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The SR-71 Blackbird Astro-Nav System worked by tracking the stars

theaviationgeekclub.com

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Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#161

Post by dekhn »

malfist wrote: post #15

When I first heard of astro-nav (for missiles) I thought it was so future sounding, like star trek. But now that I do astrophotography regularly it's just another tool that I use. It's super simple to do, I can take a photo of the night sky, and if I know the focal length and pixel size of my camera I can figure out exactly where my telescope is pointed in seconds, with an accuracy 2.5 arcseconds. You can even do it…

One of the clearest signs we live in a pinnacle of technology is that mathematicians and astronomers have been refining the equations that produce ephemerides for thousands of years, and the inverse problem (given star locations and time, predict location and pointing vector) to the point where an average human with average hardware can instantly estimate things that would have taken the world's most powerful observatories a great deal of effort to do.

https://en.wikipedia.org/wiki/Fundamental_ephemeris

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#162

Post by westmeal »

malfist wrote: post #87
blackhaz wrote: post #20

Earlier quoted context omitted.

OK. Now please do the same with a Z-80 class CPU.

I don't know the technical specs of the Z-80, but even on a minipc's ancient and super cheap CPU (I think it's a 2 point something ghz dual core) it can do a solve in seconds. If you know roughly where you're looking, the lookup is really quite quick.

GHz? Try 2 MHz.

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#163

Post by stevenwoo »

jpm_sd wrote: post #43
porphyra wrote: post #39

How does it see the stars? Even modern CMOS image sensors cannot see the stars during the day.

Probably by flying above most of the atmosphere.

The navigator verified their position with the device while they were still on the ground.

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#164

Post by dekhn »

defrost wrote: post #116
freyfogle wrote: post #53

Earlier quoted context omitted.

We had a great example of this in a recent Geomob London talk: ask Google Maps to route from Wairiki, Fiji (-16.8048, 179.9893) to Welagi, Fiji (-16.7349, -179.9439), two villages on the coastal road of the same island and about 10km apart, but as you can see from the coordinates on opposite sides of the anti-meridian. You get "Sorry, we could not calculate driving directions from "Wairiki, Fiji" to "Welagi, Fiji" ht…

It astounds me that this is still a problem in 2023. I company I coded for surveyed the entire Fiji region in 1997, drafting aircraft at 80m ground clearance in long lines across both sides of the 180 longitude line. Every single bit of commercial US GIS software available at the time was garbage and failed to correctly compute line lengths that straddled the 180 longitude line, areas of rectangles that did the same,…

Your software was written in the Buffyverse?

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#165

Post by pyinstallwoes »

labcomputer wrote: post #130
TheRealPomax wrote: post #119

[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…

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…

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]).

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#166

Post by avs733 »

missedthecue wrote: post #109
13of40 wrote: post #101

I'm going to replay this from memory without googling it, just to keep the late 20th century air of some people swapping unverifiable facts in a pub: There was an old friend of my mother's, now deceased, who was a retired engineer. He claimed to be the guy who invented the 2-part tuna can, and was known for handing out end chunks of titanium castings left over from some business he ran. At any rate, he had a couple o…

My father worked on the guidance system for the Trident nuclear missile at Singer-Kearfott for many years and this is mostly accurate. There was a lot of geometry involved. They used to test launch the missiles back during the cold war from submarines off of California. The missile would cross the continental US and land in the water off Florida. The Soviets would invariably send "fishing" boats to measure the tests…

As far as I’m aware test shots of both us Air Force and navy icbms off the west coast went to Kwajalein atoll.

Some other interesting details…

Naval icbms have star tracking for alignment and correction but land based ones do not.

The reasoning is that they are all dead reckoning…which requires knowing the initial location. The silo doesn’t move, but the subs do. Their location is tracked with another inertial guidance system but that adds error, so they use the star tracking.

Overall, the inertial navigation systems in modern icbms are not the limited in accuracy.

My favorite…there was a lot of pushback inside the us military on investing in increasing the accuracy of naval ICBMs. Doing so was seen as communicating/implying a shift in strategic goal from targeting cities (lower required accuracy) to targeting hardened facilities (higher required accuracy). That was seen as confrontational because sun based nuclear weapons had been generally treated as a second strike capability, but the primary reason to have the ability to target hardened facilities was for a first strike

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#167

Post by Merad »

dmix wrote: post #151
jimt1234 wrote: post #103

Earlier quoted context omitted.

There's an A-12 Blackbird at the San Diego Air & Space Museum in Balboa Park. Not an expert, but my understanding is the A-12 was basically the beta version that eventually lead to the SR-71. https://sandiegoairandspace.org/collection/item/lockheed-a-1...

That looks a bit smaller than I expected, from my uninformed opinion. Maybe it's just the angle of the photos. How did it fly for 10hrs (based on the parent article) at high speeds without a ton of space for fuel?

Lots of aerial refueling. 10 hours in the air, yes, but not on a single tank of fuel.

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#168

Post by dwighttk »

dmix wrote: post #151
jimt1234 wrote: post #103

Earlier quoted context omitted.

There's an A-12 Blackbird at the San Diego Air & Space Museum in Balboa Park. Not an expert, but my understanding is the A-12 was basically the beta version that eventually lead to the SR-71. https://sandiegoairandspace.org/collection/item/lockheed-a-1...

That looks a bit smaller than I expected, from my uninformed opinion. Maybe it's just the angle of the photos. How did it fly for 10hrs (based on the parent article) at high speeds without a ton of space for fuel?

"The SR-71 needed to be able to fix its position within 1,885 feet (575 m) and within 300 ft (91 m) of the center of its flight path while traveling at high speeds for up to ten hours in the air."

They refueled in-flight, and the high speeds they are referring to probably weren't sustained for 10 hours... I think it is more like "while traveling at high speeds and for up to ten hours"...

Re: The SR-71 Blackbird Astro-Nav System worked by tracking the stars

#170

Post by snypher »

Tommstein wrote: post #160
someplaceguy wrote: post #138

Earlier quoted context omitted.

> 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-14...

> 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 ... 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.

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.
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