3 ms·

I believe it has inertial systems because it’s designed to be mounted to aircraft carriers, which constantly pitch and roll about, making it harder to get a fix

I believe it has inertial systems because it’s designed to be mounted to aircraft carriers, which constantly pitch and roll about, making it harder to get a fix. If you have an accurate true north compass reading, an accurate clock and assume that you’re at sea level, star positions give you all the information you need to find your location on earth. The SR71 used a similar system. https://theaviationgeekclub.com/the-sr-71-blackbird-astro-nav-system-aka-r2-d2-worked-by-tracking-the-stars-and-was-so-powerful-that-it-could-see-the-stars-even-in-daylight/amp/ https://theaviationgeekclub.com/the-sr-71-blackbird-astro-na...

perihelions 1y ago
I'm not following your argument; magnetic north and celestial north are redundant. I don't understand what you think a magnetic orientation adds to a star tracker. The stars look exactly the same from everywhere on Earth. The only thing looking at them can tell you is an absolute rotational orientation—nothing more. Old-style navigation was based on the differential orientation of stars relative to the local orientation of the Earth's apparent horizon, but that measurement's not accurate enough for modern GNSS. (I don't think). I don't know how the SR-71 guidance works, but your link describes it as having an inertial component.
- Amezarak 1y ago
  I may be betraying my ignorance here but I don’t understand what you mean when you say the stars look the same everywhere on earth. You can definitely determine latitude from what stars are visible in the night sky along with their position in the sky, as, for example, the Southern Cross is only visible in the southern hemisphere; the converse is famously true for the North Star. It makes sense to me you don’t gain anything from having a compass (assuming clear nights) though, since you have the celestial north. Google claims that human-based celestial navigation done correctly can be accurate to 1 nautical mile, which seems good enough for navigation but not for any weapons targeting. Definitely way worse than GNSS. I would assume there are automated systems out there that can reduce the sources of error together get it down to less than that but probably not several orders of magnitude.
  - perihelions 1y ago
    > "for example, the Southern Cross is only visible in the southern hemisphere" It's in the same place everywhere; it's the position of the Earth, which you're implicitly measuring by pointing out that it's obstructing your view, that's different. The things above the horizon are visible; the things below the horizon are not. That's not facetious; the core idea of this type of navigation is to measure the position of the horizon with high precision, which is hard. Very hard if you're surrounded by mountains. ("Which hemisphere am I on?" is... low precision, if easy to measure). Those classic methods aren't pure astrometry: they're differential astrometry that measure the horizon against stars, the relative orientation of the two.
    - teamonkey 1y ago
      You don’t need to see the horizon at all, you just need to establish a vector that points Up. Traditional instruments used the horizon, but now we have gyroscopes, gravity sensors and satellite surveys of the Earth’s density and magnetic field, none of which require GPS or networking. You don’t need a wide field of view either, you just need an enough pairs of stars that the plate solver can provide an accurate reading of what in the celestial sphere the camera is actually looking at.
      - deleted 1y ago
        [deleted]
throw0101d 1y ago
> If you have an accurate true north compass reading, an accurate clock and assume that you’re at sea level, star positions give you all the information you need to find your location on earth. After stopping for a little while, the US Naval College brought back sextant training: * https://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back-navigation-by-the-stars-for-officers https://www.npr.org/2016/02/22/467210492/u-s-navy-brings-bac...

perihelions 1y ago

I'm not following your argument; magnetic north and celestial north are redundant. I don't understand what you think a magnetic orientation adds to a star tracker. The stars look exactly the same from everywhere on Earth. The only thing looking at them can tell you is an absolute rotational orientation—nothing more. Old-style navigation was based on the differential orientation of stars relative to the local orientation of the Earth's apparent horizon, but that measurement's not accurate enough for modern GNSS. (I don't think). I don't know how the SR-71 guidance works, but your link describes it as having an inertial component.

Amezarak 1y ago
I may be betraying my ignorance here but I don’t understand what you mean when you say the stars look the same everywhere on earth. You can definitely determine latitude from what stars are visible in the night sky along with their position in the sky, as, for example, the Southern Cross is only visible in the southern hemisphere; the converse is famously true for the North Star. It makes sense to me you don’t gain anything from having a compass (assuming clear nights) though, since you have the celestial north. Google claims that human-based celestial navigation done correctly can be accurate to 1 nautical mile, which seems good enough for navigation but not for any weapons targeting. Definitely way worse than GNSS. I would assume there are automated systems out there that can reduce the sources of error together get it down to less than that but probably not several orders of magnitude.
- perihelions 1y ago
  > "for example, the Southern Cross is only visible in the southern hemisphere" It's in the same place everywhere; it's the position of the Earth, which you're implicitly measuring by pointing out that it's obstructing your view, that's different. The things above the horizon are visible; the things below the horizon are not. That's not facetious; the core idea of this type of navigation is to measure the position of the horizon with high precision, which is hard. Very hard if you're surrounded by mountains. ("Which hemisphere am I on?" is... low precision, if easy to measure). Those classic methods aren't pure astrometry: they're differential astrometry that measure the horizon against stars, the relative orientation of the two.
  - teamonkey 1y ago
    You don’t need to see the horizon at all, you just need to establish a vector that points Up. Traditional instruments used the horizon, but now we have gyroscopes, gravity sensors and satellite surveys of the Earth’s density and magnetic field, none of which require GPS or networking. You don’t need a wide field of view either, you just need an enough pairs of stars that the plate solver can provide an accurate reading of what in the celestial sphere the camera is actually looking at.
    - deleted 1y ago
      [deleted]

throw0101d 1y ago

> If you have an accurate true north compass reading, an accurate clock and assume that you’re at sea level, star positions give you all the information you need to find your location on earth. After stopping for a little while, the US Naval College brought back sextant training: * https://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back-navigation-by-the-stars-for-officers https://www.npr.org/2016/02/22/467210492/u-s-navy-brings-bac...