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Background

In the early days of my interest in weather satellites, I was not familiar with the sorts of images that might be obtained from satellites, and it was the depths of winter so the amount of daylight information was minimal - hence the need for a ground track prediction program.  There was also the problem that whilst information was readily available for the NOAA satellites, there is a great scarcity of information on Meteor and Resurs satellites, not to mention less frequent visitors like Okean and Sich.   I started out using the WinOrbit program, which is an excellent piece of freeware, but I didn't like its multiple window arrangement.  I could find no software that did ground track prediction.

My starting point was the Turbo Pascal software that Dr T S Kelso (at: http://www.celestrak.com) chose to make available.   Once I had understood this, and reproduced the test cases correctly, I was able to extend the software into the more object-oriented model used by Borland's Delphi, and predict orbits.  It took a little more maths to predict the ground track of the sensor in the satellite - think of it as a broom sweeping across the ground.

The final step was to display the ground track in a meaningful way.   Fortunately, I knew that there was a database on the Web giving the height all over the world at 5 minute intervals - not too dissimilar to the satellite resolution.   Once I had worked out the maths, I combined the ground sweep with another program I had written to produce what looks like a map page tailored to the exact co-ordinates sweep out by a satellite during its pass.  The final problem to solve before distributing the program was how to distribute the topography database.  It is 18MB uncompressed, 11MB as a Zip file.  Even if I gave the URL, I know this size of download would put people off.  So I decided that since absolute accuracy wasn't essential, I could simply distribute the database as a large image, which could be compressed into a JPEG for distribution.  By choosing carefully, I created a high-resolution and low-resolution map that occupies only 98KB and 34Kb respectively.  These images are 1800 x 900, and 900 x 450 pixels in size.

For registered users, there are now a number of background options, including the ability to have an alternative background, and to have the background regularly updated from an external source.