3 ms·

I wonder if it'd make sense these days to put optical navigation beacons in space. This OP solution relies on inertial navigation, because, the star-tracking pa

I wonder if it'd make sense these days to put optical navigation beacons in space. This OP solution relies on inertial navigation, because, the star-tracking part is only a reference for orientation in space, which is an incomplete coordinate set. But if you were to put *artificial* stars close to Earth, you'd gain an additional parallax measurement, and obtain a complete basis.

sneak 1y ago
How would they be powered to be bright enough to be visible? If you switched to parts of the EM spectrum where you don’t need high power output, you’re back at radio waves and you’ve simply reinvented GPS.
- wongarsu 1y ago
  Really bright geostationary satellites to reflect sunlight might work. At geostationary they should be far enough out that you always have line of sight to one that isn't in earth's shadow. You'd need to specifically design them to have a huge surface area to get enough brightness. Maybe that'd make them impractical
- perihelions 1y ago
  Optical isn't particularly power-inefficient, is it? The equivalent of an optical magnitude +5 star, in 500 km low orbit, is (if I didn't err) only 1 kilowatt of isotropic power in a solar spectrum. Much less if it's a non-isotropic beam pointed towards earth. +5 is the brightest ~1,600 stars, likely similar to whatever any star tracker would be using.
  - sneak 1y ago
    24kWh daily is a lot of energy. For reference, the current GPS satellites are 1000kg and transmit with 50W (1.2kWh daily). Here on the ground, for rough calculations, 50kWh is around 450kg in batteries alone. There might be lighter chemistries but this is just to get you in the ballpark. I think a 1kW light on 24/7 in orbit would need to be a very large vehicle (2-4x the size of the current Navstar satellites) with huge batteries and gigantic solar arrays. Falcon 9 can probably do it, FH definitely can. An RTG might work but those are out of fashion near Earth these days for obvious reasons. In any case it would be quite costly (you’d need lots) and its resistance to jamming would benefit your adversaries just as much as it would benefit you. Not to mention, the astronomers would hate you.
    - perihelions 1y ago
      I don't know why you're worrying about batteries; LEO satellites spend only a few minutes in Earth's shadow at a time. The current model (v2 mini) Starlink satellite has 105 meter^2 solar arrays, and there's thousands of those orbiting already. That should be in the 10's of kilowatts on each Starlink. (Those solar arrays are so large they're already artificial stars of their own, though they're not designed to be). https://en.wikipedia.org/wiki/Starlink#v2_(initial_deployment) https://en.wikipedia.org/wiki/Starlink#v2_(initial_deploymen... And to repeat what I might have underemphasized: this is an estimate of isotropic power, if you were radiating uniformly in all 4pi directions.
teamonkey 1y ago
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.

sneak 1y ago

How would they be powered to be bright enough to be visible? If you switched to parts of the EM spectrum where you don’t need high power output, you’re back at radio waves and you’ve simply reinvented GPS.

wongarsu 1y ago
Really bright geostationary satellites to reflect sunlight might work. At geostationary they should be far enough out that you always have line of sight to one that isn't in earth's shadow. You'd need to specifically design them to have a huge surface area to get enough brightness. Maybe that'd make them impractical
perihelions 1y ago
Optical isn't particularly power-inefficient, is it? The equivalent of an optical magnitude +5 star, in 500 km low orbit, is (if I didn't err) only 1 kilowatt of isotropic power in a solar spectrum. Much less if it's a non-isotropic beam pointed towards earth. +5 is the brightest ~1,600 stars, likely similar to whatever any star tracker would be using.
- sneak 1y ago
  24kWh daily is a lot of energy. For reference, the current GPS satellites are 1000kg and transmit with 50W (1.2kWh daily). Here on the ground, for rough calculations, 50kWh is around 450kg in batteries alone. There might be lighter chemistries but this is just to get you in the ballpark. I think a 1kW light on 24/7 in orbit would need to be a very large vehicle (2-4x the size of the current Navstar satellites) with huge batteries and gigantic solar arrays. Falcon 9 can probably do it, FH definitely can. An RTG might work but those are out of fashion near Earth these days for obvious reasons. In any case it would be quite costly (you’d need lots) and its resistance to jamming would benefit your adversaries just as much as it would benefit you. Not to mention, the astronomers would hate you.
  - perihelions 1y ago
    I don't know why you're worrying about batteries; LEO satellites spend only a few minutes in Earth's shadow at a time. The current model (v2 mini) Starlink satellite has 105 meter^2 solar arrays, and there's thousands of those orbiting already. That should be in the 10's of kilowatts on each Starlink. (Those solar arrays are so large they're already artificial stars of their own, though they're not designed to be). https://en.wikipedia.org/wiki/Starlink#v2_(initial_deployment) https://en.wikipedia.org/wiki/Starlink#v2_(initial_deploymen... And to repeat what I might have underemphasized: this is an estimate of isotropic power, if you were radiating uniformly in all 4pi directions.

teamonkey 1y ago

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.