[time-nuts] : L1 GPS timing signal(s) into local time on computer(s)

Hal Murray hmurray at megapathdsl.net
Tue Aug 21 15:00:00 EDT 2012


kuzetsa at gmail.com said:
> ... I would presume that the fixed location used for above calculations
> would be relative to the position of the antenna?

A side effect of figuring out where you are is figuring out when you are 
there.

There are 4 unknowns: X, Y, Z, T, so you need 4 equations.  You get one 
equation from each satellite so you need 4 satellites.  If you assume you are 
on the surface of the earth, you can get away with only 3 satellites.

Yes, that tells you where the antenna is located.

If you know where you are, you only need 1 satellite to get the time.


> I read somewhere that even compensating for the length of the
> antenna cabling is important?

> Wow. Okay. The user manual actual considers this cable delay to be worth
> mention? 

Sure.  The speed of light in air/vacuum is 1 ft/ns.  Coax (and fiber) is 
slower.  Junk coax is roughly half as fast.  Good coax (foam) is roughly 
2/3rds as fast.  So it's easy to get 100 ns but unlikely to get more than a 
microsecond on an amateur budget.

Whether that is important for you depends upon your application and the 
length of the antenna cable.  With a modern not super-expensive scope, it's 
easy to see 1 ns offsets, so cable lengths could be important on something as 
simple as comparing the PPS outputs from 2 GPS systems.


Don't forget to consider the lengths of other cables in your setup.

I remember getting an interesting lesson in this area many years ago.  We had 
a couple of scope probes with long cables.  I was using one long one and one 
normal one and looking at high speed digital signals.  The offset was enough 
to confuse me.


-- 
These are my opinions.  I hate spam.







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