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