit: NASA.

References:

  • 2012 - Pulsar Navigation and X-ray Communication Demonstrations with the NICER Payload on the ISS http://hdl.handle.net/2060/20120016975

  • 1981 - Navigation using X-ray pulsars NTRS Doc.ID 19810018591 Chester, T. J., Butman, S. A.

Approximately one dozen X-ray pulsars are presently known which emit strong stable pulses with periods of 0.7 to approximately 1000 s. By comparing the arrival times of these pulses at a spacecraft and at the Earth (via an Earth orbiting satellite), a three dimensional position of the spacecraft can be determined. One day of data from a small onboard X-ray detector yields a three dimensional position accurate to approximately 150 km. This accuracy is independent of spacecraft distance from the Earth. Present techniques for determining the two spacecraft coordinates other than range measure angles and thus degrade with increasing spacecraft range. Thus navigation using X-ray pulsars will always be superior to present techniques in measuring these two coordinates for sufficiently distant spacecraft. At present, the break even point occurs near the orbit of Jupiter. The Crab pulsar can also be used to obtain one transverse coordinate with an accuracy of approximately 20 km.

For attitude determination, star trackers are and will be enough in the foreseeable future.

Related multimedia:

Deer Hunter
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  • @Chris - "position themselves" is ambiguous. I want to know not only where I am, but also where I should point the nozzles to, when I have to start Mars orbit insertion. – Deer Hunter Aug 02 '13 at 17:49
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    I would argue that it is not, but certainly "navigate" (in the title) is not ambiguous. –  Aug 02 '13 at 17:51
  • How about "when I am"? Say you've had some "unpredictable event" happen to you and knocks you and your instruments out while on interstellar voyage. How would you determine your time relative to your point of origin, and with what precision would that be possible? Let's assume you can't calculate it based on your attitude alone (i.e. your path could have changed without your knowledge for whatever reasons). – TildalWave Aug 02 '13 at 17:52
  • @TildalWave - XNAV (...and time determination). In an interstellar voyage, if anything goes wrong with the (redundant, diverse sets of) instruments, chances are the voyage's doomed. – Deer Hunter Aug 02 '13 at 17:56
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    An analogy relating XNAV to GPS might be useful for understanding here... they're basically the same principal at heart. –  Aug 02 '13 at 18:00
  • @DeerHunter - I was thinking more in a way if there's any observable and time-predictable phenomena going on in the not too distant Universe (say, within our Milky Way galaxy) that could be used to determine with some precision the time, say at least to years in precision? – TildalWave Aug 02 '13 at 18:03
  • @Chris - not exactly. Pulsars don't use optimized pseudo-random sequences, hence the difficulties of filtering fixes. – Deer Hunter Aug 02 '13 at 18:11
  • @TildalWave - that would be star movement in the nearest vicinity of Sol. – Deer Hunter Aug 02 '13 at 18:12
  • @DeerHunter clearly. I was just suggesting that it would be a good analogy for anyone not familiar with the subject. –  Aug 02 '13 at 18:12
  • Thank you. This is fascinating stuff. "By comparing the arrival times of these pulses at a spacecraft and at the Earth (via an Earth orbiting satellite)": does this necessitate contact with Earth? Or is the craft supposed to carry a library of expected arrival times on Earth? – coleopterist Aug 02 '13 at 18:36
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    @coleopterist - having a separate X-ray detector somewhere else (in LEO or on the Moon etc.) always helps (in terms of accuracy). However, you can do it on your own. And: there is no library. The pulsars are assumed fixed, and their pulses follow at equal intervals. – Deer Hunter Aug 02 '13 at 18:49
  • There is one more key thing that isn't often mentioned, what time is it. That is important to know where Earth is, so you know where to point your dish. Any thoughts on that subject? – PearsonArtPhoto Aug 03 '13 at 11:34
  • @PearsonArtPhoto - you simply count pulses AND keep a nice atomic clock. NIST is working on miniaturization of atomic clocks, by the way... – Deer Hunter Aug 03 '13 at 16:49
  • @DeerHunter: And if your computer reboots? Then what? – PearsonArtPhoto Aug 03 '13 at 18:10
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    @PearsonArtPhoto - keep the counter isolated from your flight computer :) Simple, rugged, shielded, redundant. – Deer Hunter Aug 03 '13 at 18:19
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This is currently an unsolved problem. In addition to the X-ray pulsars described by Deer Hunter, NASA funded a study into using exoplanets as a source for navigation data:

This proposal puts forth an innovative star tracker hardware sensor that allows for autonomous calculation of a spacecraft's orbit by employing Doppler Spectroscopy and Astrometric techniques. The proposed advanced star tracker provides onboard deep space orbit self determination capabilities through the use of specialized reference stars that have exoplanet companions. The motion of exoplanets around a reference star's barycenter provides a stable, highly predictable natural signal pattern. An advanced exoplanet star tracker enhances mission capabilities for future manned and unmanned space vehicles as well as reducing Deep Space Network (DSN) tracking requirements and resources.

This is far from ready, though:

Estimated Technology Readiness Level (TRL) at beginning and end of contract:
Begin: 2
End: 3 (i.e. it's in the 'research to prove feasbility' phase)

Hobbes
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