NavList:

Hewitt you wrote:
"Speaking of dim, I have a vague notion that at one time Air Force fighter/bombers were equipped with a navigation device that automatically tracked three stars day or night to give a cockpit position readout."

Large high-altitude USAF aircraft have these, including bombers. They're star tracker systems. The early versions were basically theodolites equipped with narrow field small telescopes that would slew to the expected position of the star, based on an input estimated position and orientation, and then "zero in" by finding the brightest spot with a photoelectric sensor. With a digital camera system, you can get rid of the mechanical slewing system since you can use a wide field optical system. If it's dark, you don't even need an estimated position since you can pattern match off the whole field of stars. These things can get a very accurate fix on the absolute orientation of the aircraft relative to the distant stars. To turn that into a position, they need a vertical which means the star tracking system has to be hooked into an inertial navigation system or something similar (basically just a refined electronic bubble level with a high-pass filter to average out the motions of the aircraft).

At high altitude, there's much less trouble seeing the stars in daylight. At sea level, it's a tougher problem. It's odd, really. The stars are always there -all of them --even in the middle of the brightest day-- unless their light is actually obscured by clouds or scattered by haze. If I take a digital photo of the sky, it's a question of finding those pixels that are slightly brighter than the others. We have a signal to noise problem. Of the forty photos I took yesterday of the Moon, a slim which I really could not find without the pointing aid from the software on my phone, only a couple failed to show Venus clearly. It was bright and obvious in many of the photos but quite invisible to the naked eye. So how low can we go? What is the faintest star detectable from sea level in digital photos taken with an off-the-shelf digital SLR camera in good daylight conditions? If we can get down to magnitude 1.5, that's roughly ten to twelve stars and planets at a time. Four or five would be close enough to the horizon to image in the same frame with the horizon (and high enough to be clearly visible) without using exotic lenses.

Needless to say, there's still that big nagging question: WHY?! Why do this?? This would be a lot of fun to see in action, and it might be interesting to program. If there's enough entertainment value in it, it might come about regardless of practicality. If it needs a practical excuse, I suppose if it could be sufficiently off-the-shelf and really easy to use, it might serve as that backup for "the day all the satellites die" (sounds like a movie). We will see what turns up.

-FER

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