[LEAPSECS] Crunching Bulletin B numbers (POSIX time)

[Richard Langley]

The mention of the needs of the military for precise/accurate time  
reminded me of an article in my GPS World column from a number of  
years back. I've put it and Judah Levine's article on the legal  
traceability of time on our website:

http://gge.unb.ca/Resources/gpsworld.november00.pdf

http://gge.unb.ca/Resources/gpsworld.january01.pdf

-- Richard

On 22-Feb-11, at 5:43 PM, Warner Losh wrote:


> On 02/19/2011 16:23, Ian Batten wrote:

>>

>>>>

>>>> I think that before conjecturing the requires of isolated  

>>>> machines which have no source of leap seconds (not even manual  

>>>> application via a commandline interface "leap_next_31_dec") and  

>>>> yet are attached to better than 0.02ppm clocks, someone should  

>>>> provide an example of an isolated machine that is attached to a  

>>>> 0.02ppm clock.  And why everyone else should deal with the  

>>>> complexity to save that one computer the $100 bill for a GPS  

>>>> receiver.

>>>

>>> The issue isn't money, it's access to the sky.

>>

>> WWV, MSF or DCF77.

>>

>> If people need to operate machines in Faraday Cages, but are not  

>> prepared to supply a single manual update of one bit of information  

>> per six months, I'm not sure to what extent the rest of the  

>> community should have to engage in huge complexity in order to  

>> pander to them.

>

> WWV, MFS and DCF77 do not publish a list of leap seconds, nor the  

> current UTC-TAI offset.  In addition, they are unauthenticated  

> sources of data.  Many government/military applications require a  

> source of time you can trust, not one that happens to be available  

> that the enemy could jam/rebroadcast.

>

> GPS + SAASM gives you that, but you still have to wait for the  

> almanac to know utc time.  For hot systems, that data can be  

> cached.  For cold spares, you wait between 10 and 20 minutes to get  

> it.

>

> Warner

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| Richard B. Langley                            E-mail:  

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| Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/ 

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