The only thing I needed to add was the ability to determine the apparent magnitude (brightness) of an object, and I'd be set. Gpredict had almost everything I needed, the SGP model implementation, the prediction logic, etc. I couldn't find a suitable or mature library in python or PHP implementing the SGP models, so I decided to port code from Gpredict, a real time satellite tracker application written in C. Kelso, who runs Celestrak, has a great series of columns explaining all of this in detail. (NOTE: Since these models trade computational speed for accuracy, you need to update your TLE files every few days to make sure your predictions are accurate). The TLE data can then be used to predict a satellite's position and velocity using Simplified Perturbations Models (SGP/SGP4/SDP4). (You can download these very small TLE files through sites like Celestrak). It then distributes these observations as Keplerian orbital elements in the Two Line Element Set (TLE) format. NORAD, which has responsibility for tracking all man-made objects in orbit around Earth, uses the the Space Surveillance Network for collecting observational data of these objects. But before you can do this, you first need to know the orbital path of the satellite. Then, you need to convert those coordinates to a topocentric-horizon or azimuth-elevation coordinate system that can be used to pinpoint the object in the sky. The process involves calculating the coordinates (position and velocity) of the observer, the satellite, and the sun at a given point in time using a common coordinate system called Earth Centered Intertial (ECI). So, the first step was to figure out how to to generate the predictions myself. So I looked around for existing data APIs that I could use, but couldn't find anything other than scraping existing sites (which is not only fragile, but some have no published data policy). The biggest piece of my project was going to be getting the actual satellite data. So a few months ago I decided to try and build my own. I started looking at existing software to help remind me (and others) when the next pass of the ISS would be, and while there are a few good websites out there for satellite prediction, none of them did exactly what I wanted. Ever since the first time I saw the International Space Station fly by, I've felt that it is something everyone should experience. It's not only a technical marvel to enjoy, but for me it helps keep things in perspective and my thoughts on the big picture. Seeing something in space fly overhead with the naked eye is pretty awesome.
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