Monday, June 25, 2007



GPS stands for Global Positioning System. People mainly use it in their cars for navigation—the GPS satellite system can pinpoint where you are on the globe. The system was initially designed for, and is operated by, the US military. To pretend you know all about the system, say all the following without batting an eyelid.

  • “It's all about satellites and offsets, you know! Four satellite signals compute your position in three dimensions and the time offset in the receiver clock.”

  • “GPS is everywhere! Soon you will have GPS built into your smartphone and, you know, you'll know whether I am in my bathroom or in my bedroom.” (This is not realistic, but remember you are bluffing!)

  • “It’s a wonderful thought, all those satellites out there monitoring where I am. The GPS Operational Constellation has 24 satellites orbiting Earth every 12 hours. The orbit altitude of those satellites is crazy! And with all those orbital planes equally spaced—60 degrees apart... in fact, they're inclined at about 55 degrees with respect to the equatorial plane.” (You're got to speak fast when you're saying this. Make sure you say the keywords—“orbital planes” and so on—loudly.)

  • When someone asks you how it's all controlled, say with a wave of the hand, “It's all from Colorado. The Schriever Air Force Base is where it's at.”

  • Now if someone gets inquisitive and asks how it all works, you'll need a good memory. Memorise the following: “Position is determined from multiple pseudo-range measurements at a single measurement epoch. The pseudo-range measurements are used together with SV position estimates based on the ephemeris data each SV sends. That's how they figure out where you are.”

    Simple, eh?

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