Thursday, September 07, 2017

John Sloan on Time and Space

Once again my friends and colleagues at Gogo Business Aviation in Broomfield Colorado indulged me, in the form of my alter ego John Sloan, by letting me come and give a talk entitled Time & Space on the historical connection between precision timekeeping and navigation. They taped it and put it on the 'tube. I had a great time, and hopefully I wasn't the only one. (I did, in my enthusiasm, misspeak a couple of times, and have made some corrective remarks below.) Among lots of other things, I describe how both GPS and atomic clocks work.



A big thank you to the folks at Gogo for sponsoring this.

(Updated 2018-01-26)

Corrections
  1. Despite my saying that the escapement wheel is not a gear (and it's not), I incorrectly referred to it as a gear once or twice.
  2. When I gave the example about longitude, it would be 5 hours and 11 minutes before GMT, not after.
  3. One of my Ph.D. physicist colleagues remarked that it's not really a different electron orbital shell to which the electron transitions but instead a hyperfine transition. (I'll have to look that up.)
  4. In my excitement, I said neutrons have a positive charge when of course I meant it is protons which have a positive charge. (I've know that since probably kindergarten.)
  5. The GPS IIF satellite has three atomic clocks. not four: two rubidium and one cesium. An earlier model GPS satellite I've studied has two of each.
  6. There are a variety of algorithms used by GPS to determine your position; I described one class of algorithms. There are simpler closed-form solutions that work for time measurements from just three satellites. And there are even simpler algorithms that work for two satellites, but which don't compute altitude. The iterative solution I describe is the most general one, solving for all three spatial coordinates, and for as many satellites as can be observed.