> 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]
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.
> 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.
> 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).
Not at all, your comment was strictly better for including that link, thanks for including it. It was neat to briefly skim the PDF, even if I don't have time to read the 76 pages of technical documentation.
> 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.
Maybe. But that whole paragraph is hard to understand. How were they doing alignment in a hangar to start with, were these roofless hangars?
> How were they doing alignment in a hangar to start with, were these roofless hangars?
I don't think so.
You can read more about alignment in page 7, paragraph 10A-23 and the following pages. It'd usually involve using test equipment and manually inputting the current coordinates (e.g. heading, position, altitude, date/time, ...) into the system, from what I understand.
Regardless, the paragraph I mentioned in my previous comment clearly instructs to enable ASTRO mode after the alignment is completed and the airplane is taxiing in the open. It also mentions that in ASTRO mode, the system can confuse the ceiling lights of the hangar for stars (hence why it needs to be disabled inside the hangar), which means that ASTRO mode does track stars on the ground.
Now that's a money quote. The following sentence, which says that "If star tracking has not commenced before takeoff, it should start at an altitude where cloud cover and sky brightness conditions have improved.", leads one to believe that it was possible for the sky to be too bright to track stars from the ground sometimes (since it basically says exactly that), but it now seems clear ground tracking during the day was possible. Maybe it depended on the latitude, time of day, and position of the Sun relative to the stars (low latitude in the middle of the day without sufficiently bright stars far enough away from the Sun = sky too bright, higher latitude early or late in the day with sufficiently bright stars far enough away from the Sun = track from the ground, any other combination = results may vary). Actually, I just saw that the bottom of page 42 describes how to handle situations where the sky was too bright for specific stars.
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.
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...