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Sun Sensors for Attitude Determination

I'm interested in developing a sun sensor (or maybe a horizon sensor) for a low-cost, amateur, high altitude project. However, I'm having some difficulty determining the best setup for the sun sensor. The majority of the sensing will occur over 20km (about 65,000 ft), so the effects of radiation from the sky can be considered minimal.

I was considering two methods:

The first involves 5 photocells mounted at 90 degree angles to each other (one pointed vertically), with a light shield mounted below allowing a half-angle of around 60 degrees (i.e. blocking the light from the earth, so long as the sensor is 30 degrees from vertical or less). Measuring the resistance of all 5 photocells should yield (with the appropriate calculations) the position of the sun wrt the photocells. One concern I have about this method is the large variation of performance of photocells (regardless of having the same part number), and would probably require some careful calibration.

The other method is to have a sheilded field of, say, X by X photocells and a single opening centered over it. This method is much more "coarse", expensive, and harder to make, but less suseptable to non-solar radiation. I'm aware that most "non-amateur" sun sensors use CCDs instead of photocells, but I believe that is a little beyond my experience (and budget).

Does anyone else have knowledge of other techniques involved in sun sensors? Are any photocells or other light sensors better (or more precise) than others? Also, are there any wavelength filters I should consider? Any info is greatly appreciated!

Thanks in advance! Dave

Reply to
David Harper
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Are you familiar with these papers:

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/steen

Reply to
Steen

The new CMOS imager chips are much easier to deal with than CCDs -- for example, they have on-chip A/D conversion, so the outside interface is all digital -- and also, if memory serves, quite a bit cheaper. Their imaging performance isn't as good as CCDs, but for seeing the Sun that's not important.

--
"Think outside the box -- the box isn't our friend."    |   Henry Spencer
                                -- George Herbert       | henry@spsystems.net
Reply to
Henry Spencer

snipped-for-privacy@gmail.com (David Harper) :

Using Google I found

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this is single axis but uses cheap leds as the sensors, this suggest that you can make a 3 by 3 or 5 by 5 sensor without breaking the bank.

Also lots more hits on:

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Reply to
Earl Colby Pottinger

I'm not sure I understand fully your need, but it did stimulate a thought. I'd consider the idea of an optical system designed to image the entire sky, but with a very poor focus so that the sun is spread across 30% or so of the image plane. (I'm kind of thinking a defocused fish-eye lens.) A quad-detector placed at this plane will provide four separate signals, the sum of which provides your reference level. Orienting the quad detector optimally against the ecliptic for the largest sensitivity, as the sun crosses over, may take some thought or experiment or both, but I imagine that this can be arranged into a small self-contained system. Whether or not it provides the precision or repeatability you need... is another story.

I'd tend to want to use a narrow part of the (sub)visible band with Si detectors, perhaps in the 800nm or 880nm (far enough above the bandgap that there is little temperature dependence of the wavelength response function) only because they are very good (good NF, stable, cheap, widely available, high impedance and low leakage against offset voltage, etc.) But I really have no experience here, at all.

I'm curious about the answers, so I'll read along with you.

Jon

Reply to
Jonathan Kirwan

Genrally, some form of calibration is requied. Since the sun is just so damm bright there really is little problem in designing one that works at sea level and then calibrating it on a fine day. But keep in mind thermal stablity from the start.

Greg

Reply to
greg

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