5 ms·

[it] was so powerful that it could see the stars even in daylight That's a very odd thing to say: when you're at 85,000 feet (as noted, the SR-71's cruising al

[it] was so powerful that it could see the stars even in daylight That's a very odd thing to say: when you're at 85,000 feet (as noted, the SR-71's cruising altitude), the concept of a blue "daylight" that blocks the view of space only exists below you, not above you [1]. There is not enough Rayleigh scattering at that height to get in the way of a camera looking for stars to map to a star chart. It's pretty much why they used star navigation: they're always visible when you're halfway to space. [1] http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

labcomputer 3y ago
I don’t think that’s quite true. There are actually some interesting patents related to the star sensor in the devices, particularly related sky background gradients. Northrop’s patents are more interesting than Lockheed’s, IMHO. In all cases the patents mention using an IR-pass filter to improve contrast. The sensor is essentially an analog lock-in/synchronous detector with a rotating shutter and a wedge prism that nutates the star field about the boresight. the patented component in most cases is the shutter (there are different patterns, and Northrop came up with some clever designs). You end up with a frequency modulated signal from the photomultiplier tube (carrier is from the shutter, modulation frequency the difference between the prism and shutter). By measuring the phase and magnitude of the modulated signal, you can steer the telescope onto the star, and the “coding gain” from the lock-in is substantial. Edit, previous comments: https://news.ycombinator.com/item?id=27084261 https://news.ycombinator.com/item?id=27084261 https://news.ycombinator.com/item?id=23238437 https://news.ycombinator.com/item?id=23238437
- pyinstallwoes 3y ago
  I have recently been bitten by the modular synthesis bug, and it is wild to me how these terms seem to have correspondence even though I am learning it from the audio domain. I guess a signal is a signal and that is the power of analog. For context: learning about carriers, modulation frequency in relation to audio synthesis [acknowledging there is more than one type of 'FM' in relation to audio but nonetheless similar]).
ezconnect 3y ago
They also need an initial fix before take off and that could be day or night depending on the mission.
- frud 3y ago
  I think that in order to calculate your latitude and longitude position using what amount to high-tech sextant readings you have to know your local down vector to a high degree of accuracy. That is, the system judges the position of the Blackbird by comparing its local down vector with the angles to the star sightings. If the down vector measurement is off by an arcsecond, the measured latitude and longitude position on the earth will also be off by an arcsecond (at sea level, about 600 feet). It seems to me that there's an interesting problem that the Blackbird goes so fast that its down vector changes dramatically over the course of a typical flight, so I'm wondering how it accurately updates its down vector reference. Jets can go through arbitrary and wobbly 3-d trajectories, accelerating in any direction, so it can't be a simple measurement of gravity. It would have to be a accelerometer combined with inertial and gyro and elevation readings all summed up.
  - umeshunni 3y ago
    What does 'down vector' mean in this context?
    - frud 3y ago
      A very precise estimation of which direction gravity is pulling down towards. Which is almost, but not exactly, the vector towards the center of the earth. And it also needs to be disentangled from the suborbital centrifugal/centripetal forces experienced by the movement of the Blackbird.
  - ezconnect 3y ago
    Can it just look at the front and back horizon to estimate the local down vector?
    - 3y ago
someplaceguy 3y ago
> when you're at 85,000 feet (as noted, the SR-71's cruising altitude), the concept of a blue "daylight" that blocks the view of space only exists below you, not above you The Astro-Nav System didn't work only at cruising altitude, it tracked stars even when the SR-71 was on the ground. [1] [1] https://airandspace.si.edu/webimages/collections/full/NAS-14V2%20ANS%20System.pdf https://airandspace.si.edu/webimages/collections/full/NAS-14...
- Tommstein 3y ago
  > The Astro-Nav System didn't work only at cruising altitude, it tracked stars even when the SR-71 was on the ground. [1] > > [1] https://airandspace.si.edu/webimages/collections/full/NAS-14 https://airandspace.si.edu/webimages/collections/full/NAS-14... I don't necessarily doubt you, but linking to a 76-page PDF without any indication of where the reader is supposed to find support for your claims is not nearly as convincing as you think it is.
  - snypher 3y ago
    The whole document supports the claim, it details ground astroinertial startup which would not occur if it could not see stars from the ground. The fact they made this document available is very useful to those who are actually invested in the subject.
  - someplaceguy 3y ago
    > I don't necessarily doubt you, but linking to a 76-page PDF without any indication of where the reader is supposed to find support for your claims is not nearly as convincing as you think it is. It's funny you say that, because based on almost all the assertions you read here on HN which are taken for granted without any links to support them, you'd think linking to an authoritative PDF would be more convincing than if I hadn't linked to it. But based on your comment it seems like it works the opposite way (unless I point to the exact place supporting my assertion, presumably). But sure, fine, I'll take your bait. I only skimmed the document, so I can't point you to the best place where to look for it. But I can point you to page 31, paragraph 10A-69 which says the following: "When alignment is performed in a hanger, there is a possibility that the system will track false stars (ceiling lights, etc). To prevent this from happening, the INERTIAL ONLY mode is selected and enabled after completion of an alignment. After the airplane taxies into the open, ASTRO INERTIAL mode is selected and enabled". I'm pretty sure this supports my assertion.
nullc 3y ago
if you use a SWIR camera stars are pretty visible in the daytime even at ground level. Scattering is related to a fourth power of frequency so by the time you're down to IR the amount of scattered light is pretty low. (which is also why the sky is blue-- or really UV, if we could see it) The software for plate solving is pretty good. I've had some fun taking movie screenshots and figuring out if the stars are real, and if they're the right hemisphere. (can't be used for location without the time and orientation of the camera)

labcomputer 3y ago

I don’t think that’s quite true. There are actually some interesting patents related to the star sensor in the devices, particularly related sky background gradients. Northrop’s patents are more interesting than Lockheed’s, IMHO. In all cases the patents mention using an IR-pass filter to improve contrast. The sensor is essentially an analog lock-in/synchronous detector with a rotating shutter and a wedge prism that nutates the star field about the boresight. the patented component in most cases is the shutter (there are different patterns, and Northrop came up with some clever designs). You end up with a frequency modulated signal from the photomultiplier tube (carrier is from the shutter, modulation frequency the difference between the prism and shutter). By measuring the phase and magnitude of the modulated signal, you can steer the telescope onto the star, and the “coding gain” from the lock-in is substantial. Edit, previous comments: https://news.ycombinator.com/item?id=27084261 https://news.ycombinator.com/item?id=27084261 https://news.ycombinator.com/item?id=23238437 https://news.ycombinator.com/item?id=23238437

pyinstallwoes 3y ago
I have recently been bitten by the modular synthesis bug, and it is wild to me how these terms seem to have correspondence even though I am learning it from the audio domain. I guess a signal is a signal and that is the power of analog. For context: learning about carriers, modulation frequency in relation to audio synthesis [acknowledging there is more than one type of 'FM' in relation to audio but nonetheless similar]).

ezconnect 3y ago

They also need an initial fix before take off and that could be day or night depending on the mission.

frud 3y ago
I think that in order to calculate your latitude and longitude position using what amount to high-tech sextant readings you have to know your local down vector to a high degree of accuracy. That is, the system judges the position of the Blackbird by comparing its local down vector with the angles to the star sightings. If the down vector measurement is off by an arcsecond, the measured latitude and longitude position on the earth will also be off by an arcsecond (at sea level, about 600 feet). It seems to me that there's an interesting problem that the Blackbird goes so fast that its down vector changes dramatically over the course of a typical flight, so I'm wondering how it accurately updates its down vector reference. Jets can go through arbitrary and wobbly 3-d trajectories, accelerating in any direction, so it can't be a simple measurement of gravity. It would have to be a accelerometer combined with inertial and gyro and elevation readings all summed up.
- umeshunni 3y ago
  What does 'down vector' mean in this context?
  - frud 3y ago
    A very precise estimation of which direction gravity is pulling down towards. Which is almost, but not exactly, the vector towards the center of the earth. And it also needs to be disentangled from the suborbital centrifugal/centripetal forces experienced by the movement of the Blackbird.
- ezconnect 3y ago
  Can it just look at the front and back horizon to estimate the local down vector?
  - 3y ago

someplaceguy 3y ago

> when you're at 85,000 feet (as noted, the SR-71's cruising altitude), the concept of a blue "daylight" that blocks the view of space only exists below you, not above you The Astro-Nav System didn't work only at cruising altitude, it tracked stars even when the SR-71 was on the ground. [1] [1] https://airandspace.si.edu/webimages/collections/full/NAS-14V2%20ANS%20System.pdf https://airandspace.si.edu/webimages/collections/full/NAS-14...

Tommstein 3y ago
> The Astro-Nav System didn't work only at cruising altitude, it tracked stars even when the SR-71 was on the ground. [1] > > [1] https://airandspace.si.edu/webimages/collections/full/NAS-14 https://airandspace.si.edu/webimages/collections/full/NAS-14... I don't necessarily doubt you, but linking to a 76-page PDF without any indication of where the reader is supposed to find support for your claims is not nearly as convincing as you think it is.
- snypher 3y ago
  The whole document supports the claim, it details ground astroinertial startup which would not occur if it could not see stars from the ground. The fact they made this document available is very useful to those who are actually invested in the subject.
- someplaceguy 3y ago
  > I don't necessarily doubt you, but linking to a 76-page PDF without any indication of where the reader is supposed to find support for your claims is not nearly as convincing as you think it is. It's funny you say that, because based on almost all the assertions you read here on HN which are taken for granted without any links to support them, you'd think linking to an authoritative PDF would be more convincing than if I hadn't linked to it. But based on your comment it seems like it works the opposite way (unless I point to the exact place supporting my assertion, presumably). But sure, fine, I'll take your bait. I only skimmed the document, so I can't point you to the best place where to look for it. But I can point you to page 31, paragraph 10A-69 which says the following: "When alignment is performed in a hanger, there is a possibility that the system will track false stars (ceiling lights, etc). To prevent this from happening, the INERTIAL ONLY mode is selected and enabled after completion of an alignment. After the airplane taxies into the open, ASTRO INERTIAL mode is selected and enabled". I'm pretty sure this supports my assertion.

nullc 3y ago

if you use a SWIR camera stars are pretty visible in the daytime even at ground level. Scattering is related to a fourth power of frequency so by the time you're down to IR the amount of scattered light is pretty low. (which is also why the sky is blue-- or really UV, if we could see it) The software for plate solving is pretty good. I've had some fun taking movie screenshots and figuring out if the stars are real, and if they're the right hemisphere. (can't be used for location without the time and orientation of the camera)