[GreenKeys] Accurate clocks...

[Don Robert House]

Thanks for reminding me Craig.

I also remember that at one time there was a lieutenant in the US Air  
Force responsible for dithering the GPS signals.
Supposedly the USAF wanted the signals to not be as accurate as they  
could be for defense reasons.

The story at the phone company went something like this...

If an attacker was using a missile dependent on GPS for navigation and  
fire it at the White House in Washington DC,
the person could steer the missile to a specific window in the  
mansion.  So by dithering the signal they could no
longer pick the window but still take out the whole building.

I wonder if the good lieutenant still spends his days and nights  
dithering?

Tee hee,

Don



On 9 Jan 2009, at 4:22 AM, Craig Sawyers wrote:

> AHH yes now I recall...  Cesium.
> That is the element that is accurate to one second over 400 years.

The constellation of orbital satellites that provide the signals for GPS
navigation use Caesium clocks.  However, the precise frequency has to be
corrected for the effects of both special relativitity (because of  
relative
motion between the satellites and the receiver) and general relativity
(because the clocks and the receiver are in different locations in the
earth's gravity field).

Although this amounts to only a few tens of microseconds a day, the
registered position of the receiver would drift by around 6 miles a  
day if
relativistic effects were not corrected for.

Craig

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