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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

#121

Post by defrost »

dheera wrote: post #75
_moof wrote: post #35

Earlier quoted context omitted.

The International Date Line and the 180-degree line of longitude have bitten a _lot_ of folks who don't think to test at the boundaries.

Why didn't they just use UTC for everything? No time zone no bite

It's a longitude issue, not a time zone problem.

The distance along the zero latitude equator from -179.99 W to +179.99 E should be relatively easy to compute and yet a large proportion of software in the mapping domain screws that up - even software that controls drones and| used by commercial pilots in the US, etc.

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

#123

Post by pests »

seryoiupfurds wrote: post #91
mk_stjames wrote: post #86

Earlier quoted context omitted.

Interesting, is the ring road around Tavuki Island THE only road that falls on the anti-meridian, at all? A quick look along the line and it looks like it. It's wild that on google maps, even dropping a pin on the Tavuki road and trying to drag it across the line, it just stops, hitting this invisible wall. Something about this experience makes me want to go there....

Wow, even zooming in and out on that island makes Google Maps skip into the middle of the ocean. Edit: and the nearby Rabi Island has a discontinuity at the meridian in map view. Zooming in on it in satellite view sent the map into a crazy refreshing state with flickering grid lines.

These real-life effects by our decisions in technology sometimes half way around the world are very interesting to me. Similar to how some poeple are unlucky enough to live in their communities geographic center that becomes the default location for GPS lookups for that location.

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

#124

Post by photoGrant »

gmiller123456 wrote: post #108
porphyra wrote: post #39

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

Modern, and even old, sensors absolutely can see stars during the day. It's really a question of magnification. When you magnify the image, the pixels a star falls on don't change much, but the background gets darker because the background is spread over more pixels. I was talking to a grad student who was having an issue getting descent flat frames (images of a uniform field, used to account for dust and optical tra…

Put a lens cap on! Then shoot.

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

#125

Post by sonofhans »

demondemidi wrote: post #122

I don't think they used enough rivets. https://theaviationgeekclub.com/wp-content/uploads/2023/09/S...

That’s basically the law in aviation — rivet everything. My college roommate was studying to be a pilot, and I swear half his first year he did nothing but rivet.

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

#126

Post by junofan »

elzbardico wrote: post #61
jacurtis wrote: post #32

> the flight plan was recorded on a punched tape that told the aircraft where to go, when to turn, and when to turn the sensors on and off. This is equally fascinating to me. Before each flight, someone had to generate a flight plan and punch it onto a physical tape, then load it into the plan. T No mid-flight "re-calculating route" if stuff goes wrong. One of the most interesting and under-appreciated aspects of ear…

America was not critical defeating Germany. The soviets did this one.

Strange considering Jodl surrendered to Eisenhower.

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

#128

Post by doctor_eval »

defrost wrote: post #121
dheera wrote: post #75

Earlier quoted context omitted.

Why didn't they just use UTC for everything? No time zone no bite

It's a longitude issue, not a time zone problem. The distance along the zero latitude equator from -179.99 W to +179.99 E should be relatively easy to compute and yet a large proportion of software in the mapping domain screws that up - even software that controls drones and| used by commercial pilots in the US, etc.

Ok so to clarify it’s not the international date line but the latitude? Which makes more sense to me because the international date line is not actually a straight line.

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

#129

Post by gibspaulding »

For anyone interested in the SR-71 (and other Cold War era spy planes), Skunkworks by Joe Rich is a fascinating read with lots of fun details about their development.

Also, fun fact - in 2025 we will cross over into a period of history where the SR-71 first flew closer to the Wright Flyer than to the present day.

Edit: Ben Rich, not Joe Rich. Thanks runjake!

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

#130

Post by labcomputer »

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 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=23238437

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