[GreenKeys] Accurate clocks...

[Larry DeSoto]

That's true but the real purpose of dithering was to protect the ICBM silos. 
Close enough for them had to be *real* close.

Larry



----- Original Message ----- 
From: "Don Robert House" <Packard42 at gmail.com>
To: "Craig Sawyers" <c.sawyers at tech-enterprise.com>
Cc: <greenkeys at mailman.qth.net>
Sent: Friday, January 09, 2009 2:51 PM
Subject: Re: [GreenKeys] Accurate clocks...


> 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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