Last night (about 8:20 PM Pacfic time, Sept 13) I was watching the ISS pass above where I lived and noticed that it passed very close to a star in the constellation Bootes and another star in the Big Dipper. I was curious how close Celestia would be in simulating what I actually saw. I downloaded and input the latest ISS orbital elements and ran Celestia. Celestia did a good job of similulating the pass of the star in Bootes, but missed predicting the pass of the star in the Big Dipper (the prediction was higher in the sky than where it actually was). I was curious whether this might be do to:
1) limitations in the accuracy of the orbital elements
2) limitations in the prediction software, or
3) something else??
Thanks
Accuray of ISS predictions
Dan,
Please read the "Preliminary User's FAQ", especially Q/A#23
It's mostly due to limitations in the shape of the model of the Earth. Celestia models latitude, longitude and altitude positions on the Earth as if it were a sphere. It isn't. The ISS is in a very low orbit. As a result, even small differences in positions of the Earth's surface will cause large differences in the observed direction of the ISS.
Sorry.
Please read the "Preliminary User's FAQ", especially Q/A#23
It's mostly due to limitations in the shape of the model of the Earth. Celestia models latitude, longitude and altitude positions on the Earth as if it were a sphere. It isn't. The ISS is in a very low orbit. As a result, even small differences in positions of the Earth's surface will cause large differences in the observed direction of the ISS.
Sorry.
Selden
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Topic authorGuest Dan
There's no reason you couldn't try enabling the oblate feature. Please let us know if it helps.
I seem to recall that Celestia's "atmosphere" function was being drawn as a sphere and didn't conform to the surface of an oblate spheroid. I know that Chris was working on the problem, but I'm not sure if it was completely fixed.
Unfortunately, the earth isn't a simple oblate spheroid, either. For example, the generation of accurate maps requires the use of multiple offset oblate spheroids, using different shapes and offsets for different locations. One discussion of this is at http://exchange.manifold.net/manifold/manuals/5_userman/mfd50The_Earth_as_an_Ellipsoid.htm
I seem to recall that Celestia's "atmosphere" function was being drawn as a sphere and didn't conform to the surface of an oblate spheroid. I know that Chris was working on the problem, but I'm not sure if it was completely fixed.
Unfortunately, the earth isn't a simple oblate spheroid, either. For example, the generation of accurate maps requires the use of multiple offset oblate spheroids, using different shapes and offsets for different locations. One discussion of this is at http://exchange.manifold.net/manifold/manuals/5_userman/mfd50The_Earth_as_an_Ellipsoid.htm
Selden
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granthutchison
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Topic authorGuest Dan
As was suggested, I tried running Celestia with oblateness turned on. This made no perceptible difference in the Celestia predictions.
One other observation. I mentioned in my previous post, that Celestia did do a good job of predicting where the ISS was in Bootes. It appeared to me that at particular moment, the angle formed by ISS, me, and the horizon was about 90 degrees. As ISS moved further north toward the Big Dipper and the horizon, the viewing angle became more oblique and Celestia was not "bending" down the ISS orbit enough to predict what I was seeing.
This is probably just another way of describing what Seldon was talking about.
Dan
One other observation. I mentioned in my previous post, that Celestia did do a good job of predicting where the ISS was in Bootes. It appeared to me that at particular moment, the angle formed by ISS, me, and the horizon was about 90 degrees. As ISS moved further north toward the Big Dipper and the horizon, the viewing angle became more oblique and Celestia was not "bending" down the ISS orbit enough to predict what I was seeing.
This is probably just another way of describing what Seldon was talking about.
Dan