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Comment by toomuchtodo | Hacker News Reader

> Now... navigation on or over land by performing SLAM on ground features... that would be interesting.

I'd more be interested in guidance using x-ray emissions from pulsars and neutron stars. Constant reference points that are almost impossible to jam.

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Even better are distant quasi-stellar radio sources (e.g., [0]). The detectable pulsars and neutron stars are close enough (inside our galaxy) that they will not form a fixed reference frame, due to slow motion across the sky. Though, I suppose they're probably good enough over short time intervals, depending on the needed accuracy.

There's also the issue of X-rays not penetrating the Earth's atmosphere. One could try triangulating based on time of arrival for pulsar radio pulses, but that requires a reasonable amount of collecting area with a radio telescope and substantial processing power.

[0] https://en.wikipedia.org/wiki/International_Celestial_Refere...

Edit: slight clarification

Anyone have any idea how much extra "positional" info one could extract out of the position of the sun + an accurate clock?

Granted, I know you couldn't quite get an exact 3d positional reading from just those. But the way I see it, these are complementary systems, and if possible, this extra "data-point" can add accuracy by further narrowing down possible actual locations. Sort of like overlapping 3d volumes (from different systems, with differing levels of accuracy), that allow you to triangulate your position. Each data-source incrementally removing "definitely-wrong" or "very-innacurate" data points from your end calculation.

My M.S. thesis was essentially an attempt to answer this question. The answer, it turns out, is that the readings you get from single bright celestial sources (sun, moon) aren't accurate enough to enable navigation primarily from this data, even assuming perfect timekeeping.

You can use multiple readings of multiple celestial sources (stars) to navigate reliably, but then you've shifted the problem. With a single reference source your main problem is the sensitivity to error (slight angular errors translate to miles). But if you use starlight your primary difficulty is simply measuring the light sources in an affordable way (i.e. short of taking images in real time and doing real time image processing and graph construction on them).

The atmosphere is totally opaque to X-rays[0], so this wouldn't work from the ground.

[0] http://commons.wikimedia.org/wiki/File:Atmospheric_electroma...

True, but you could just use stars. The SR-71's celestial navigation unit could get a fix during the day while it's sitting on the ground: https://books.google.com/books?id=xwPFC3GtcL8C&pg=PA132&lpg=...

The Nortronics NAS-14V2 was designed in the early 60s. I would love to see the insides of one of those things.

Clouds, fog and smoke are the obvious issues. Dense tree canopies too.

Sure, those are all obstacles. But the way I see it, they can all complement each other. I.e. One of the posters mentioned drift of accuracy for some alternate positioning methods. You could then periodically, when possible, use this more-accurate but obstacle-susceptible system to "correct" the drift.

One of my bucket list projects is to build a celestial navigation unit using a prosumer-grade CCD and BeagleBoard grade hardware (star maps stored on MicroSD card, computation done onboard providing NEMA output).

The SR-71's celestial nav unit was a work of art. Kudos for the reference.

Regarding technical viability: what's their luminosity? It's quite hard to compete with solar radiation. x-rays also can't be resolved (there's no small x-ray lens), so I assume you need some kind of shielded detector, which should be vulnerable to all kinds of radiation sources and isotropic -- so a lot of noise and no resolution.

Regarding jamming resistance, if the signal is weak enough considering it's lack of resolve-ability (contrary to advanced GPS antennas which reject ground sources due to multipath and interference), it should be easy to direct an x-ray source at a receiver it and neutralize it.

> Regarding jamming resistance, if the signal is weak enough considering it's lack of resolve-ability (contrary to advanced GPS antennas which reject ground sources due to multipath and interference), it should be easy to direct an x-ray source at a receiver it and neutralize it.

Receivers are entirely passive. You would need to know with certainty location of the receiver to attempt to jam it, unless you're just using an omnidirectional transmitter (hello inverse square law!).

You could aim an x-ray beam at an airplane or car for example. Even with an omnidirectional antenna the signal of distant x-ray sources is so faint you most certainly could overpower it for large distances.

I also didn't realize but in this case unless the x ray sources have very specific, high frequency, detectable temporal radiation patterns you'd rely primarily on their location, and as I've said it's hard to resolve x ray images.

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