When you turn on your GPS receiver - what happens? Well, lots of stuff. But primarily, GPS receivers work in a two stage process:
- Look for available satellites to track
- Do everything else
What I do want to discuss is how long you can use an almanac for analysis. Your receiver will usually download a new almanac when it sees that a new one is available, so it always has the freshest data. When you do analysis though, sometimes you may not have the latest almanac (or the one correct for the time of analysis - a related problem). So, is it ok to use any old almanac for analysis? Since they are not the most accurate ephemeris representations, should I be using them for analysis anyway? I'll answer these questions and show some interesting graphics that bring the point home.