[time-nuts] Where does the source time for GPS come from?

[Tom Van Baak]

Hi Sean,

(I)

The one official, legally traceable, source of precise time & frequency for the United States has always been NIST, based on their master clock(s) in Boulder Colorado. Many PhD's, over generations, from the Dept of Commerce, have brought you this amazing service. Yes, they excel in building ever more precise clocks; for greater USA commercial corroboration and prosperity.

The informal, but highly accurate, irresistible, and utterly ubiquitous source of precise time & frequency is GPS, which is a US military asset, based on master clock(s) in Washington DC, and also master clock(s) Schriever Air Force Base, Colorado. Many PhD's, over generations, from the Dept of Defense, have brought you this amazing service. Yes, they excel in building ever more precise PNT: Positioning, Navigation, and Timing systems; for USA defense and offense.

There are also other satellite time & navigation systems, with varying degrees of accuracy, trust, intent, commercial and military control, such as GLONASS (Russia), BeiDou (China), Galileo (EU), as well as Japan and India.

As time nuts in the early 21st century, we are showered with free precise time; we live in an unprecedented era where extreme precision flows through the air and down from the sky without regulation and for bargain prices.

(II)

Atomic clocks are good, but not perfect. All of these systems of atomic clocks regularly compare themselves with 350+ other atomic clocks around the world, under the coordination and authority of BIPM, based in Paris, France. So the same people that tell you how long a *meter* really is, or how massive a *kilogram* really is, or how much a *volt* really is, also have the job of telling us how long a *second* really is. That's the basis of precise time.

Note also the "C" in UTC is "coordinated", meaning both coordination amongst some astronomical clocks (earth, moon, sun) and also coordination amongst many terrestrial clocks. No one clock ever owns "the right time", not even spinning earth ball, so they are pooled together with a best-guess algorithm. So there is no truth, only coordination. That's why UTC is the standard; not perfect; but the standard.

(III)

To answer your question:

1) At any instant your GPS receiver simply gives you "the time" based on its plain cheap crystal oscillator. It will be close, but wrong.

2) But, that little crystal oscillator is gradually, numerically adjusted based on average coordinated reception of signals from up to a dozen fancy atomic GPS clocks in space. They will be very close, but wrong, at some level.

3) And, those atomic clocks in space are gradually, occasionally numerically adjusted to match the master clock at USNO. The master clock is closest, but even it will be wrong.

4) That GPS master clock, hydrogen maser MC2, at USNO is gradually, numerically adjusted based on the weighted mean of dozens of commercial cesium clocks, and three world-class, custom-designed, atomic rubidium fountain clocks at the USNO laboratory. It's a beauty, one of the modern wonders of the world; you should visit. USNO is the best in the world for PNT, but still not "perfect".

5) This whole set of USNO clocks is gradually, numerically adjusted based on the weighted mean of atomic clocks around the world. Any of those clocks will be close, but each still wrong if you look close enough. It's when you combine *all* of them that you get something better.

6) All of those clocks together are still a guess -- but since there are *no better clocks in the universe* to compare against -- this set of 350+ clocks on earth are called "right". And that is UTC. Everyone follows UTC. Someone has to tell us what the right meter is, the right kg, the right volt. BIPM tell us what the right time is.

So while it is true no one knows perfectly what time it is exactly now, through this multi-layered, world-wide, cooperative system -- everyone gets the benefit of everyone else's best clock. Everyone tries to be as close as possible, and yet everyone known they will be wrong at some level.

This is the mystery and allure of the world of precise time. Welcome to time nuts.

/tvb


----- Original Message ----- 
From: "Sean Gallagher" <sean at wetstonetech.com>
To: <time-nuts at febo.com>
Sent: Tuesday, April 12, 2016 12:11 PM
Subject: [time-nuts] Where does the source time for GPS come from?


>I recently wrote a paper for school on precision time and from my 
> research it seems that the US Naval Observatory is the main source of 
> time for the United States. So I would assume that this is also true for 
> the Master Control Station in Colorado and that they get their source 
> time from the Observatory as well? I emailed the GPS.gov site and got an 
> answer that they do indeed have clocks there but he was unsure of their 
> source time because he was the webmaster for the site and so is not 
> intricately tied in to the system.
> 
> What's throwing me off though is that my former boss wrote up something 
> for work where he states that the satellites get their time from BIPM? 
> He is older and wiser than I am but I think that at this point I have 
> probably done more research on it all so that's why I am not accepting 
> his word immediately but also not writing it off.
> 
> NIST is another option but it appears as if they are more about research 
> correct?
> 
> Respectfully,
> 
> Sean Gallagher
> Malware Analyst
> _______________________________________________