Satellites Replace Gas Stations    

   Well...in that your Global Positioning System 
device directs you to your destination, you no 
longer have to get directions from a gas station 
attendant.

   Unless your old habits are hard to break.

   A couple years ago we were in South Tampa looking for Gandy Blvd. Thinking we might have passed it by, we visited a gas station on the corner. 

   "Which way to Gandy Blvd?" "This is it," the man said. And so we were...at it.

   With your vehicle's GPS device, or smartphone, you are in contact with at least four satellites. There are 32 of them orbiting at 12,600 miles above Earth, circling the planet about every 12 hours.

   Independent-minded China has its own system.

   Satellites broadcast radio-waves at intervals indicating the time and their location. Your GPS device calculates how long it took for each of three signals to arrive (at the speed of light). It checks its position, comparing the three transit times.

   Each satellite generates a circle, and where the circles overlap, cross and intersect, that is your location. Your programmed maps can direct you from there. 

   The science is a bit more complicated. Timing, gravitational mass and different clock speeds all come into play. 

   What you need to understand is Einstein's general theory of relativity (of gravity) and his special theory of relativity. We won't get into that here, due to lack of space. Okay?  😊 

 The Physics of Everyday Things 

by James Kakalios

Professor of Physics, Univ. of Minnesota