Comments on: Engineer Guy vs. The Atomic Clock

By: ChrisW

ChrisW — Wed, 13 Jun 2012 21:19:29 +0000

They are tracked by ground stations and
told where they are.

By: MrDan

MrDan — Wed, 13 Jun 2012 18:08:53 +0000

3 satellites will get you 2 points, if you can use common sense (“I am stood on the Earth”), then you know which of those points is you. If not, you need a 4th satellite.

Now, the question is, how do the GPS satellites know where they are?

By: miroslava von schlochbaum

miroslava von schlochbaum — Wed, 13 Jun 2012 18:01:17 +0000

ok… all fair’nuff… but what happens if those two points happen to both be acceptable solutions (by being on the earth’s surface, or near the ISS’s ‘normal’ altitude)? (“that can’t happen.” ..ok, but why? “go visit a text book on spherical geometry and stop bothering us”)

By: ChrisW

ChrisW — Wed, 13 Jun 2012 17:59:47 +0000

True story about atomic clocks. The TV station I work at had a Rubidium atomic clock which we used to synchronize with the TV network. One day it failed and reverted to the quartz crystal oscillator. We didn’t notice any problem, but NASA called us to let us know it had failed. They had been using our broadcast signal as a frequency reference for some equipment. They gave us a Cesium clock to replace our failed Rubidium clock.

By: ChrisW

ChrisW — Wed, 13 Jun 2012 17:50:32 +0000

The ISS can just ignore the point which doesn’t fit their normal altitude.

By: ChrisW

ChrisW — Wed, 13 Jun 2012 17:47:28 +0000

Good question. Usually, one of the points is not near the Earth’s surface. The other is your position.
His explanation was good but a little off. If only one satellite is received you know nothing about your position unless you have your own synchronized atomic clock. Distance is calculated using “pseudoranges”, and four satellites are required, I think.

By: miroslava von schlochbaum

miroslava von schlochbaum — Wed, 13 Jun 2012 17:34:03 +0000

That’s interesting – thank you. so you’re saying GPS can’t be used by say… the space station?

By: MrDan

MrDan — Wed, 13 Jun 2012 17:18:30 +0000

Because of the 2 points, only one of them is on the surface of Earth. The other is in space somewhere.

By: miroslava von schlochbaum

miroslava von schlochbaum — Wed, 13 Jun 2012 16:56:59 +0000

indeed such a wondrous fine video is worthy to post adjacent redundant links to it.

one miniscule quibble with the otherwise excellent video: two (3space) sphere surfaces intersect to produce an uncertainty of a (2 space) circle; a third sphere intersects with that circle at two points, how is that enough to establish a single point? …probably time constraints in the video prevent making that clear to simple folks such as me.

By: Claudio Bonifazi (@ClaudioBonifazi)

Claudio Bonifazi (@ClaudioBonifazi) — Wed, 13 Jun 2012 13:32:33 +0000

i love this guy.