For new readers and those who request to be“好友 good friends”  please read my 公告栏 first I am often asked by my friends in the humanities (e.g.,  Literature or History scholars) “I know you don’t drive trains, but what you really do?”. Of course, I could give the short stock answer: Engineers use sciences to solve problems of civilization and humankind. This would be accurate but dry and not very informative. Recently, I came up with a longer explanation that gives the questioner a deeper understanding and appreciation of what we do.I start off by mentioning the GPS which nowadays are found in most  autos and most people regardless of their background are familiar.  I then ask them if they know how sailors centuries ago uses fixed stars  and sextant to locate their position in the middle of an ocean.  Oh, that is “triangulation”. I then say that GPS utilizes the same idea except  instead of fixed stars which are not always available; we use 24 satellites  that circulate the earth constantly that no matter where you are  at least three of the 24 are in your line of sight. At that point I introduce the complication that while fixed stars are essentially stationery, the satellites are moving very fast, how do you triangulate with respect to several fast moving objects. Engineers figured out a fast computational scheme to do this which are generally useful for any kind of estimation (positions included) in a moving and changing environment so  long as the laws of motion are known (satellites move according to Newton’s Law of motion  which everyone has heard of). At that point the listener would say, so that’s how GPSworks. I then throw in another complication to impress them. Everyone has heard of Einstein theory of relativity, both the special and the general kind,  which few understand. But everyone  has heard that due to the theory of relativity you slow down aging in a fast moving object since time slows down when you are speeding. But we are seldom  concerned since we never move fast enough to matter.  But satellites do with respect to our automobile. Their clock runs slower than our  clock in the car. In order to accurately account for the motion of the satellite  in our triangulation calculation we must use time and make a relativistic correction in our computation. Actually the relativistic effect and correction required are even more complicated (see http://physicscentral.com/explore/writers/will.cfm I thank Weibo Gong of U Mass for this reference) . Otherwise, our position determination can be off by  miles per day instead of in “feet” of the current GPS accuracy. Thus, here we  have illustrated how ancient idea of geometrical science combined with sophisticated theory of physics to enable clever engineering to produce something of great benefit to humankind. This feat was judged so excellent that two Nobel-equivalent prizes for engineering, the Draper Prize, was awarded for conceiving the idea (in 2003) and for working out the computational scheme (in 2008 see footnote below). Listeners to this tale now walks away with a deeper  appreciation  of how we serve society and justify our existence. Footnote:If the listener is slightly more scientifically oriented and remembers his/herhigh school algebra, you can always go on to explain the essence of estimation   http://blog.sciencenet.cn/blog-1565-851754.html  .  Now the listener will feel that s/he for the first time  understands a Nobel-prize result.

Note added 6/3/2017

If you are really interested in the history of GPS you can consult the book “GPS –Declassified” by Frazier and Richardson, or the C-Span video 5/5/2017  on the same subject. It greatly expanded on the detail engineering and history behindthis dual use military-civilian product which I glossed over in this short introduction.