3 ms·

Compared to the rest of the engineering, I don’t think the computation is all that hard. It doesn’t need to be that fast since you only need to do a correction

Compared to the rest of the engineering, I don’t think the computation is all that hard. It doesn’t need to be that fast since you only need to do a correction maybe every 10-20 minutes or so. The math was innovative in that they invented the Kalman filter for these types of computations. You pick your star such that it’s by far the brightest one in its neighborhood. So the image processing is just finding the brightest pixel. From what I’ve read (mostly the excellent book “Inventing Accuracy” the main challenge is very precisely aligning the telescope with the “stable member” in the gyroscope.

supportengineer 3y ago
Did the units in question even have pixels?
- cameldrv 3y ago
  Not exactly. The SR-71 system had an expected position of the star, and then it would mechanically scan the telescope in a rectangular spiral scan pattern with a single pixel photodetector until it found a star with the expected brightness. This could take up to 22 minutes from a semi-cold start (roughly, you know your heading within 3 degrees, and your position within 1 degree lat/lon). [1] Once the system had shot a couple of stars and the IMU was dialed in, it could do an update in about 30 sec. The Trident I used a Vidicon tube (what they used to use for TV cameras before CCDs were invented) [2] The Trident approach was to shoot a single star once in the flight. The Trident II uses a CCD. The first version of the guidance system for that missile (The Draper Mark 6) used a 90x90 CCD [3]. That has been replaced with the Mark 6 Mod 1, which they say has increased resolution but they don't give a figure [4] [1] NAS-14V2 ANS System Manual, Page 10A-49 https://airandspace.si.edu/webimages/collections/full/NAS-14V2%20ANS%20System.pdf https://airandspace.si.edu/webimages/collections/full/NAS-14... [2] Inventing Accuracy, Page 290 [3] The Trident II (Mk-6) Guidance System, Page 1460 https://sci-hub.se/https://doi.org/10.2514/6.1991-2761 https://sci-hub.se/https://doi.org/10.2514/6.1991-2761 [4] Modifying the MARK 6 Guidance System Part 1 https://www.electronicdesign.com/home/article/21201730/modifying-the-mark-6-guidance-system-part-1 https://www.electronicdesign.com/home/article/21201730/modif...

supportengineer 3y ago

Did the units in question even have pixels?

cameldrv 3y ago
Not exactly. The SR-71 system had an expected position of the star, and then it would mechanically scan the telescope in a rectangular spiral scan pattern with a single pixel photodetector until it found a star with the expected brightness. This could take up to 22 minutes from a semi-cold start (roughly, you know your heading within 3 degrees, and your position within 1 degree lat/lon). [1] Once the system had shot a couple of stars and the IMU was dialed in, it could do an update in about 30 sec. The Trident I used a Vidicon tube (what they used to use for TV cameras before CCDs were invented) [2] The Trident approach was to shoot a single star once in the flight. The Trident II uses a CCD. The first version of the guidance system for that missile (The Draper Mark 6) used a 90x90 CCD [3]. That has been replaced with the Mark 6 Mod 1, which they say has increased resolution but they don't give a figure [4] [1] NAS-14V2 ANS System Manual, Page 10A-49 https://airandspace.si.edu/webimages/collections/full/NAS-14V2%20ANS%20System.pdf https://airandspace.si.edu/webimages/collections/full/NAS-14... [2] Inventing Accuracy, Page 290 [3] The Trident II (Mk-6) Guidance System, Page 1460 https://sci-hub.se/https://doi.org/10.2514/6.1991-2761 https://sci-hub.se/https://doi.org/10.2514/6.1991-2761 [4] Modifying the MARK 6 Guidance System Part 1 https://www.electronicdesign.com/home/article/21201730/modifying-the-mark-6-guidance-system-part-1 https://www.electronicdesign.com/home/article/21201730/modif...