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GPS Works like Birthday Cards

Feb
2
2010

Recently I celebrated a birthday which got me thinking… You can loosely describe how GPS works by thinking about the way you receive birthday cards in the mail.

Ignoring for a moment that each card has the recipient address on it, let’s look at a bunch of birthday cards and consider the postmark date and the return address. Each friend is like a satellite, sending me a card. All of the cards arrive on my birthday. I can look at the postmark and make an estimate how far away I am from the sender by the amount of time between the postmark and my birthday. Let’s say I got the following cards with the following postmarks and return addresses.

  • Ohio, five days ago
  • California, three days ago
  • Wyoming, one day ago
  • Kansas, one day ago

So where would I live in this hypothetical example? If you pull up a map and look for a state near Wyoming and Kansas, but also between Ohio and California, slightly closer to California then Colorado becomes the determined location.

GPS works in a similar way. Signals (birthday cards) are constantly being sent from the satellites with the current time and an almanac of where all of the other satellites are. The GPS receiver calculates the difference in time between when the signal was sent (postmark) and when it was received, then figures out the distance based on the signal traveling at near light speed. Knowing the distance to each satellite and where each satellite is relative to earth, your position can be triangulated, and then fully computed.

And while the audience is for kids, I’ve always liked the following video.

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