[GreenKeys] Accurate clocks...

[Brooke Clarke]

Hi Don:

The story I heard is that as the GPS satellites were being put up and long 
before the constellation was operational the surveyors figured out a number of 
ways to make use the L2 Precision Positioning Service without knowing the 
military crypto code.  That's when Selective Availability was turned on.  They 
made the civilian ephemeris less accurate and dithered the clock.

I think it was year 2000 when SA was turned off permanently and it's been off 
since.  Between those two times SA was turned off when there was a need for 
more accurate results, like when looking for a down aircraft using civilian GPS 
receivers or for the first desert wars.

Anti Spoofing (AS) is still on and a crypto key is needed to make use of the L2 
signals in the normal way.

Have Fun,

Brooke Clarke
http://www.prc68.com

Don Robert House wrote:
> 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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