Two nights ago I found myself at 38,000 feet over the Atlantic enroute to France. Since my 2-year-old daughter seatmate on my right was finally asleep, I snuck a quick glance through the window to see if I could glimpse any stars. A few were visible, but not enough for me to detect any of the five constellations I (sort of) know.
Too bad, because my seatmate on my left was not asleep, and he and I had been talking about star tracking as an aircraft guidance tool. During the course of our conversation, Bart mentioned that back during the mid-sixties, he had worked for Northrop on the guidance platform for the then ultra-secret SR-71 Blackbird spyplane. Being the complete airplane geek that I am -- and knowing that the SR-71 is hands-down the coolest aircraft to ever fly -- I became fascinated with Bart's story.
Since client of the original SR-71 program (at that time simply called "Oxcart") was the CIA, and the CIA's only interest in the aircraft was its ability to take extremely precise photos at extremely precise coordinates, it was critical that the plane always knew exactly where it was. Inertial guidance systems of the time were okay, but nowhere near good enough for Oxcart's mission. Enter Northrop and the star-tracker.
In order for Oxcart to pinpoint its position over the earth down to a matter of yards, Northrop developed and installed a star-based guidance system. So by looking up, this system could tell the plane exactly where to look down.
Recalling my recent conversation with astronomer Ron Marzke, I asked Bart how the star-tracker actually "looked" for stars. Did it scan a wide slice of sky, searching for a specific constellation? Did it focus on a particular piece of the heavens and wait for a certain star to pass by? It turns out neither. Here's the way it worked: once in stable flight miles above the earth, Oxcart's inertial guidance system fed rough coordinates to the star-tracker.
The star tracker then began to sweep it's narrow-beam telescope over the patch off sky where it expected to find a given bright star, say Sirius, for example. Once it located a celestial object of the right intensity to potentially be Sirius, it locked that in, then started another narrow sweep where it expected another star to be. If it found that, the pattern repeated until it had one-by-one located enough objects of the right intensity in the right location to "know" exactly where it was. The system then fed this precise location to the aircraft's pilot so he could fly precisely the desired ground track, and to the cameras so that they could take their pictures at the right time.
Presto: smile Ivan (or Muamar, Nassar, or Kim Ill) you've had a Kodak moment.
Now I think that having a chance to sit for a couple hours next to anyone who was intimate with the SR-71 is exciting enough, but what made this conversation particularly serendipitous was that I had earlier in the flight been reading up on the human vision system. It turns out that there is an small area on the human eye's cornea that has the greatest concentration of light receptors. This area is called the fovea, and is the only part of our eye that sees in detail. The fovea is in fact so small that our detailed field of view is no bigger than the size of our thumbnail held out at arm's length. (The reason we think we're seeing so much more of our world in detail is because our eyes are constantly making tiny movements and piecing together a bigger picture.)
What got to me was when Bart said that the Oxcart's star-tracking system's field of view was... well, about the size of a thumbnail held out at arm's length.
Funny how things just fit sometimes.