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I was reading one of the questions about a version of GPS around Mars, and I started wondering about a Solar system wide positioning system -- large satellites with accurate clocks and powerful transmitters in a range of locations around the solar system. Then it occurred to me that pulsars might already provide what's needed, and with quite a bit more transmitter power (admittedly at larger ranges) than we can muster on a space probe.

So, question -- how hard would it be for space probes in the solar system to establish relative positions and ultimately construct a reference frame by comparing pulsar timings? How large a receiver would be needed? Are there other difficulties?

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It looks like this falls into the "complicated but plausible, and being investigated" category - this is from a recent overview paper, "Pulsar timing and its applications", RN Manchester, 2017. Journal of Physics; arXiv.

Another interesting application of precision pulsar timing is to navigation of spacecraft that are distant from the Earth, even outside the solar system. [...] Analysis of a realistic simulation has shown that position location with an accuracy of better than 20 km is possible using observations of just four MSPs. Autonomous operation of the system is possible, but accuracy is improved with updates of the pulsar parameters from Earth-based observations. It is interesting to note that in 2016 the Chinese launched a satellite, XPNAV, dedicated to exploring X-ray pulsar navigation, and that NASA's recently launched NICER mission on the International Space Station has a project (SEXTANT) devoted to this topic as well.

There is a writeup of the 2018 NICER/SEXTANT project here; in the first run, they took 78 measurements of four pulsars over one day, and were able to find a location accurate to within ten miles, which is pretty good going. I haven't found an update on XPNAV.

However, there are definitely logistical questions. NICER is described as "about the size of a washing machine" (photo), and per the CRS-11 manifest it massed about 370kg. I don't know what its power requirements are, but I'm guessing they're not small - easy for the ISS, challenging for a smaller probe.

Granted you could probably strip it down quite a bit if you were just using it for navigation, rather than navigation being tacked on to scientific observing, but it feels like there is still a long way to go before you could bolt one of these onto something like Cassini or New Horizons.

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  • $\begingroup$ Did you find anything about solving for ambiguities? The signals are perfectly periodic, so there seems to be no way to get an unique solution without prior knowledge of the rough position. $\endgroup$ – asdfex Aug 8 at 16:42
  • $\begingroup$ @asdfex I didn't delve that deeply, I confess! $\endgroup$ – Andrew Aug 8 at 16:58
  • $\begingroup$ No problem, just asking. Power Requirements shouldn't be that large. The detector technology is quite low power and also the electronics are not really complicated. I'd guesstimate 20W should suffice. Not much, but still a lot for a deep space probe. The size might be a problem - and it needs to be large, photon hit rate is only about 0.05Hz... $\endgroup$ – asdfex Aug 8 at 17:14

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