Since the intended use of this program is Amateur Radio satellite tracking (or possibly naked-eye visual observation), great accuracy is not really necessary. Typical Amateur antenna systems (less than 20 dB gain) have beamwidths of 10 degrees or more. Thus, even a low-orbit satellite (at say 600 km altitude) need only be located to within 50 km or so!

There are 3 aspects to the accuracy of tracking predictions:

I. Numerical accuracy of calculations: This includes digital precision (loosely, rounding-off errors), uncertainty in parameter estimates, and the way in which parameters are incorporated into the model (i.e. some parameters may have more "leverage" to influence the results than others). For what it's worth, all calculations, including trig functions, in

A more serious limitation is that all times are rounded to the nearest second by the Windows date conversion routine I used. Since a satellite in orbit has an average velocity of about 8 km/sec, this implies a possible (apparent) position error of up to 8 km at the nominal time, depending on the reference epoch time and the actual time of calculation. I'm still thinking about how to deal with this.

II. Orbital model accuracy: There are varying degrees of detail in representing the orbital motion of a satellite, depending on the required accuracy.

III. Observer model accuracy: Once the satellite's position is known to the desired degree of accuracy, we need to compute where to look (or point the antenna). Once again, there are levels of accuracy here, depending on the model. Accuracy here of better than 1 km is possible, depending upon how well the observer coordinates are known.

IV. Other non-idealities: The apparent position of a satellite may differ significantly from the true position, due to atmospheric distortion, especially at radio frequencies. See the related discussion of the Horizon.

Other aspects of accuracy relate to the presentation of the results:

The Maps: The accuracy of the on-screen display depends not only on the resolution of the display adaptor (how many independent pixels exist on the screen, like 1024x768) but also on the underlying map information. The maps are all generated by expanding, contracting, or distorting an internal bitmap which has a resolution of about 1000x500 pixels. Thus, accuracy better than about 0.3 degrees (about 20 miles at the equator) is impossible. In practice there are some problems with the bitmap, particularly in the antarctic regions, and near the International Date Line. Small islands have also been omitted in many cases. So don't expect too much.

Furthermore, numerical values are generally rounded off for printing or display, to reflect a reasonable level of accuracy (a percent or so). Just because the computer calculates 14 or so digits, doesn't mean they are all

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